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<h1><a href="https://google.github.io/styleguide/">styleguide</a></h1>
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<h1 id="google-python-style-guide">Google Python Style Guide</h1>
<!-- markdown="1" is required for GitHub Pages to render the TOC properly. -->
<details>
<summary>Table of Contents</summary>
<ul>
<li><a href="#s1-background">1 Background</a></li>
<li><a href="#s2-python-language-rules">2 Python Language Rules</a>
<ul>
<li><a href="#s2.1-lint">2.1 Lint</a></li>
<li><a href="#s2.2-imports">2.2 Imports</a></li>
<li><a href="#s2.3-packages">2.3 Packages</a></li>
<li><a href="#s2.4-exceptions">2.4 Exceptions</a></li>
<li><a href="#s2.5-global-variables">2.5 Mutable Global State</a></li>
<li><a href="#s2.6-nested">2.6 Nested/Local/Inner Classes and Functions</a></li>
<li><a href="#s2.7-comprehensions">2.7 Comprehensions &amp; Generator Expressions</a></li>
<li><a href="#s2.8-default-iterators-and-operators">2.8 Default Iterators and Operators</a></li>
<li><a href="#s2.9-generators">2.9 Generators</a></li>
<li><a href="#s2.10-lambda-functions">2.10 Lambda Functions</a></li>
<li><a href="#s2.11-conditional-expressions">2.11 Conditional Expressions</a></li>
<li><a href="#s2.12-default-argument-values">2.12 Default Argument Values</a></li>
<li><a href="#s2.13-properties">2.13 Properties</a></li>
<li><a href="#s2.14-truefalse-evaluations">2.14 True/False Evaluations</a></li>
<li><a href="#s2.16-lexical-scoping">2.16 Lexical Scoping</a></li>
<li><a href="#s2.17-function-and-method-decorators">2.17 Function and Method Decorators</a></li>
<li><a href="#s2.18-threading">2.18 Threading</a></li>
<li><a href="#s2.19-power-features">2.19 Power Features</a></li>
<li><a href="#s2.20-modern-python">2.20 Modern Python: from __future__ imports</a></li>
<li><a href="#s2.21-type-annotated-code">2.21 Type Annotated Code</a></li>
</ul>
</li>
<li><a href="#s3-python-style-rules">3 Python Style Rules</a>
<ul>
<li><a href="#s3.1-semicolons">3.1 Semicolons</a></li>
<li><a href="#s3.2-line-length">3.2 Line length</a></li>
<li><a href="#s3.3-parentheses">3.3 Parentheses</a></li>
<li><a href="#s3.4-indentation">3.4 Indentation</a>
<ul>
<li><a href="#s3.4.1-trailing-commas">3.4.1 Trailing commas in sequences of items?</a></li>
</ul>
</li>
<li><a href="#s3.5-blank-lines">3.5 Blank Lines</a></li>
<li><a href="#s3.6-whitespace">3.6 Whitespace</a></li>
<li><a href="#s3.7-shebang-line">3.7 Shebang Line</a></li>
<li><a href="#s3.8-comments-and-docstrings">3.8 Comments and Docstrings</a>
<ul>
<li><a href="#s3.8.1-comments-in-doc-strings">3.8.1 Docstrings</a></li>
<li><a href="#s3.8.2-comments-in-modules">3.8.2 Modules</a></li>
<li><a href="#s3.8.2.1-test-modules">3.8.2.1 Test modules</a></li>
<li><a href="#s3.8.3-functions-and-methods">3.8.3 Functions and Methods</a></li>
<li><a href="#s3.8.3.1-overridden-methods">3.8.3.1 Overridden Methods</a></li>
<li><a href="#s3.8.4-comments-in-classes">3.8.4 Classes</a></li>
<li><a href="#s3.8.5-block-and-inline-comments">3.8.5 Block and Inline Comments</a></li>
<li><a href="#s3.8.6-punctuation-spelling-and-grammar">3.8.6 Punctuation, Spelling, and Grammar</a></li>
</ul>
</li>
<li><a href="#s3.10-strings">3.10 Strings</a>
<ul>
<li><a href="#s3.10.1-logging">3.10.1 Logging</a></li>
<li><a href="#s3.10.2-error-messages">3.10.2 Error Messages</a></li>
</ul>
</li>
<li><a href="#s3.11-files-sockets-closeables">3.11 Files, Sockets, and similar Stateful Resources</a></li>
<li><a href="#s3.12-todo-comments">3.12 TODO Comments</a></li>
<li><a href="#s3.13-imports-formatting">3.13 Imports formatting</a></li>
<li><a href="#s3.14-statements">3.14 Statements</a></li>
<li><a href="#s3.15-accessors">3.15 Accessors</a></li>
<li><a href="#s3.16-naming">3.16 Naming</a>
<ul>
<li><a href="#s3.16.1-names-to-avoid">3.16.1 Names to Avoid</a></li>
<li><a href="#s3.16.2-naming-conventions">3.16.2 Naming Conventions</a></li>
<li><a href="#s3.16.3-file-naming">3.16.3 File Naming</a></li>
<li><a href="#s3.16.4-guidelines-derived-from-guidos-recommendations">3.16.4 Guidelines derived from Guido’s Recommendations</a></li>
</ul>
</li>
<li><a href="#s3.17-main">3.17 Main</a></li>
<li><a href="#s3.18-function-length">3.18 Function length</a></li>
<li><a href="#s3.19-type-annotations">3.19 Type Annotations</a>
<ul>
<li><a href="#s3.19.1-general-rules">3.19.1 General Rules</a></li>
<li><a href="#s3.19.2-line-breaking">3.19.2 Line Breaking</a></li>
<li><a href="#s3.19.3-forward-declarations">3.19.3 Forward Declarations</a></li>
<li><a href="#s3.19.4-default-values">3.19.4 Default Values</a></li>
<li><a href="#s3.19.5-nonetype">3.19.5 NoneType</a></li>
<li><a href="#s3.19.6-type-aliases">3.19.6 Type Aliases</a></li>
<li><a href="#s3.19.7-ignoring-types">3.19.7 Ignoring Types</a></li>
<li><a href="#s3.19.8-typing-variables">3.19.8 Typing Variables</a></li>
<li><a href="#s3.19.9-tuples-vs-lists">3.19.9 Tuples vs Lists</a></li>
<li><a href="#s3.19.10-typevars">3.19.10 Type variables</a></li>
<li><a href="#s3.19.11-string-types">3.19.11 String types</a></li>
<li><a href="#s3.19.12-imports-for-typing">3.19.12 Imports For Typing</a></li>
<li><a href="#s3.19.13-conditional-imports">3.19.13 Conditional Imports</a></li>
<li><a href="#s3.19.14-circular-dependencies">3.19.14 Circular Dependencies</a></li>
<li><a href="#s3.19.15-generics">3.19.15 Generics</a></li>
<li><a href="#s3.19.16-build-dependencies">3.19.16 Build Dependencies</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#4-parting-words">4 Parting Words</a></li>
</ul>
</details>
<p><a id="s1-background"></a>
<a id="1-background"></a></p>
<p><a id="background"></a></p>
<h2 id="1-background">1 Background</h2>
<p>Python is the main dynamic language used at Google. This style guide is a list
of <em>dos and don’ts</em> for Python programs.</p>
<p>To help you format code correctly, we’ve created a <a href="/styleguide/google_python_style.vim">settings file for Vim</a>. For Emacs, the default settings should be fine.</p>
<p>Many teams use the <a href="https://github.com/psf/black">Black</a> or <a href="https://github.com/google/pyink">Pyink</a>
auto-formatter to avoid arguing over formatting.</p>
<p><a id="s2-python-language-rules"></a>
<a id="2-python-language-rules"></a></p>
<p><a id="python-language-rules"></a></p>
<h2 id="2-python-language-rules">2 Python Language Rules</h2>
<p><a id="s2.1-lint"></a>
<a id="21-lint"></a></p>
<p><a id="lint"></a></p>
<h3 id="21-lint">2.1 Lint</h3>
<p>Run <code class="language-plaintext highlighter-rouge">pylint</code> over your code using this <a href="https://google.github.io/styleguide/pylintrc">pylintrc</a>.</p>
<p><a id="s2.1.1-definition"></a>
<a id="211-definition"></a></p>
<p><a id="lint-definition"></a></p>
<h4 id="211-definition">2.1.1 Definition</h4>
<p><code class="language-plaintext highlighter-rouge">pylint</code>
is a tool for finding bugs and style problems in Python source code. It finds
problems that are typically caught by a compiler for less dynamic languages like
C and C++. Because of the dynamic nature of Python, some
warnings may be incorrect; however, spurious warnings should be fairly
infrequent.</p>
<p><a id="s2.1.2-pros"></a>
<a id="212-pros"></a></p>
<p><a id="lint-pros"></a></p>
<h4 id="212-pros">2.1.2 Pros</h4>
<p>Catches easy-to-miss errors like typos, using-vars-before-assignment, etc.</p>
<p><a id="s2.1.3-cons"></a>
<a id="213-cons"></a></p>
<p><a id="lint-cons"></a></p>
<h4 id="213-cons">2.1.3 Cons</h4>
<p><code class="language-plaintext highlighter-rouge">pylint</code>
isn’t perfect. To take advantage of it, sometimes we’ll need to write around it,
suppress its warnings or fix it.</p>
<p><a id="s2.1.4-decision"></a>
<a id="214-decision"></a></p>
<p><a id="lint-decision"></a></p>
<h4 id="214-decision">2.1.4 Decision</h4>
<p>Make sure you run
<code class="language-plaintext highlighter-rouge">pylint</code>
on your code.</p>
<p>Suppress warnings if they are inappropriate so that other issues are not hidden.
To suppress warnings, you can set a line-level comment:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">def</span> <span class="nf">do_PUT</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="c1"># WSGI name, so pylint: disable=invalid-name
</span> <span class="p">...</span>
</code></pre></div></div>
<p><code class="language-plaintext highlighter-rouge">pylint</code>
warnings are each identified by symbolic name (<code class="language-plaintext highlighter-rouge">empty-docstring</code>)
Google-specific warnings start with <code class="language-plaintext highlighter-rouge">g-</code>.</p>
<p>If the reason for the suppression is not clear from the symbolic name, add an
explanation.</p>
<p>Suppressing in this way has the advantage that we can easily search for
suppressions and revisit them.</p>
<p>You can get a list of
<code class="language-plaintext highlighter-rouge">pylint</code>
warnings by doing:</p>
<div class="language-shell highlighter-rouge"><div class="highlight"><pre class="highlight"><code>pylint <span class="nt">--list-msgs</span>
</code></pre></div></div>
<p>To get more information on a particular message, use:</p>
<div class="language-shell highlighter-rouge"><div class="highlight"><pre class="highlight"><code>pylint <span class="nt">--help-msg</span><span class="o">=</span>invalid-name
</code></pre></div></div>
<p>Prefer <code class="language-plaintext highlighter-rouge">pylint: disable</code> to the deprecated older form <code class="language-plaintext highlighter-rouge">pylint: disable-msg</code>.</p>
<p>Unused argument warnings can be suppressed by deleting the variables at the
beginning of the function. Always include a comment explaining why you are
deleting it. “Unused.” is sufficient. For example:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">def</span> <span class="nf">viking_cafe_order</span><span class="p">(</span><span class="n">spam</span><span class="p">:</span> <span class="nb">str</span><span class="p">,</span> <span class="n">beans</span><span class="p">:</span> <span class="nb">str</span><span class="p">,</span> <span class="n">eggs</span><span class="p">:</span> <span class="nb">str</span> <span class="o">|</span> <span class="bp">None</span> <span class="o">=</span> <span class="bp">None</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">str</span><span class="p">:</span>
<span class="k">del</span> <span class="n">beans</span><span class="p">,</span> <span class="n">eggs</span> <span class="c1"># Unused by vikings.
</span> <span class="k">return</span> <span class="n">spam</span> <span class="o">+</span> <span class="n">spam</span> <span class="o">+</span> <span class="n">spam</span>
</code></pre></div></div>
<p>Other common forms of suppressing this warning include using ‘<code class="language-plaintext highlighter-rouge">_</code>’ as the
identifier for the unused argument or prefixing the argument name with
‘<code class="language-plaintext highlighter-rouge">unused_</code>’, or assigning them to ‘<code class="language-plaintext highlighter-rouge">_</code>’. These forms are allowed but no longer
encouraged. These break callers that pass arguments by name and do not enforce
that the arguments are actually unused.</p>
<p><a id="s2.2-imports"></a>
<a id="22-imports"></a></p>
<p><a id="imports"></a></p>
<h3 id="22-imports">2.2 Imports</h3>
<p>Use <code class="language-plaintext highlighter-rouge">import</code> statements for packages and modules only, not for individual types,
classes, or functions.</p>
<p><a id="s2.2.1-definition"></a>
<a id="221-definition"></a></p>
<p><a id="imports-definition"></a></p>
<h4 id="221-definition">2.2.1 Definition</h4>
<p>Reusability mechanism for sharing code from one module to another.</p>
<p><a id="s2.2.2-pros"></a>
<a id="222-pros"></a></p>
<p><a id="imports-pros"></a></p>
<h4 id="222-pros">2.2.2 Pros</h4>
<p>The namespace management convention is simple. The source of each identifier is
indicated in a consistent way; <code class="language-plaintext highlighter-rouge">x.Obj</code> says that object <code class="language-plaintext highlighter-rouge">Obj</code> is defined in
module <code class="language-plaintext highlighter-rouge">x</code>.</p>
<p><a id="s2.2.3-cons"></a>
<a id="223-cons"></a></p>
<p><a id="imports-cons"></a></p>
<h4 id="223-cons">2.2.3 Cons</h4>
<p>Module names can still collide. Some module names are inconveniently long.</p>
<p><a id="s2.2.4-decision"></a>
<a id="224-decision"></a></p>
<p><a id="imports-decision"></a></p>
<h4 id="224-decision">2.2.4 Decision</h4>
<ul>
<li>Use <code class="language-plaintext highlighter-rouge">import x</code> for importing packages and modules.</li>
<li>Use <code class="language-plaintext highlighter-rouge">from x import y</code> where <code class="language-plaintext highlighter-rouge">x</code> is the package prefix and <code class="language-plaintext highlighter-rouge">y</code> is the module
name with no prefix.</li>
<li>Use <code class="language-plaintext highlighter-rouge">from x import y as z</code> in any of the following circumstances:
<ul>
<li>Two modules named <code class="language-plaintext highlighter-rouge">y</code> are to be imported.</li>
<li><code class="language-plaintext highlighter-rouge">y</code> conflicts with a top-level name defined in the current module.</li>
<li><code class="language-plaintext highlighter-rouge">y</code> conflicts with a common parameter name that is part of the public
API (e.g., <code class="language-plaintext highlighter-rouge">features</code>).</li>
<li><code class="language-plaintext highlighter-rouge">y</code> is an inconveniently long name.</li>
<li><code class="language-plaintext highlighter-rouge">y</code> is too generic in the context of your code (e.g., <code class="language-plaintext highlighter-rouge">from
storage.file_system import options as fs_options</code>).</li>
</ul>
</li>
<li>Use <code class="language-plaintext highlighter-rouge">import y as z</code> only when <code class="language-plaintext highlighter-rouge">z</code> is a standard abbreviation (e.g., <code class="language-plaintext highlighter-rouge">import
numpy as np</code>).</li>
</ul>
<p>For example the module <code class="language-plaintext highlighter-rouge">sound.effects.echo</code> may be imported as follows:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">sound.effects</span> <span class="kn">import</span> <span class="n">echo</span>
<span class="p">...</span>
<span class="n">echo</span><span class="p">.</span><span class="n">EchoFilter</span><span class="p">(</span><span class="nb">input</span><span class="p">,</span> <span class="n">output</span><span class="p">,</span> <span class="n">delay</span><span class="o">=</span><span class="mf">0.7</span><span class="p">,</span> <span class="n">atten</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span>
</code></pre></div></div>
<p>Do not use relative names in imports. Even if the module is in the same package,
use the full package name. This helps prevent unintentionally importing a
package twice.</p>
<p><a id="imports-exemptions"></a></p>
<h5 id="2241-exemptions">2.2.4.1 Exemptions</h5>
<p>Exemptions from this rule:</p>
<ul>
<li>Symbols from the following modules are used to support static analysis and
type checking:
<ul>
<li><a href="#typing-imports"><code class="language-plaintext highlighter-rouge">typing</code> module</a></li>
<li><a href="#typing-imports"><code class="language-plaintext highlighter-rouge">collections.abc</code> module</a></li>
<li><a href="https://github.com/python/typing_extensions/blob/main/README.md"><code class="language-plaintext highlighter-rouge">typing_extensions</code> module</a></li>
</ul>
</li>
<li>Redirects from the
<a href="https://six.readthedocs.io/#module-six.moves">six.moves module</a>.</li>
</ul>
<p><a id="s2.3-packages"></a>
<a id="23-packages"></a></p>
<p><a id="packages"></a></p>
<h3 id="23-packages">2.3 Packages</h3>
<p>Import each module using the full pathname location of the module.</p>
<p><a id="s2.3.1-pros"></a>
<a id="231-pros"></a></p>
<p><a id="packages-pros"></a></p>
<h4 id="231-pros">2.3.1 Pros</h4>
<p>Avoids conflicts in module names or incorrect imports due to the module search
path not being what the author expected. Makes it easier to find modules.</p>
<p><a id="s2.3.2-cons"></a>
<a id="232-cons"></a></p>
<p><a id="packages-cons"></a></p>
<h4 id="232-cons">2.3.2 Cons</h4>
<p>Makes it harder to deploy code because you have to replicate the package
hierarchy. Not really a problem with modern deployment mechanisms.</p>
<p><a id="s2.3.3-decision"></a>
<a id="233-decision"></a></p>
<p><a id="packages-decision"></a></p>
<h4 id="233-decision">2.3.3 Decision</h4>
<p>All new code should import each module by its full package name.</p>
<p>Imports should be as follows:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="c1"># Reference absl.flags in code with the complete name (verbose).
</span> <span class="kn">import</span> <span class="nn">absl.flags</span>
<span class="kn">from</span> <span class="nn">doctor.who</span> <span class="kn">import</span> <span class="n">jodie</span>
<span class="n">_FOO</span> <span class="o">=</span> <span class="n">absl</span><span class="p">.</span><span class="n">flags</span><span class="p">.</span><span class="n">DEFINE_string</span><span class="p">(...)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="c1"># Reference flags in code with just the module name (common).
</span> <span class="kn">from</span> <span class="nn">absl</span> <span class="kn">import</span> <span class="n">flags</span>
<span class="kn">from</span> <span class="nn">doctor.who</span> <span class="kn">import</span> <span class="n">jodie</span>
<span class="n">_FOO</span> <span class="o">=</span> <span class="n">flags</span><span class="p">.</span><span class="n">DEFINE_string</span><span class="p">(...)</span>
</code></pre></div></div>
<p><em>(assume this file lives in <code class="language-plaintext highlighter-rouge">doctor/who/</code> where <code class="language-plaintext highlighter-rouge">jodie.py</code> also exists)</em></p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span>
<span class="c1"># Unclear what module the author wanted and what will be imported. The actual
</span> <span class="c1"># import behavior depends on external factors controlling sys.path.
</span> <span class="c1"># Which possible jodie module did the author intend to import?
</span> <span class="kn">import</span> <span class="nn">jodie</span>
</code></pre></div></div>
<p>The directory the main binary is located in should not be assumed to be in
<code class="language-plaintext highlighter-rouge">sys.path</code> despite that happening in some environments. This being the case,
code should assume that <code class="language-plaintext highlighter-rouge">import jodie</code> refers to a third-party or top-level
package named <code class="language-plaintext highlighter-rouge">jodie</code>, not a local <code class="language-plaintext highlighter-rouge">jodie.py</code>.</p>
<p><a id="s2.4-exceptions"></a>
<a id="24-exceptions"></a></p>
<p><a id="exceptions"></a></p>
<h3 id="24-exceptions">2.4 Exceptions</h3>
<p>Exceptions are allowed but must be used carefully.</p>
<p><a id="s2.4.1-definition"></a>
<a id="241-definition"></a></p>
<p><a id="exceptions-definition"></a></p>
<h4 id="241-definition">2.4.1 Definition</h4>
<p>Exceptions are a means of breaking out of normal control flow to handle errors
or other exceptional conditions.</p>
<p><a id="s2.4.2-pros"></a>
<a id="242-pros"></a></p>
<p><a id="exceptions-pros"></a></p>
<h4 id="242-pros">2.4.2 Pros</h4>
<p>The control flow of normal operation code is not cluttered by error-handling
code. It also allows the control flow to skip multiple frames when a certain
condition occurs, e.g., returning from N nested functions in one step instead of
having to plumb error codes through.</p>
<p><a id="s2.4.3-cons"></a>
<a id="243-cons"></a></p>
<p><a id="exceptions-cons"></a></p>
<h4 id="243-cons">2.4.3 Cons</h4>
<p>May cause the control flow to be confusing. Easy to miss error cases when making
library calls.</p>
<p><a id="s2.4.4-decision"></a>
<a id="244-decision"></a></p>
<p><a id="exceptions-decision"></a></p>
<h4 id="244-decision">2.4.4 Decision</h4>
<p>Exceptions must follow certain conditions:</p>
<ul>
<li>
<p>Make use of built-in exception classes when it makes sense. For example,
raise a <code class="language-plaintext highlighter-rouge">ValueError</code> to indicate a programming mistake like a violated
precondition, such as may happen when validating function arguments.</p>
</li>
<li>
<p>Do not use <code class="language-plaintext highlighter-rouge">assert</code> statements in place of conditionals or validating
preconditions. They must not be critical to the application logic. A litmus
test would be that the <code class="language-plaintext highlighter-rouge">assert</code> could be removed without breaking the code.
<code class="language-plaintext highlighter-rouge">assert</code> conditionals are
<a href="https://docs.python.org/3/reference/simple_stmts.html#the-assert-statement">not guaranteed</a>
to be evaluated. For <a href="https://pytest.org">pytest</a> based tests, <code class="language-plaintext highlighter-rouge">assert</code> is
okay and expected to verify expectations. For
example:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">connect_to_next_port</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">minimum</span><span class="p">:</span> <span class="nb">int</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">int</span><span class="p">:</span>
<span class="s">"""Connects to the next available port.
Args:
minimum: A port value greater or equal to 1024.
Returns:
The new minimum port.
Raises:
ConnectionError: If no available port is found.
"""</span>
<span class="k">if</span> <span class="n">minimum</span> <span class="o">&lt;</span> <span class="mi">1024</span><span class="p">:</span>
<span class="c1"># Note that this raising of ValueError is not mentioned in the doc
</span> <span class="c1"># string's "Raises:" section because it is not appropriate to
</span> <span class="c1"># guarantee this specific behavioral reaction to API misuse.
</span> <span class="k">raise</span> <span class="nb">ValueError</span><span class="p">(</span><span class="sa">f</span><span class="s">'Min. port must be at least 1024, not </span><span class="si">{</span><span class="n">minimum</span><span class="si">}</span><span class="s">.'</span><span class="p">)</span>
<span class="n">port</span> <span class="o">=</span> <span class="bp">self</span><span class="p">.</span><span class="n">_find_next_open_port</span><span class="p">(</span><span class="n">minimum</span><span class="p">)</span>
<span class="k">if</span> <span class="n">port</span> <span class="ow">is</span> <span class="bp">None</span><span class="p">:</span>
<span class="k">raise</span> <span class="nb">ConnectionError</span><span class="p">(</span>
<span class="sa">f</span><span class="s">'Could not connect to service on port </span><span class="si">{</span><span class="n">minimum</span><span class="si">}</span><span class="s"> or higher.'</span><span class="p">)</span>
<span class="c1"># The code does not depend on the result of this assert.
</span> <span class="k">assert</span> <span class="n">port</span> <span class="o">&gt;=</span> <span class="n">minimum</span><span class="p">,</span> <span class="p">(</span>
<span class="sa">f</span><span class="s">'Unexpected port </span><span class="si">{</span><span class="n">port</span><span class="si">}</span><span class="s"> when minimum was </span><span class="si">{</span><span class="n">minimum</span><span class="si">}</span><span class="s">.'</span><span class="p">)</span>
<span class="k">return</span> <span class="n">port</span>
</code></pre></div> </div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">connect_to_next_port</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">minimum</span><span class="p">:</span> <span class="nb">int</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">int</span><span class="p">:</span>
<span class="s">"""Connects to the next available port.
Args:
minimum: A port value greater or equal to 1024.
Returns:
The new minimum port.
"""</span>
<span class="k">assert</span> <span class="n">minimum</span> <span class="o">&gt;=</span> <span class="mi">1024</span><span class="p">,</span> <span class="s">'Minimum port must be at least 1024.'</span>
<span class="c1"># The following code depends on the previous assert.
</span> <span class="n">port</span> <span class="o">=</span> <span class="bp">self</span><span class="p">.</span><span class="n">_find_next_open_port</span><span class="p">(</span><span class="n">minimum</span><span class="p">)</span>
<span class="k">assert</span> <span class="n">port</span> <span class="ow">is</span> <span class="ow">not</span> <span class="bp">None</span>
<span class="c1"># The type checking of the return statement relies on the assert.
</span> <span class="k">return</span> <span class="n">port</span>
</code></pre></div> </div>
</li>
<li>
<p>Libraries or packages may define their own exceptions. When doing so they
must inherit from an existing exception class. Exception names should end in
<code class="language-plaintext highlighter-rouge">Error</code> and should not introduce repetition (<code class="language-plaintext highlighter-rouge">foo.FooError</code>).</p>
</li>
<li>
<p>Never use catch-all <code class="language-plaintext highlighter-rouge">except:</code> statements, or catch <code class="language-plaintext highlighter-rouge">Exception</code> or
<code class="language-plaintext highlighter-rouge">StandardError</code>, unless you are</p>
<ul>
<li>re-raising the exception, or</li>
<li>creating an isolation point in the program where exceptions are not
propagated but are recorded and suppressed instead, such as protecting a
thread from crashing by guarding its outermost block.</li>
</ul>
<p>Python is very tolerant in this regard and <code class="language-plaintext highlighter-rouge">except:</code> will really catch
everything including misspelled names, sys.exit() calls, Ctrl+C interrupts,
unittest failures and all kinds of other exceptions that you simply don’t
want to catch.</p>
</li>
<li>
<p>Minimize the amount of code in a <code class="language-plaintext highlighter-rouge">try</code>/<code class="language-plaintext highlighter-rouge">except</code> block. The larger the body
of the <code class="language-plaintext highlighter-rouge">try</code>, the more likely that an exception will be raised by a line of
code that you didn’t expect to raise an exception. In those cases, the
<code class="language-plaintext highlighter-rouge">try</code>/<code class="language-plaintext highlighter-rouge">except</code> block hides a real error.</p>
</li>
<li>
<p>Use the <code class="language-plaintext highlighter-rouge">finally</code> clause to execute code whether or not an exception is
raised in the <code class="language-plaintext highlighter-rouge">try</code> block. This is often useful for cleanup, i.e., closing a
file.</p>
</li>
</ul>
<p><a id="s2.5-global-variables"></a>
<a id="25-global-variables"></a>
<a id="s2.5-global-state"></a>
<a id="25-global-state"></a></p>
<p><a id="global-variables"></a></p>
<h3 id="25-mutable-global-state">2.5 Mutable Global State</h3>
<p>Avoid mutable global state.</p>
<p><a id="s2.5.1-definition"></a>
<a id="251-definition"></a></p>
<p><a id="global-variables-definition"></a></p>
<h4 id="251-definition">2.5.1 Definition</h4>
<p>Module-level values or class attributes that can get mutated during program
execution.</p>
<p><a id="s2.5.2-pros"></a>
<a id="252-pros"></a></p>
<p><a id="global-variables-pros"></a></p>
<h4 id="252-pros">2.5.2 Pros</h4>
<p>Occasionally useful.</p>
<p><a id="s2.5.3-cons"></a>
<a id="253-cons"></a></p>
<p><a id="global-variables-cons"></a></p>
<h4 id="253-cons">2.5.3 Cons</h4>
<ul>
<li>
<p>Breaks encapsulation: Such design can make it hard to achieve valid
objectives. For example, if global state is used to manage a database
connection, then connecting to two different databases at the same time
(such as for computing differences during a migration) becomes difficult.
Similar problems easily arise with global registries.</p>
</li>
<li>
<p>Has the potential to change module behavior during the import, because
assignments to global variables are done when the module is first imported.</p>
</li>
</ul>
<p><a id="s2.5.4-decision"></a>
<a id="254-decision"></a></p>
<p><a id="global-variables-decision"></a></p>
<h4 id="254-decision">2.5.4 Decision</h4>
<p>Avoid mutable global state.</p>
<p>In those rare cases where using global state is warranted, mutable global
entities should be declared at the module level or as a class attribute and made
internal by prepending an <code class="language-plaintext highlighter-rouge">_</code> to the name. If necessary, external access to
mutable global state must be done through public functions or class methods. See
<a href="#s3.16-naming">Naming</a> below. Please explain the design reasons why mutable
global state is being used in a comment or a doc linked to from a comment.</p>
<p>Module-level constants are permitted and encouraged. For example:
<code class="language-plaintext highlighter-rouge">_MAX_HOLY_HANDGRENADE_COUNT = 3</code> for an internal use constant or
<code class="language-plaintext highlighter-rouge">SIR_LANCELOTS_FAVORITE_COLOR = "blue"</code> for a public API constant. Constants
must be named using all caps with underscores. See <a href="#s3.16-naming">Naming</a>
below.</p>
<p><a id="s2.6-nested"></a>
<a id="26-nested"></a></p>
<p><a id="nested-classes-functions"></a></p>
<h3 id="26-nestedlocalinner-classes-and-functions">2.6 Nested/Local/Inner Classes and Functions</h3>
<p>Nested local functions or classes are fine when used to close over a local
variable. Inner classes are fine.</p>
<p><a id="s2.6.1-definition"></a>
<a id="261-definition"></a></p>
<p><a id="nested-classes-functions-definition"></a></p>
<h4 id="261-definition">2.6.1 Definition</h4>
<p>A class can be defined inside of a method, function, or class. A function can be
defined inside a method or function. Nested functions have read-only access to
variables defined in enclosing scopes.</p>
<p><a id="s2.6.2-pros"></a>
<a id="262-pros"></a></p>
<p><a id="nested-classes-functions-pros"></a></p>
<h4 id="262-pros">2.6.2 Pros</h4>
<p>Allows definition of utility classes and functions that are only used inside of
a very limited scope. Very
<a href="https://en.wikipedia.org/wiki/Abstract_data_type">ADT</a>-y. Commonly used for
implementing decorators.</p>
<p><a id="s2.6.3-cons"></a>
<a id="263-cons"></a></p>
<p><a id="nested-classes-functions-cons"></a></p>
<h4 id="263-cons">2.6.3 Cons</h4>
<p>Nested functions and classes cannot be directly tested. Nesting can make the
outer function longer and less readable.</p>
<p><a id="s2.6.4-decision"></a>
<a id="264-decision"></a></p>
<p><a id="nested-classes-functions-decision"></a></p>
<h4 id="264-decision">2.6.4 Decision</h4>
<p>They are fine with some caveats. Avoid nested functions or classes except when
closing over a local value other than <code class="language-plaintext highlighter-rouge">self</code> or <code class="language-plaintext highlighter-rouge">cls</code>. Do not nest a function
just to hide it from users of a module. Instead, prefix its name with an _ at
the module level so that it can still be accessed by tests.</p>
<p><a id="s2.7-comprehensions"></a>
<a id="s2.7-list_comprehensions"></a>
<a id="27-list_comprehensions"></a>
<a id="list_comprehensions"></a>
<a id="list-comprehensions"></a></p>
<p><a id="comprehensions"></a></p>
<h3 id="27-comprehensions--generator-expressions">2.7 Comprehensions &amp; Generator Expressions</h3>
<p>Okay to use for simple cases.</p>
<p><a id="s2.7.1-definition"></a>
<a id="271-definition"></a></p>
<p><a id="comprehensions-definition"></a></p>
<h4 id="271-definition">2.7.1 Definition</h4>
<p>List, Dict, and Set comprehensions as well as generator expressions provide a
concise and efficient way to create container types and iterators without
resorting to the use of traditional loops, <code class="language-plaintext highlighter-rouge">map()</code>, <code class="language-plaintext highlighter-rouge">filter()</code>, or <code class="language-plaintext highlighter-rouge">lambda</code>.</p>
<p><a id="s2.7.2-pros"></a>
<a id="272-pros"></a></p>
<p><a id="comprehensions-pros"></a></p>
<h4 id="272-pros">2.7.2 Pros</h4>
<p>Simple comprehensions can be clearer and simpler than other dict, list, or set
creation techniques. Generator expressions can be very efficient, since they
avoid the creation of a list entirely.</p>
<p><a id="s2.7.3-cons"></a>
<a id="273-cons"></a></p>
<p><a id="comprehensions-cons"></a></p>
<h4 id="273-cons">2.7.3 Cons</h4>
<p>Complicated comprehensions or generator expressions can be hard to read.</p>
<p><a id="s2.7.4-decision"></a>
<a id="274-decision"></a></p>
<p><a id="comprehensions-decision"></a></p>
<h4 id="274-decision">2.7.4 Decision</h4>
<p>Comprehensions are allowed, however multiple <code class="language-plaintext highlighter-rouge">for</code> clauses or filter expressions
are not permitted. Optimize for readability, not conciseness.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="n">result</span> <span class="o">=</span> <span class="p">[</span><span class="n">mapping_expr</span> <span class="k">for</span> <span class="n">value</span> <span class="ow">in</span> <span class="n">iterable</span> <span class="k">if</span> <span class="n">filter_expr</span><span class="p">]</span>
<span class="n">result</span> <span class="o">=</span> <span class="p">[</span>
<span class="n">is_valid</span><span class="p">(</span><span class="n">metric</span><span class="o">=</span><span class="p">{</span><span class="s">'key'</span><span class="p">:</span> <span class="n">value</span><span class="p">})</span>
<span class="k">for</span> <span class="n">value</span> <span class="ow">in</span> <span class="n">interesting_iterable</span>
<span class="k">if</span> <span class="n">a_longer_filter_expression</span><span class="p">(</span><span class="n">value</span><span class="p">)</span>
<span class="p">]</span>
<span class="n">descriptive_name</span> <span class="o">=</span> <span class="p">[</span>
<span class="n">transform</span><span class="p">({</span><span class="s">'key'</span><span class="p">:</span> <span class="n">key</span><span class="p">,</span> <span class="s">'value'</span><span class="p">:</span> <span class="n">value</span><span class="p">},</span> <span class="n">color</span><span class="o">=</span><span class="s">'black'</span><span class="p">)</span>
<span class="k">for</span> <span class="n">key</span><span class="p">,</span> <span class="n">value</span> <span class="ow">in</span> <span class="n">generate_iterable</span><span class="p">(</span><span class="n">some_input</span><span class="p">)</span>
<span class="k">if</span> <span class="n">complicated_condition_is_met</span><span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="n">value</span><span class="p">)</span>
<span class="p">]</span>
<span class="n">result</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
<span class="k">for</span> <span class="n">y</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">):</span>
<span class="k">if</span> <span class="n">x</span> <span class="o">*</span> <span class="n">y</span> <span class="o">&gt;</span> <span class="mi">10</span><span class="p">:</span>
<span class="n">result</span><span class="p">.</span><span class="n">append</span><span class="p">((</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">))</span>
<span class="k">return</span> <span class="p">{</span>
<span class="n">x</span><span class="p">:</span> <span class="n">complicated_transform</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">long_generator_function</span><span class="p">(</span><span class="n">parameter</span><span class="p">)</span>
<span class="k">if</span> <span class="n">x</span> <span class="ow">is</span> <span class="ow">not</span> <span class="bp">None</span>
<span class="p">}</span>
<span class="k">return</span> <span class="p">(</span><span class="n">x</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span>
<span class="n">unique_names</span> <span class="o">=</span> <span class="p">{</span><span class="n">user</span><span class="p">.</span><span class="n">name</span> <span class="k">for</span> <span class="n">user</span> <span class="ow">in</span> <span class="n">users</span> <span class="k">if</span> <span class="n">user</span> <span class="ow">is</span> <span class="ow">not</span> <span class="bp">None</span><span class="p">}</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span>
<span class="n">result</span> <span class="o">=</span> <span class="p">[(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span> <span class="k">for</span> <span class="n">y</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> <span class="k">if</span> <span class="n">x</span> <span class="o">*</span> <span class="n">y</span> <span class="o">&gt;</span> <span class="mi">10</span><span class="p">]</span>
<span class="k">return</span> <span class="p">(</span>
<span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">z</span><span class="p">)</span>
<span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="k">for</span> <span class="n">y</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="k">if</span> <span class="n">x</span> <span class="o">!=</span> <span class="n">y</span>
<span class="k">for</span> <span class="n">z</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="k">if</span> <span class="n">y</span> <span class="o">!=</span> <span class="n">z</span>
<span class="p">)</span>
</code></pre></div></div>
<p><a id="s2.8-default-iterators-and-operators"></a></p>
<p><a id="default-iterators-operators"></a></p>
<h3 id="28-default-iterators-and-operators">2.8 Default Iterators and Operators</h3>
<p>Use default iterators and operators for types that support them, like lists,
dictionaries, and files.</p>
<p><a id="s2.8.1-definition"></a>
<a id="281-definition"></a></p>
<p><a id="default-iterators-operators-definition"></a></p>
<h4 id="281-definition">2.8.1 Definition</h4>
<p>Container types, like dictionaries and lists, define default iterators and
membership test operators (“in” and “not in”).</p>
<p><a id="s2.8.2-pros"></a>
<a id="282-pros"></a></p>
<p><a id="default-iterators-operators-pros"></a></p>
<h4 id="282-pros">2.8.2 Pros</h4>
<p>The default iterators and operators are simple and efficient. They express the
operation directly, without extra method calls. A function that uses default
operators is generic. It can be used with any type that supports the operation.</p>
<p><a id="s2.8.3-cons"></a>
<a id="283-cons"></a></p>
<p><a id="default-iterators-operators-cons"></a></p>
<h4 id="283-cons">2.8.3 Cons</h4>
<p>You can’t tell the type of objects by reading the method names (unless the
variable has type annotations). This is also an advantage.</p>
<p><a id="s2.8.4-decision"></a>
<a id="284-decision"></a></p>
<p><a id="default-iterators-operators-decision"></a></p>
<h4 id="284-decision">2.8.4 Decision</h4>
<p>Use default iterators and operators for types that support them, like lists,
dictionaries, and files. The built-in types define iterator methods, too. Prefer
these methods to methods that return lists, except that you should not mutate a
container while iterating over it.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="k">for</span> <span class="n">key</span> <span class="ow">in</span> <span class="n">adict</span><span class="p">:</span> <span class="p">...</span>
<span class="k">if</span> <span class="n">obj</span> <span class="ow">in</span> <span class="n">alist</span><span class="p">:</span> <span class="p">...</span>
<span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">afile</span><span class="p">:</span> <span class="p">...</span>
<span class="k">for</span> <span class="n">k</span><span class="p">,</span> <span class="n">v</span> <span class="ow">in</span> <span class="n">adict</span><span class="p">.</span><span class="n">items</span><span class="p">():</span> <span class="p">...</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="k">for</span> <span class="n">key</span> <span class="ow">in</span> <span class="n">adict</span><span class="p">.</span><span class="n">keys</span><span class="p">():</span> <span class="p">...</span>
<span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">afile</span><span class="p">.</span><span class="n">readlines</span><span class="p">():</span> <span class="p">...</span>
</code></pre></div></div>
<p><a id="s2.9-generators"></a>
<a id="29-generators"></a></p>
<p><a id="generators"></a></p>
<h3 id="29-generators">2.9 Generators</h3>
<p>Use generators as needed.</p>
<p><a id="s2.9.1-definition"></a>
<a id="291-definition"></a></p>
<p><a id="generators-definition"></a></p>
<h4 id="291-definition">2.9.1 Definition</h4>
<p>A generator function returns an iterator that yields a value each time it
executes a yield statement. After it yields a value, the runtime state of the
generator function is suspended until the next value is needed.</p>
<p><a id="s2.9.2-pros"></a>
<a id="292-pros"></a></p>
<p><a id="generators-pros"></a></p>
<h4 id="292-pros">2.9.2 Pros</h4>
<p>Simpler code, because the state of local variables and control flow are
preserved for each call. A generator uses less memory than a function that
creates an entire list of values at once.</p>
<p><a id="s2.9.3-cons"></a>
<a id="293-cons"></a></p>
<p><a id="generators-cons"></a></p>
<h4 id="293-cons">2.9.3 Cons</h4>
<p>Local variables in the generator will not be garbage collected until the
generator is either consumed to exhaustion or itself garbage collected.</p>
<p><a id="s2.9.4-decision"></a>
<a id="294-decision"></a></p>
<p><a id="generators-decision"></a></p>
<h4 id="294-decision">2.9.4 Decision</h4>
<p>Fine. Use “Yields:” rather than “Returns:” in the docstring for generator
functions.</p>
<p>If the generator manages an expensive resource, make sure to force the clean up.</p>
<p>A good way to do the clean up is by wrapping the generator with a context
manager <a href="https://peps.python.org/pep-0533/">PEP-0533</a>.</p>
<p><a id="s2.10-lambda-functions"></a>
<a id="210-lambda-functions"></a></p>
<p><a id="lambdas"></a></p>
<h3 id="210-lambda-functions">2.10 Lambda Functions</h3>
<p>Okay for one-liners. Prefer generator expressions over <code class="language-plaintext highlighter-rouge">map()</code> or <code class="language-plaintext highlighter-rouge">filter()</code>
with a <code class="language-plaintext highlighter-rouge">lambda</code>.</p>
<p><a id="s2.10.1-definition"></a>
<a id="2101-definition"></a></p>
<p><a id="lambdas-definition"></a></p>
<h4 id="2101-definition">2.10.1 Definition</h4>
<p>Lambdas define anonymous functions in an expression, as opposed to a statement.</p>
<p><a id="s2.10.2-pros"></a>
<a id="2102-pros"></a></p>
<p><a id="lambdas-pros"></a></p>
<h4 id="2102-pros">2.10.2 Pros</h4>
<p>Convenient.</p>
<p><a id="s2.10.3-cons"></a>
<a id="2103-cons"></a></p>
<p><a id="lambdas-cons"></a></p>
<h4 id="2103-cons">2.10.3 Cons</h4>
<p>Harder to read and debug than local functions. The lack of names means stack
traces are more difficult to understand. Expressiveness is limited because the
function may only contain an expression.</p>
<p><a id="s2.10.4-decision"></a>
<a id="2104-decision"></a></p>
<p><a id="lambdas-decision"></a></p>
<h4 id="2104-decision">2.10.4 Decision</h4>
<p>Lambdas are allowed. If the code inside the lambda function spans multiple lines
or is longer than 60-80 chars, it might be better to define it as a regular
<a href="#lexical-scoping">nested function</a>.</p>
<p>For common operations like multiplication, use the functions from the <code class="language-plaintext highlighter-rouge">operator</code>
module instead of lambda functions. For example, prefer <code class="language-plaintext highlighter-rouge">operator.mul</code> to
<code class="language-plaintext highlighter-rouge">lambda x, y: x * y</code>.</p>
<p><a id="s2.11-conditional-expressions"></a>
<a id="211-conditional-expressions"></a></p>
<p><a id="conditional-expressions"></a></p>
<h3 id="211-conditional-expressions">2.11 Conditional Expressions</h3>
<p>Okay for simple cases.</p>
<p><a id="s2.11.1-definition"></a>
<a id="2111-definition"></a></p>
<p><a id="conditional-expressions-definition"></a></p>
<h4 id="2111-definition">2.11.1 Definition</h4>
<p>Conditional expressions (sometimes called a “ternary operator”) are mechanisms
that provide a shorter syntax for if statements. For example: <code class="language-plaintext highlighter-rouge">x = 1 if cond
else 2</code>.</p>
<p><a id="s2.11.2-pros"></a>
<a id="2112-pros"></a></p>
<p><a id="conditional-expressions-pros"></a></p>
<h4 id="2112-pros">2.11.2 Pros</h4>
<p>Shorter and more convenient than an if statement.</p>
<p><a id="s2.11.3-cons"></a>
<a id="2113-cons"></a></p>
<p><a id="conditional-expressions-cons"></a></p>
<h4 id="2113-cons">2.11.3 Cons</h4>
<p>May be harder to read than an if statement. The condition may be difficult to
locate if the expression is long.</p>
<p><a id="s2.11.4-decision"></a>
<a id="2114-decision"></a></p>
<p><a id="conditional-expressions-decision"></a></p>
<h4 id="2114-decision">2.11.4 Decision</h4>
<p>Okay to use for simple cases. Each portion must fit on one line:
true-expression, if-expression, else-expression. Use a complete if statement
when things get more complicated.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="n">one_line</span> <span class="o">=</span> <span class="s">'yes'</span> <span class="k">if</span> <span class="n">predicate</span><span class="p">(</span><span class="n">value</span><span class="p">)</span> <span class="k">else</span> <span class="s">'no'</span>
<span class="n">slightly_split</span> <span class="o">=</span> <span class="p">(</span><span class="s">'yes'</span> <span class="k">if</span> <span class="n">predicate</span><span class="p">(</span><span class="n">value</span><span class="p">)</span>
<span class="k">else</span> <span class="s">'no, nein, nyet'</span><span class="p">)</span>
<span class="n">the_longest_ternary_style_that_can_be_done</span> <span class="o">=</span> <span class="p">(</span>
<span class="s">'yes, true, affirmative, confirmed, correct'</span>
<span class="k">if</span> <span class="n">predicate</span><span class="p">(</span><span class="n">value</span><span class="p">)</span>
<span class="k">else</span> <span class="s">'no, false, negative, nay'</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span>
<span class="n">bad_line_breaking</span> <span class="o">=</span> <span class="p">(</span><span class="s">'yes'</span> <span class="k">if</span> <span class="n">predicate</span><span class="p">(</span><span class="n">value</span><span class="p">)</span> <span class="k">else</span>
<span class="s">'no'</span><span class="p">)</span>
<span class="n">portion_too_long</span> <span class="o">=</span> <span class="p">(</span><span class="s">'yes'</span>
<span class="k">if</span> <span class="n">some_long_module</span><span class="p">.</span><span class="n">some_long_predicate_function</span><span class="p">(</span>
<span class="n">really_long_variable_name</span><span class="p">)</span>
<span class="k">else</span> <span class="s">'no, false, negative, nay'</span><span class="p">)</span>
</code></pre></div></div>
<p><a id="s2.12-default-argument-values"></a>
<a id="212-default-argument-values"></a></p>
<p><a id="default-arguments"></a></p>
<h3 id="212-default-argument-values">2.12 Default Argument Values</h3>
<p>Okay in most cases.</p>
<p><a id="s2.12.1-definition"></a>
<a id="2121-definition"></a></p>
<p><a id="default-arguments-definition"></a></p>
<h4 id="2121-definition">2.12.1 Definition</h4>
<p>You can specify values for variables at the end of a function’s parameter list,
e.g., <code class="language-plaintext highlighter-rouge">def foo(a, b=0):</code>. If <code class="language-plaintext highlighter-rouge">foo</code> is called with only one argument, <code class="language-plaintext highlighter-rouge">b</code> is set
to 0. If it is called with two arguments, <code class="language-plaintext highlighter-rouge">b</code> has the value of the second
argument.</p>
<p><a id="s2.12.2-pros"></a>
<a id="2122-pros"></a></p>
<p><a id="default-arguments-pros"></a></p>
<h4 id="2122-pros">2.12.2 Pros</h4>
<p>Often you have a function that uses lots of default values, but on rare
occasions you want to override the defaults. Default argument values provide an
easy way to do this, without having to define lots of functions for the rare
exceptions. As Python does not support overloaded methods/functions, default
arguments are an easy way of “faking” the overloading behavior.</p>
<p><a id="s2.12.3-cons"></a>
<a id="2123-cons"></a></p>
<p><a id="default-arguments-cons"></a></p>
<h4 id="2123-cons">2.12.3 Cons</h4>
<p>Default arguments are evaluated once at module load time. This may cause
problems if the argument is a mutable object such as a list or a dictionary. If
the function modifies the object (e.g., by appending an item to a list), the
default value is modified.</p>
<p><a id="s2.12.4-decision"></a>
<a id="2124-decision"></a></p>
<p><a id="default-arguments-decision"></a></p>
<h4 id="2124-decision">2.12.4 Decision</h4>
<p>Okay to use with the following caveat:</p>
<p>Do not use mutable objects as default values in the function or method
definition.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="o">=</span><span class="bp">None</span><span class="p">):</span>
<span class="k">if</span> <span class="n">b</span> <span class="ow">is</span> <span class="bp">None</span><span class="p">:</span>
<span class="n">b</span> <span class="o">=</span> <span class="p">[]</span>
<span class="n">Yes</span><span class="p">:</span> <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">:</span> <span class="n">Sequence</span> <span class="o">|</span> <span class="bp">None</span> <span class="o">=</span> <span class="bp">None</span><span class="p">):</span>
<span class="k">if</span> <span class="n">b</span> <span class="ow">is</span> <span class="bp">None</span><span class="p">:</span>
<span class="n">b</span> <span class="o">=</span> <span class="p">[]</span>
<span class="n">Yes</span><span class="p">:</span> <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">:</span> <span class="n">Sequence</span> <span class="o">=</span> <span class="p">()):</span> <span class="c1"># Empty tuple OK since tuples are immutable.
</span> <span class="p">...</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">absl</span> <span class="kn">import</span> <span class="n">flags</span>
<span class="n">_FOO</span> <span class="o">=</span> <span class="n">flags</span><span class="p">.</span><span class="n">DEFINE_string</span><span class="p">(...)</span>
<span class="n">No</span><span class="p">:</span> <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="o">=</span><span class="p">[]):</span>
<span class="p">...</span>
<span class="n">No</span><span class="p">:</span> <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="o">=</span><span class="n">time</span><span class="p">.</span><span class="n">time</span><span class="p">()):</span> <span class="c1"># Is `b` supposed to represent when this module was loaded?
</span> <span class="p">...</span>
<span class="n">No</span><span class="p">:</span> <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="o">=</span><span class="n">_FOO</span><span class="p">.</span><span class="n">value</span><span class="p">):</span> <span class="c1"># sys.argv has not yet been parsed...
</span> <span class="p">...</span>
<span class="n">No</span><span class="p">:</span> <span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">:</span> <span class="n">Mapping</span> <span class="o">=</span> <span class="p">{}):</span> <span class="c1"># Could still get passed to unchecked code.
</span> <span class="p">...</span>
</code></pre></div></div>
<p><a id="s2.13-properties"></a>
<a id="213-properties"></a></p>
<p><a id="properties"></a></p>
<h3 id="213-properties">2.13 Properties</h3>
<p>Properties may be used to control getting or setting attributes that require
trivial computations or logic. Property implementations must match the general
expectations of regular attribute access: that they are cheap, straightforward,
and unsurprising.</p>
<p><a id="s2.13.1-definition"></a>
<a id="2131-definition"></a></p>
<p><a id="properties-definition"></a></p>
<h4 id="2131-definition">2.13.1 Definition</h4>
<p>A way to wrap method calls for getting and setting an attribute as a standard
attribute access.</p>
<p><a id="s2.13.2-pros"></a>
<a id="2132-pros"></a></p>
<p><a id="properties-pros"></a></p>
<h4 id="2132-pros">2.13.2 Pros</h4>
<ul>
<li>Allows for an attribute access and assignment API rather than
<a href="#getters-and-setters">getter and setter</a> method calls.</li>
<li>Can be used to make an attribute read-only.</li>
<li>Allows calculations to be lazy.</li>
<li>Provides a way to maintain the public interface of a class when the
internals evolve independently of class users.</li>
</ul>
<p><a id="s2.13.3-cons"></a>
<a id="2133-cons"></a></p>
<p><a id="properties-cons"></a></p>
<h4 id="2133-cons">2.13.3 Cons</h4>
<ul>
<li>Can hide side-effects much like operator overloading.</li>
<li>Can be confusing for subclasses.</li>
</ul>
<p><a id="s2.13.4-decision"></a>
<a id="2134-decision"></a></p>
<p><a id="properties-decision"></a></p>
<h4 id="2134-decision">2.13.4 Decision</h4>
<p>Properties are allowed, but, like operator overloading, should only be used when
necessary and match the expectations of typical attribute access; follow the
<a href="#getters-and-setters">getters and setters</a> rules otherwise.</p>
<p>For example, using a property to simply both get and set an internal attribute
isn’t allowed: there is no computation occurring, so the property is unnecessary
(<a href="#getters-and-setters">make the attribute public instead</a>). In comparison,
using a property to control attribute access or to calculate a <em>trivially</em>
derived value is allowed: the logic is simple and unsurprising.</p>
<p>Properties should be created with the <code class="language-plaintext highlighter-rouge">@property</code>
<a href="#s2.17-function-and-method-decorators">decorator</a>. Manually implementing a
property descriptor is considered a <a href="#power-features">power feature</a>.</p>
<p>Inheritance with properties can be non-obvious. Do not use properties to
implement computations a subclass may ever want to override and extend.</p>
<p><a id="s2.14-truefalse-evaluations"></a>
<a id="214-truefalse-evaluations"></a></p>
<p><a id="truefalse-evaluations"></a></p>
<h3 id="214-truefalse-evaluations">2.14 True/False Evaluations</h3>
<p>Use the “implicit” false if at all possible (with a few caveats).</p>
<p><a id="s2.14.1-definition"></a>
<a id="2141-definition"></a></p>
<p><a id="truefalse-evaluations-definition"></a></p>
<h4 id="2141-definition">2.14.1 Definition</h4>
<p>Python evaluates certain values as <code class="language-plaintext highlighter-rouge">False</code> when in a boolean context. A quick
“rule of thumb” is that all “empty” values are considered false, so <code class="language-plaintext highlighter-rouge">0, None,
[], {}, ''</code> all evaluate as false in a boolean context.</p>
<p><a id="s2.14.2-pros"></a>
<a id="2142-pros"></a></p>
<p><a id="truefalse-evaluations-pros"></a></p>
<h4 id="2142-pros">2.14.2 Pros</h4>
<p>Conditions using Python booleans are easier to read and less error-prone. In
most cases, they’re also faster.</p>
<p><a id="s2.14.3-cons"></a>
<a id="2143-cons"></a></p>
<p><a id="truefalse-evaluations-cons"></a></p>
<h4 id="2143-cons">2.14.3 Cons</h4>
<p>May look strange to C/C++ developers.</p>
<p><a id="s2.14.4-decision"></a>
<a id="2144-decision"></a></p>
<p><a id="truefalse-evaluations-decision"></a></p>
<h4 id="2144-decision">2.14.4 Decision</h4>
<p>Use the “implicit” false if possible, e.g., <code class="language-plaintext highlighter-rouge">if foo:</code> rather than <code class="language-plaintext highlighter-rouge">if foo !=
[]:</code>. There are a few caveats that you should keep in mind though:</p>
<ul>
<li>
<p>Always use <code class="language-plaintext highlighter-rouge">if foo is None:</code> (or <code class="language-plaintext highlighter-rouge">is not None</code>) to check for a <code class="language-plaintext highlighter-rouge">None</code> value.
E.g., when testing whether a variable or argument that defaults to <code class="language-plaintext highlighter-rouge">None</code>
was set to some other value. The other value might be a value that’s false
in a boolean context!</p>
</li>
<li>
<p>Never compare a boolean variable to <code class="language-plaintext highlighter-rouge">False</code> using <code class="language-plaintext highlighter-rouge">==</code>. Use <code class="language-plaintext highlighter-rouge">if not x:</code>
instead. If you need to distinguish <code class="language-plaintext highlighter-rouge">False</code> from <code class="language-plaintext highlighter-rouge">None</code> then chain the
expressions, such as <code class="language-plaintext highlighter-rouge">if not x and x is not None:</code>.</p>
</li>
<li>
<p>For sequences (strings, lists, tuples), use the fact that empty sequences
are false, so <code class="language-plaintext highlighter-rouge">if seq:</code> and <code class="language-plaintext highlighter-rouge">if not seq:</code> are preferable to <code class="language-plaintext highlighter-rouge">if len(seq):</code>
and <code class="language-plaintext highlighter-rouge">if not len(seq):</code> respectively.</p>
</li>
<li>
<p>When handling integers, implicit false may involve more risk than benefit
(i.e., accidentally handling <code class="language-plaintext highlighter-rouge">None</code> as 0). You may compare a value which is
known to be an integer (and is not the result of <code class="language-plaintext highlighter-rouge">len()</code>) against the
integer 0.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="k">if</span> <span class="ow">not</span> <span class="n">users</span><span class="p">:</span>
<span class="k">print</span><span class="p">(</span><span class="s">'no users'</span><span class="p">)</span>
<span class="k">if</span> <span class="n">i</span> <span class="o">%</span> <span class="mi">10</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="bp">self</span><span class="p">.</span><span class="n">handle_multiple_of_ten</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="o">=</span><span class="bp">None</span><span class="p">):</span>
<span class="k">if</span> <span class="n">x</span> <span class="ow">is</span> <span class="bp">None</span><span class="p">:</span>
<span class="n">x</span> <span class="o">=</span> <span class="p">[]</span>
</code></pre></div> </div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">users</span><span class="p">)</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">print</span><span class="p">(</span><span class="s">'no users'</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">i</span> <span class="o">%</span> <span class="mi">10</span><span class="p">:</span>
<span class="bp">self</span><span class="p">.</span><span class="n">handle_multiple_of_ten</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="o">=</span><span class="bp">None</span><span class="p">):</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">x</span> <span class="ow">or</span> <span class="p">[]</span>
</code></pre></div> </div>
</li>
<li>
<p>Note that <code class="language-plaintext highlighter-rouge">'0'</code> (i.e., <code class="language-plaintext highlighter-rouge">0</code> as string) evaluates to true.</p>
</li>
<li>
<p>Note that Numpy arrays may raise an exception in an implicit boolean
context. Prefer the <code class="language-plaintext highlighter-rouge">.size</code> attribute when testing emptiness of a <code class="language-plaintext highlighter-rouge">np.array</code>
(e.g. <code class="language-plaintext highlighter-rouge">if not users.size</code>).</p>
</li>
</ul>
<p><a id="s2.16-lexical-scoping"></a>
<a id="216-lexical-scoping"></a></p>
<p><a id="lexical-scoping"></a></p>
<h3 id="216-lexical-scoping">2.16 Lexical Scoping</h3>
<p>Okay to use.</p>
<p><a id="s2.16.1-definition"></a>
<a id="2161-definition"></a></p>
<p><a id="lexical-scoping-definition"></a></p>
<h4 id="2161-definition">2.16.1 Definition</h4>
<p>A nested Python function can refer to variables defined in enclosing functions,
but cannot assign to them. Variable bindings are resolved using lexical scoping,
that is, based on the static program text. Any assignment to a name in a block
will cause Python to treat all references to that name as a local variable, even
if the use precedes the assignment. If a global declaration occurs, the name is
treated as a global variable.</p>
<p>An example of the use of this feature is:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">def</span> <span class="nf">get_adder</span><span class="p">(</span><span class="n">summand1</span><span class="p">:</span> <span class="nb">float</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">Callable</span><span class="p">[[</span><span class="nb">float</span><span class="p">],</span> <span class="nb">float</span><span class="p">]:</span>
<span class="s">"""Returns a function that adds numbers to a given number."""</span>
<span class="k">def</span> <span class="nf">adder</span><span class="p">(</span><span class="n">summand2</span><span class="p">:</span> <span class="nb">float</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">float</span><span class="p">:</span>
<span class="k">return</span> <span class="n">summand1</span> <span class="o">+</span> <span class="n">summand2</span>
<span class="k">return</span> <span class="n">adder</span>
</code></pre></div></div>
<p><a id="s2.16.2-pros"></a>
<a id="2162-pros"></a></p>
<p><a id="lexical-scoping-pros"></a></p>
<h4 id="2162-pros">2.16.2 Pros</h4>
<p>Often results in clearer, more elegant code. Especially comforting to
experienced Lisp and Scheme (and Haskell and ML and …) programmers.</p>
<p><a id="s2.16.3-cons"></a>
<a id="2163-cons"></a></p>
<p><a id="lexical-scoping-cons"></a></p>
<h4 id="2163-cons">2.16.3 Cons</h4>
<p>Can lead to confusing bugs, such as this example based on
<a href="https://peps.python.org/pep-0227/">PEP-0227</a>:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">i</span> <span class="o">=</span> <span class="mi">4</span>
<span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">x</span><span class="p">:</span> <span class="n">Iterable</span><span class="p">[</span><span class="nb">int</span><span class="p">]):</span>
<span class="k">def</span> <span class="nf">bar</span><span class="p">():</span>
<span class="k">print</span><span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s">''</span><span class="p">)</span>
<span class="c1"># ...
</span> <span class="c1"># A bunch of code here
</span> <span class="c1"># ...
</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">x</span><span class="p">:</span> <span class="c1"># Ah, i *is* local to foo, so this is what bar sees
</span> <span class="k">print</span><span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s">''</span><span class="p">)</span>
<span class="n">bar</span><span class="p">()</span>
</code></pre></div></div>
<p>So <code class="language-plaintext highlighter-rouge">foo([1, 2, 3])</code> will print <code class="language-plaintext highlighter-rouge">1 2 3 3</code>,
not <code class="language-plaintext highlighter-rouge">1 2 3 4</code>.</p>
<p><a id="s2.16.4-decision"></a>
<a id="2164-decision"></a></p>
<p><a id="lexical-scoping-decision"></a></p>
<h4 id="2164-decision">2.16.4 Decision</h4>
<p>Okay to use.</p>
<p><a id="s2.17-function-and-method-decorators"></a>
<a id="217-function-and-method-decorators"></a>
<a id="function-and-method-decorators"></a></p>
<p><a id="decorators"></a></p>
<h3 id="217-function-and-method-decorators">2.17 Function and Method Decorators</h3>
<p>Use decorators judiciously when there is a clear advantage. Avoid <code class="language-plaintext highlighter-rouge">staticmethod</code>
and limit use of <code class="language-plaintext highlighter-rouge">classmethod</code>.</p>
<p><a id="s2.17.1-definition"></a>
<a id="2171-definition"></a></p>
<p><a id="decorators-definition"></a></p>
<h4 id="2171-definition">2.17.1 Definition</h4>
<p><a href="https://docs.python.org/3/glossary.html#term-decorator">Decorators for Functions and Methods</a>
(a.k.a “the <code class="language-plaintext highlighter-rouge">@</code> notation”). One common decorator is <code class="language-plaintext highlighter-rouge">@property</code>, used for
converting ordinary methods into dynamically computed attributes. However, the
decorator syntax allows for user-defined decorators as well. Specifically, for
some function <code class="language-plaintext highlighter-rouge">my_decorator</code>, this:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
<span class="o">@</span><span class="n">my_decorator</span>
<span class="k">def</span> <span class="nf">method</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c1"># method body ...
</span></code></pre></div></div>
<p>is equivalent to:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">method</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c1"># method body ...
</span> <span class="n">method</span> <span class="o">=</span> <span class="n">my_decorator</span><span class="p">(</span><span class="n">method</span><span class="p">)</span>
</code></pre></div></div>
<p><a id="s2.17.2-pros"></a>
<a id="2172-pros"></a></p>
<p><a id="decorators-pros"></a></p>
<h4 id="2172-pros">2.17.2 Pros</h4>
<p>Elegantly specifies some transformation on a method; the transformation might
eliminate some repetitive code, enforce invariants, etc.</p>
<p><a id="s2.17.3-cons"></a>
<a id="2173-cons"></a></p>
<p><a id="decorators-cons"></a></p>
<h4 id="2173-cons">2.17.3 Cons</h4>
<p>Decorators can perform arbitrary operations on a function’s arguments or return
values, resulting in surprising implicit behavior. Additionally, decorators
execute at object definition time. For module-level objects (classes, module
functions, …) this happens at import time. Failures in decorator code are
pretty much impossible to recover from.</p>
<p><a id="s2.17.4-decision"></a>
<a id="2174-decision"></a></p>
<p><a id="decorators-decision"></a></p>
<h4 id="2174-decision">2.17.4 Decision</h4>
<p>Use decorators judiciously when there is a clear advantage. Decorators should
follow the same import and naming guidelines as functions. A decorator docstring
should clearly state that the function is a decorator. Write unit tests for
decorators.</p>
<p>Avoid external dependencies in the decorator itself (e.g. don’t rely on files,
sockets, database connections, etc.), since they might not be available when the
decorator runs (at import time, perhaps from <code class="language-plaintext highlighter-rouge">pydoc</code> or other tools). A
decorator that is called with valid parameters should (as much as possible) be
guaranteed to succeed in all cases.</p>
<p>Decorators are a special case of “top-level code” - see <a href="#s3.17-main">main</a> for
more discussion.</p>
<p>Never use <code class="language-plaintext highlighter-rouge">staticmethod</code> unless forced to in order to integrate with an API
defined in an existing library. Write a module-level function instead.</p>
<p>Use <code class="language-plaintext highlighter-rouge">classmethod</code> only when writing a named constructor, or a class-specific
routine that modifies necessary global state such as a process-wide cache.</p>
<p><a id="s2.18-threading"></a>
<a id="218-threading"></a></p>
<p><a id="threading"></a></p>
<h3 id="218-threading">2.18 Threading</h3>
<p>Do not rely on the atomicity of built-in types.</p>
<p>While Python’s built-in data types such as dictionaries appear to have atomic
operations, there are corner cases where they aren’t atomic (e.g. if <code class="language-plaintext highlighter-rouge">__hash__</code>
or <code class="language-plaintext highlighter-rouge">__eq__</code> are implemented as Python methods) and their atomicity should not be
relied upon. Neither should you rely on atomic variable assignment (since this
in turn depends on dictionaries).</p>
<p>Use the <code class="language-plaintext highlighter-rouge">queue</code> module’s <code class="language-plaintext highlighter-rouge">Queue</code> data type as the preferred way to communicate
data between threads. Otherwise, use the <code class="language-plaintext highlighter-rouge">threading</code> module and its locking
primitives. Prefer condition variables and <code class="language-plaintext highlighter-rouge">threading.Condition</code> instead of
using lower-level locks.</p>
<p><a id="s2.19-power-features"></a>
<a id="219-power-features"></a></p>
<p><a id="power-features"></a></p>
<h3 id="219-power-features">2.19 Power Features</h3>
<p>Avoid these features.</p>
<p><a id="s2.19.1-definition"></a>
<a id="2191-definition"></a></p>
<p><a id="power-features-definition"></a></p>
<h4 id="2191-definition">2.19.1 Definition</h4>
<p>Python is an extremely flexible language and gives you many fancy features such
as custom metaclasses, access to bytecode, on-the-fly compilation, dynamic
inheritance, object reparenting, import hacks, reflection (e.g. some uses of
<code class="language-plaintext highlighter-rouge">getattr()</code>), modification of system internals, <code class="language-plaintext highlighter-rouge">__del__</code> methods implementing
customized cleanup, etc.</p>
<p><a id="s2.19.2-pros"></a>
<a id="2192-pros"></a></p>
<p><a id="power-features-pros"></a></p>
<h4 id="2192-pros">2.19.2 Pros</h4>
<p>These are powerful language features. They can make your code more compact.</p>
<p><a id="s2.19.3-cons"></a>
<a id="2193-cons"></a></p>
<p><a id="power-features-cons"></a></p>
<h4 id="2193-cons">2.19.3 Cons</h4>
<p>It’s very tempting to use these “cool” features when they’re not absolutely
necessary. It’s harder to read, understand, and debug code that’s using unusual
features underneath. It doesn’t seem that way at first (to the original author),
but when revisiting the code, it tends to be more difficult than code that is
longer but is straightforward.</p>
<p><a id="s2.19.4-decision"></a>
<a id="2194-decision"></a></p>
<p><a id="power-features-decision"></a></p>
<h4 id="2194-decision">2.19.4 Decision</h4>
<p>Avoid these features in your code.</p>
<p>Standard library modules and classes that internally use these features are okay
to use (for example, <code class="language-plaintext highlighter-rouge">abc.ABCMeta</code>, <code class="language-plaintext highlighter-rouge">dataclasses</code>, and <code class="language-plaintext highlighter-rouge">enum</code>).</p>
<p><a id="s2.20-modern-python"></a>
<a id="220-modern-python"></a></p>
<p><a id="modern-python"></a></p>
<h3 id="220-modern-python-from-__future__-imports">2.20 Modern Python: from __future__ imports</h3>
<p>New language version semantic changes may be gated behind a special future
import to enable them on a per-file basis within earlier runtimes.</p>
<p><a id="s2.20.1-definition"></a>
<a id="2201-definition"></a></p>
<p><a id="modern-python-definition"></a></p>
<h4 id="2201-definition">2.20.1 Definition</h4>
<p>Being able to turn on some of the more modern features via <code class="language-plaintext highlighter-rouge">from __future__
import</code> statements allows early use of features from expected future Python
versions.</p>
<p><a id="s2.20.2-pros"></a>
<a id="2202-pros"></a></p>
<p><a id="modern-python-pros"></a></p>
<h4 id="2202-pros">2.20.2 Pros</h4>
<p>This has proven to make runtime version upgrades smoother as changes can be made
on a per-file basis while declaring compatibility and preventing regressions
within those files. Modern code is more maintainable as it is less likely to
accumulate technical debt that will be problematic during future runtime
upgrades.</p>
<p><a id="s2.20.3-cons"></a>
<a id="2203-cons"></a></p>
<p><a id="modern-python-cons"></a></p>
<h4 id="2203-cons">2.20.3 Cons</h4>
<p>Such code may not work on very old interpreter versions prior to the
introduction of the needed future statement. The need for this is more common in
projects supporting an extremely wide variety of environments.</p>
<p><a id="s2.20.4-decision"></a>
<a id="2204-decision"></a></p>
<p><a id="modern-python-decision"></a></p>
<h4 id="2204-decision">2.20.4 Decision</h4>
<h5 id="from-__future__-imports">from __future__ imports</h5>
<p>Use of <code class="language-plaintext highlighter-rouge">from __future__ import</code> statements is encouraged. It allows a given
source file to start using more modern Python syntax features today. Once you no
longer need to run on a version where the features are hidden behind a
<code class="language-plaintext highlighter-rouge">__future__</code> import, feel free to remove those lines.</p>
<p>In code that may execute on versions as old as 3.5 rather than &gt;= 3.7, import:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">__future__</span> <span class="kn">import</span> <span class="n">generator_stop</span>
</code></pre></div></div>
<p>For more information read the
<a href="https://docs.python.org/3/library/__future__.html">Python future statement definitions</a>
documentation.</p>
<p>Please don’t remove these imports until you are confident the code is only ever
used in a sufficiently modern environment. Even if you do not currently use the
feature a specific future import enables in your code today, keeping it in place
in the file prevents later modifications of the code from inadvertently
depending on the older behavior.</p>
<p>Use other <code class="language-plaintext highlighter-rouge">from __future__</code> import statements as you see fit.</p>
<p><a id="s2.21-type-annotated-code"></a>
<a id="s2.21-typed-code"></a>
<a id="221-type-annotated-code"></a>
<a id="typed-code"></a></p>
<p><a id="typed-code"></a></p>
<h3 id="221-type-annotated-code">2.21 Type Annotated Code</h3>
<p>You can annotate Python code with
<a href="https://docs.python.org/3/library/typing.html">type hints</a>. Type-check the code
at build time with a type checking tool like <a href="https://github.com/google/pytype">pytype</a>.
In most cases, when feasible, type annotations are in source files. For
third-party or extension modules, annotations can be in
<a href="https://peps.python.org/pep-0484/#stub-files">stub <code class="language-plaintext highlighter-rouge">.pyi</code> files</a>.</p>
<p><a id="s2.21.1-definition"></a>
<a id="2211-definition"></a></p>
<p><a id="typed-code-definition"></a></p>
<h4 id="2211-definition">2.21.1 Definition</h4>
<p>Type annotations (or “type hints”) are for function or method arguments and
return values:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="n">a</span><span class="p">:</span> <span class="nb">int</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">list</span><span class="p">[</span><span class="nb">int</span><span class="p">]:</span>
</code></pre></div></div>
<p>You can also declare the type of a variable using similar syntax:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">a</span><span class="p">:</span> <span class="n">SomeType</span> <span class="o">=</span> <span class="n">some_func</span><span class="p">()</span>
</code></pre></div></div>
<p><a id="s2.21.2-pros"></a>
<a id="2212-pros"></a></p>
<p><a id="typed-code-pros"></a></p>
<h4 id="2212-pros">2.21.2 Pros</h4>
<p>Type annotations improve the readability and maintainability of your code. The
type checker will convert many runtime errors to build-time errors, and reduce
your ability to use <a href="#power-features">Power Features</a>.</p>
<p><a id="s2.21.3-cons"></a>
<a id="2213-cons"></a></p>
<p><a id="typed-code-cons"></a></p>
<h4 id="2213-cons">2.21.3 Cons</h4>
<p>You will have to keep the type declarations up to date.
You might see type errors that you think are
valid code. Use of a
<a href="https://github.com/google/pytype">type checker</a>
may reduce your ability to use <a href="#power-features">Power Features</a>.</p>
<p><a id="s2.21.4-decision"></a>
<a id="2214-decision"></a></p>
<p><a id="typed-code-decision"></a></p>
<h4 id="2214-decision">2.21.4 Decision</h4>
<p>You are strongly encouraged to enable Python type analysis when updating code.
When adding or modifying public APIs, include type annotations and enable
checking via pytype in the build system. As static analysis is relatively new to
Python, we acknowledge that undesired side-effects (such as
wrongly
inferred types) may prevent adoption by some projects. In those situations,
authors are encouraged to add a comment with a TODO or link to a bug describing
the issue(s) currently preventing type annotation adoption in the BUILD file or
in the code itself as appropriate.</p>
<p><a id="s3-python-style-rules"></a>
<a id="3-python-style-rules"></a></p>
<p><a id="python-style-rules"></a></p>
<h2 id="3-python-style-rules">3 Python Style Rules</h2>
<p><a id="s3.1-semicolons"></a>
<a id="31-semicolons"></a></p>
<p><a id="semicolons"></a></p>
<h3 id="31-semicolons">3.1 Semicolons</h3>
<p>Do not terminate your lines with semicolons, and do not use semicolons to put
two statements on the same line.</p>
<p><a id="s3.2-line-length"></a>
<a id="32-line-length"></a></p>
<p><a id="line-length"></a></p>
<h3 id="32-line-length">3.2 Line length</h3>
<p>Maximum line length is <em>80 characters</em>.</p>
<p>Explicit exceptions to the 80 character limit:</p>
<ul>
<li>Long import statements.</li>
<li>URLs, pathnames, or long flags in comments.</li>
<li>Long string module-level constants not containing whitespace that would be
inconvenient to split across lines such as URLs or pathnames.
<ul>
<li>Pylint disable comments. (e.g.: <code class="language-plaintext highlighter-rouge"># pylint: disable=invalid-name</code>)</li>
</ul>
</li>
</ul>
<p>Do not use a backslash for
<a href="https://docs.python.org/3/reference/lexical_analysis.html#explicit-line-joining">explicit line continuation</a>.</p>
<p>Instead, make use of Python’s
<a href="http://docs.python.org/reference/lexical_analysis.html#implicit-line-joining">implicit line joining inside parentheses, brackets and braces</a>.
If necessary, you can add an extra pair of parentheses around an expression.</p>
<p>Note that this rule doesn’t prohibit backslash-escaped newlines within strings
(see <a href="#strings">below</a>).</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="n">foo_bar</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">width</span><span class="p">,</span> <span class="n">height</span><span class="p">,</span> <span class="n">color</span><span class="o">=</span><span class="s">'black'</span><span class="p">,</span> <span class="n">design</span><span class="o">=</span><span class="bp">None</span><span class="p">,</span> <span class="n">x</span><span class="o">=</span><span class="s">'foo'</span><span class="p">,</span>
<span class="n">emphasis</span><span class="o">=</span><span class="bp">None</span><span class="p">,</span> <span class="n">highlight</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
<span class="n">Yes</span><span class="p">:</span> <span class="k">if</span> <span class="p">(</span><span class="n">width</span> <span class="o">==</span> <span class="mi">0</span> <span class="ow">and</span> <span class="n">height</span> <span class="o">==</span> <span class="mi">0</span> <span class="ow">and</span>
<span class="n">color</span> <span class="o">==</span> <span class="s">'red'</span> <span class="ow">and</span> <span class="n">emphasis</span> <span class="o">==</span> <span class="s">'strong'</span><span class="p">):</span>
<span class="p">(</span><span class="n">bridge_questions</span><span class="p">.</span><span class="n">clarification_on</span>
<span class="p">.</span><span class="n">average_airspeed_of</span><span class="p">.</span><span class="n">unladen_swallow</span><span class="p">)</span> <span class="o">=</span> <span class="s">'African or European?'</span>
<span class="k">with</span> <span class="p">(</span>
<span class="n">very_long_first_expression_function</span><span class="p">()</span> <span class="k">as</span> <span class="n">spam</span><span class="p">,</span>
<span class="n">very_long_second_expression_function</span><span class="p">()</span> <span class="k">as</span> <span class="n">beans</span><span class="p">,</span>
<span class="n">third_thing</span><span class="p">()</span> <span class="k">as</span> <span class="n">eggs</span><span class="p">,</span>
<span class="p">):</span>
<span class="n">place_order</span><span class="p">(</span><span class="n">eggs</span><span class="p">,</span> <span class="n">beans</span><span class="p">,</span> <span class="n">spam</span><span class="p">,</span> <span class="n">beans</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
<span class="n">No</span><span class="p">:</span> <span class="k">if</span> <span class="n">width</span> <span class="o">==</span> <span class="mi">0</span> <span class="ow">and</span> <span class="n">height</span> <span class="o">==</span> <span class="mi">0</span> <span class="ow">and</span> \
<span class="n">color</span> <span class="o">==</span> <span class="s">'red'</span> <span class="ow">and</span> <span class="n">emphasis</span> <span class="o">==</span> <span class="s">'strong'</span><span class="p">:</span>
<span class="n">bridge_questions</span><span class="p">.</span><span class="n">clarification_on</span> \
<span class="p">.</span><span class="n">average_airspeed_of</span><span class="p">.</span><span class="n">unladen_swallow</span> <span class="o">=</span> <span class="s">'African or European?'</span>
<span class="k">with</span> <span class="n">very_long_first_expression_function</span><span class="p">()</span> <span class="k">as</span> <span class="n">spam</span><span class="p">,</span> \
<span class="n">very_long_second_expression_function</span><span class="p">()</span> <span class="k">as</span> <span class="n">beans</span><span class="p">,</span> \
<span class="n">third_thing</span><span class="p">()</span> <span class="k">as</span> <span class="n">eggs</span><span class="p">:</span>
<span class="n">place_order</span><span class="p">(</span><span class="n">eggs</span><span class="p">,</span> <span class="n">beans</span><span class="p">,</span> <span class="n">spam</span><span class="p">,</span> <span class="n">beans</span><span class="p">)</span>
</code></pre></div></div>
<p>When a literal string won’t fit on a single line, use parentheses for implicit
line joining.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">x</span> <span class="o">=</span> <span class="p">(</span><span class="s">'This will build a very long long '</span>
<span class="s">'long long long long long long string'</span><span class="p">)</span>
</code></pre></div></div>
<p>Prefer to break lines at the highest possible syntactic level. If you must break
a line twice, break it at the same syntactic level both times.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="n">bridgekeeper</span><span class="p">.</span><span class="n">answer</span><span class="p">(</span>
<span class="n">name</span><span class="o">=</span><span class="s">"Arthur"</span><span class="p">,</span> <span class="n">quest</span><span class="o">=</span><span class="n">questlib</span><span class="p">.</span><span class="n">find</span><span class="p">(</span><span class="n">owner</span><span class="o">=</span><span class="s">"Arthur"</span><span class="p">,</span> <span class="n">perilous</span><span class="o">=</span><span class="bp">True</span><span class="p">))</span>
<span class="n">answer</span> <span class="o">=</span> <span class="p">(</span><span class="n">a_long_line</span><span class="p">().</span><span class="n">of_chained_methods</span><span class="p">()</span>
<span class="p">.</span><span class="n">that_eventually_provides</span><span class="p">().</span><span class="n">an_answer</span><span class="p">())</span>
<span class="k">if</span> <span class="p">(</span>
<span class="n">config</span> <span class="ow">is</span> <span class="bp">None</span>
<span class="ow">or</span> <span class="s">'editor.language'</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">config</span>
<span class="ow">or</span> <span class="n">config</span><span class="p">[</span><span class="s">'editor.language'</span><span class="p">].</span><span class="n">use_spaces</span> <span class="ow">is</span> <span class="bp">False</span>
<span class="p">):</span>
<span class="n">use_tabs</span><span class="p">()</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="n">bridgekeeper</span><span class="p">.</span><span class="n">answer</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">"Arthur"</span><span class="p">,</span> <span class="n">quest</span><span class="o">=</span><span class="n">questlib</span><span class="p">.</span><span class="n">find</span><span class="p">(</span>
<span class="n">owner</span><span class="o">=</span><span class="s">"Arthur"</span><span class="p">,</span> <span class="n">perilous</span><span class="o">=</span><span class="bp">True</span><span class="p">))</span>
<span class="n">answer</span> <span class="o">=</span> <span class="n">a_long_line</span><span class="p">().</span><span class="n">of_chained_methods</span><span class="p">().</span><span class="n">that_eventually_provides</span><span class="p">(</span>
<span class="p">).</span><span class="n">an_answer</span><span class="p">()</span>
<span class="k">if</span> <span class="p">(</span><span class="n">config</span> <span class="ow">is</span> <span class="bp">None</span> <span class="ow">or</span> <span class="s">'editor.language'</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">config</span> <span class="ow">or</span> <span class="n">config</span><span class="p">[</span>
<span class="s">'editor.language'</span><span class="p">].</span><span class="n">use_spaces</span> <span class="ow">is</span> <span class="bp">False</span><span class="p">):</span>
<span class="n">use_tabs</span><span class="p">()</span>
</code></pre></div></div>
<p>Within comments, put long URLs on their own line if necessary.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="c1"># See details at
</span> <span class="c1"># http://www.example.com/us/developer/documentation/api/content/v2.0/csv_file_name_extension_full_specification.html
</span></code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="c1"># See details at
</span> <span class="c1"># http://www.example.com/us/developer/documentation/api/content/\
</span> <span class="c1"># v2.0/csv_file_name_extension_full_specification.html
</span></code></pre></div></div>
<p>Make note of the indentation of the elements in the line continuation examples
above; see the <a href="#s3.4-indentation">indentation</a> section for explanation.</p>
<p><a href="#docstrings">Docstring</a> summary lines must remain within the 80 character
limit.</p>
<p>In all other cases where a line exceeds 80 characters, and the
<a href="https://github.com/psf/black">Black</a> or <a href="https://github.com/google/pyink">Pyink</a>
auto-formatter does not help bring the line below the limit, the line is allowed
to exceed this maximum. Authors are encouraged to manually break the line up per
the notes above when it is sensible.</p>
<p><a id="s3.3-parentheses"></a>
<a id="33-parentheses"></a></p>
<p><a id="parentheses"></a></p>
<h3 id="33-parentheses">3.3 Parentheses</h3>
<p>Use parentheses sparingly.</p>
<p>It is fine, though not required, to use parentheses around tuples. Do not use
them in return statements or conditional statements unless using parentheses for
implied line continuation or to indicate a tuple.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="k">if</span> <span class="n">foo</span><span class="p">:</span>
<span class="n">bar</span><span class="p">()</span>
<span class="k">while</span> <span class="n">x</span><span class="p">:</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">bar</span><span class="p">()</span>
<span class="k">if</span> <span class="n">x</span> <span class="ow">and</span> <span class="n">y</span><span class="p">:</span>
<span class="n">bar</span><span class="p">()</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">x</span><span class="p">:</span>
<span class="n">bar</span><span class="p">()</span>
<span class="c1"># For a 1 item tuple the ()s are more visually obvious than the comma.
</span> <span class="n">onesie</span> <span class="o">=</span> <span class="p">(</span><span class="n">foo</span><span class="p">,)</span>
<span class="k">return</span> <span class="n">foo</span>
<span class="k">return</span> <span class="n">spam</span><span class="p">,</span> <span class="n">beans</span>
<span class="k">return</span> <span class="p">(</span><span class="n">spam</span><span class="p">,</span> <span class="n">beans</span><span class="p">)</span>
<span class="k">for</span> <span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span> <span class="ow">in</span> <span class="nb">dict</span><span class="p">.</span><span class="n">items</span><span class="p">():</span> <span class="p">...</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="k">if</span> <span class="p">(</span><span class="n">x</span><span class="p">):</span>
<span class="n">bar</span><span class="p">()</span>
<span class="k">if</span> <span class="ow">not</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>
<span class="n">bar</span><span class="p">()</span>
<span class="k">return</span> <span class="p">(</span><span class="n">foo</span><span class="p">)</span>
</code></pre></div></div>
<p><a id="s3.4-indentation"></a>
<a id="34-indentation"></a></p>
<p><a id="indentation"></a></p>
<h3 id="34-indentation">3.4 Indentation</h3>
<p>Indent your code blocks with <em>4 spaces</em>.</p>
<p>Never use tabs. Implied line continuation should align wrapped elements
vertically (see <a href="#s3.2-line-length">line length examples</a>), or use a hanging
4-space indent. Closing (round, square or curly) brackets can be placed at the
end of the expression, or on separate lines, but then should be indented the
same as the line with the corresponding opening bracket.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="c1"># Aligned with opening delimiter.
</span> <span class="n">foo</span> <span class="o">=</span> <span class="n">long_function_name</span><span class="p">(</span><span class="n">var_one</span><span class="p">,</span> <span class="n">var_two</span><span class="p">,</span>
<span class="n">var_three</span><span class="p">,</span> <span class="n">var_four</span><span class="p">)</span>
<span class="n">meal</span> <span class="o">=</span> <span class="p">(</span><span class="n">spam</span><span class="p">,</span>
<span class="n">beans</span><span class="p">)</span>
<span class="c1"># Aligned with opening delimiter in a dictionary.
</span> <span class="n">foo</span> <span class="o">=</span> <span class="p">{</span>
<span class="s">'long_dictionary_key'</span><span class="p">:</span> <span class="n">value1</span> <span class="o">+</span>
<span class="n">value2</span><span class="p">,</span>
<span class="p">...</span>
<span class="p">}</span>
<span class="c1"># 4-space hanging indent; nothing on first line.
</span> <span class="n">foo</span> <span class="o">=</span> <span class="n">long_function_name</span><span class="p">(</span>
<span class="n">var_one</span><span class="p">,</span> <span class="n">var_two</span><span class="p">,</span> <span class="n">var_three</span><span class="p">,</span>
<span class="n">var_four</span><span class="p">)</span>
<span class="n">meal</span> <span class="o">=</span> <span class="p">(</span>
<span class="n">spam</span><span class="p">,</span>
<span class="n">beans</span><span class="p">)</span>
<span class="c1"># 4-space hanging indent; nothing on first line,
</span> <span class="c1"># closing parenthesis on a new line.
</span> <span class="n">foo</span> <span class="o">=</span> <span class="n">long_function_name</span><span class="p">(</span>
<span class="n">var_one</span><span class="p">,</span> <span class="n">var_two</span><span class="p">,</span> <span class="n">var_three</span><span class="p">,</span>
<span class="n">var_four</span>
<span class="p">)</span>
<span class="n">meal</span> <span class="o">=</span> <span class="p">(</span>
<span class="n">spam</span><span class="p">,</span>
<span class="n">beans</span><span class="p">,</span>
<span class="p">)</span>
<span class="c1"># 4-space hanging indent in a dictionary.
</span> <span class="n">foo</span> <span class="o">=</span> <span class="p">{</span>
<span class="s">'long_dictionary_key'</span><span class="p">:</span>
<span class="n">long_dictionary_value</span><span class="p">,</span>
<span class="p">...</span>
<span class="p">}</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="c1"># Stuff on first line forbidden.
</span> <span class="n">foo</span> <span class="o">=</span> <span class="n">long_function_name</span><span class="p">(</span><span class="n">var_one</span><span class="p">,</span> <span class="n">var_two</span><span class="p">,</span>
<span class="n">var_three</span><span class="p">,</span> <span class="n">var_four</span><span class="p">)</span>
<span class="n">meal</span> <span class="o">=</span> <span class="p">(</span><span class="n">spam</span><span class="p">,</span>
<span class="n">beans</span><span class="p">)</span>
<span class="c1"># 2-space hanging indent forbidden.
</span> <span class="n">foo</span> <span class="o">=</span> <span class="n">long_function_name</span><span class="p">(</span>
<span class="n">var_one</span><span class="p">,</span> <span class="n">var_two</span><span class="p">,</span> <span class="n">var_three</span><span class="p">,</span>
<span class="n">var_four</span><span class="p">)</span>
<span class="c1"># No hanging indent in a dictionary.
</span> <span class="n">foo</span> <span class="o">=</span> <span class="p">{</span>
<span class="s">'long_dictionary_key'</span><span class="p">:</span>
<span class="n">long_dictionary_value</span><span class="p">,</span>
<span class="p">...</span>
<span class="p">}</span>
</code></pre></div></div>
<p><a id="s3.4.1-trailing-comma"></a>
<a id="s3.4.1-trailing-commas"></a>
<a id="s3.4.1-trailing_comma"></a>
<a id="s3.4.1-trailing_commas"></a>
<a id="341-trailing_comma"></a>
<a id="341-trailing_commas"></a>
<a id="trailing_comma"></a>
<a id="trailing_commas"></a></p>
<p><a id="trailing-comma"></a></p>
<h4 id="341-trailing-commas-in-sequences-of-items">3.4.1 Trailing commas in sequences of items?</h4>
<p>Trailing commas in sequences of items are recommended only when the closing
container token <code class="language-plaintext highlighter-rouge">]</code>, <code class="language-plaintext highlighter-rouge">)</code>, or <code class="language-plaintext highlighter-rouge">}</code> does not appear on the same line as the final
element, as well as for tuples with a single element. The presence of a trailing
comma is also used as a hint to our Python code auto-formatter
<a href="https://github.com/psf/black">Black</a> or <a href="https://github.com/google/pyink">Pyink</a>
to direct it to auto-format the container of items to one item per line when the
<code class="language-plaintext highlighter-rouge">,</code> after the final element is present.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="n">golomb3</span> <span class="o">=</span> <span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span>
<span class="n">golomb4</span> <span class="o">=</span> <span class="p">[</span>
<span class="mi">0</span><span class="p">,</span>
<span class="mi">1</span><span class="p">,</span>
<span class="mi">4</span><span class="p">,</span>
<span class="mi">6</span><span class="p">,</span>
<span class="p">]</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="n">golomb4</span> <span class="o">=</span> <span class="p">[</span>
<span class="mi">0</span><span class="p">,</span>
<span class="mi">1</span><span class="p">,</span>
<span class="mi">4</span><span class="p">,</span>
<span class="mi">6</span><span class="p">,]</span>
</code></pre></div></div>
<p><a id="s3.5-blank-lines"></a>
<a id="35-blank-lines"></a></p>
<p><a id="blank-lines"></a></p>
<h3 id="35-blank-lines">3.5 Blank Lines</h3>
<p>Two blank lines between top-level definitions, be they function or class
definitions. One blank line between method definitions and between the docstring
of a <code class="language-plaintext highlighter-rouge">class</code> and the first method. No blank line following a <code class="language-plaintext highlighter-rouge">def</code> line. Use
single blank lines as you judge appropriate within functions or methods.</p>
<p>Blank lines need not be anchored to the definition. For example, related
comments immediately preceding function, class, and method definitions can make
sense. Consider if your comment might be more useful as part of the docstring.</p>
<p><a id="s3.6-whitespace"></a>
<a id="36-whitespace"></a></p>
<p><a id="whitespace"></a></p>
<h3 id="36-whitespace">3.6 Whitespace</h3>
<p>Follow standard typographic rules for the use of spaces around punctuation.</p>
<p>No whitespace inside parentheses, brackets or braces.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="n">spam</span><span class="p">(</span><span class="n">ham</span><span class="p">[</span><span class="mi">1</span><span class="p">],</span> <span class="p">{</span><span class="s">'eggs'</span><span class="p">:</span> <span class="mi">2</span><span class="p">},</span> <span class="p">[])</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="n">spam</span><span class="p">(</span> <span class="n">ham</span><span class="p">[</span> <span class="mi">1</span> <span class="p">],</span> <span class="p">{</span> <span class="s">'eggs'</span><span class="p">:</span> <span class="mi">2</span> <span class="p">},</span> <span class="p">[</span> <span class="p">]</span> <span class="p">)</span>
</code></pre></div></div>
<p>No whitespace before a comma, semicolon, or colon. Do use whitespace after a
comma, semicolon, or colon, except at the end of the line.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="k">if</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">4</span><span class="p">:</span>
<span class="k">print</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span>
<span class="n">x</span><span class="p">,</span> <span class="n">y</span> <span class="o">=</span> <span class="n">y</span><span class="p">,</span> <span class="n">x</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="k">if</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">4</span> <span class="p">:</span>
<span class="k">print</span><span class="p">(</span><span class="n">x</span> <span class="p">,</span> <span class="n">y</span><span class="p">)</span>
<span class="n">x</span> <span class="p">,</span> <span class="n">y</span> <span class="o">=</span> <span class="n">y</span> <span class="p">,</span> <span class="n">x</span>
</code></pre></div></div>
<p>No whitespace before the open paren/bracket that starts an argument list,
indexing or slicing.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="n">spam</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="n">spam</span> <span class="p">(</span><span class="mi">1</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="nb">dict</span><span class="p">[</span><span class="s">'key'</span><span class="p">]</span> <span class="o">=</span> <span class="nb">list</span><span class="p">[</span><span class="n">index</span><span class="p">]</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="nb">dict</span> <span class="p">[</span><span class="s">'key'</span><span class="p">]</span> <span class="o">=</span> <span class="nb">list</span> <span class="p">[</span><span class="n">index</span><span class="p">]</span>
</code></pre></div></div>
<p>No trailing whitespace.</p>
<p>Surround binary operators with a single space on either side for assignment
(<code class="language-plaintext highlighter-rouge">=</code>), comparisons (<code class="language-plaintext highlighter-rouge">==, &lt;, &gt;, !=, &lt;&gt;, &lt;=, &gt;=, in, not in, is, is not</code>), and
Booleans (<code class="language-plaintext highlighter-rouge">and, or, not</code>). Use your better judgment for the insertion of spaces
around arithmetic operators (<code class="language-plaintext highlighter-rouge">+</code>, <code class="language-plaintext highlighter-rouge">-</code>, <code class="language-plaintext highlighter-rouge">*</code>, <code class="language-plaintext highlighter-rouge">/</code>, <code class="language-plaintext highlighter-rouge">//</code>, <code class="language-plaintext highlighter-rouge">%</code>, <code class="language-plaintext highlighter-rouge">**</code>, <code class="language-plaintext highlighter-rouge">@</code>).</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">1</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="n">x</span><span class="o">&lt;</span><span class="mi">1</span>
</code></pre></div></div>
<p>Never use spaces around <code class="language-plaintext highlighter-rouge">=</code> when passing keyword arguments or defining a default
parameter value, with one exception:
<a href="#typing-default-values">when a type annotation is present</a>, <em>do</em> use spaces
around the <code class="language-plaintext highlighter-rouge">=</code> for the default parameter value.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="k">def</span> <span class="nf">complex</span><span class="p">(</span><span class="n">real</span><span class="p">,</span> <span class="n">imag</span><span class="o">=</span><span class="mf">0.0</span><span class="p">):</span> <span class="k">return</span> <span class="n">Magic</span><span class="p">(</span><span class="n">r</span><span class="o">=</span><span class="n">real</span><span class="p">,</span> <span class="n">i</span><span class="o">=</span><span class="n">imag</span><span class="p">)</span>
<span class="n">Yes</span><span class="p">:</span> <span class="k">def</span> <span class="nf">complex</span><span class="p">(</span><span class="n">real</span><span class="p">,</span> <span class="n">imag</span><span class="p">:</span> <span class="nb">float</span> <span class="o">=</span> <span class="mf">0.0</span><span class="p">):</span> <span class="k">return</span> <span class="n">Magic</span><span class="p">(</span><span class="n">r</span><span class="o">=</span><span class="n">real</span><span class="p">,</span> <span class="n">i</span><span class="o">=</span><span class="n">imag</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="k">def</span> <span class="nf">complex</span><span class="p">(</span><span class="n">real</span><span class="p">,</span> <span class="n">imag</span> <span class="o">=</span> <span class="mf">0.0</span><span class="p">):</span> <span class="k">return</span> <span class="n">Magic</span><span class="p">(</span><span class="n">r</span> <span class="o">=</span> <span class="n">real</span><span class="p">,</span> <span class="n">i</span> <span class="o">=</span> <span class="n">imag</span><span class="p">)</span>
<span class="n">No</span><span class="p">:</span> <span class="k">def</span> <span class="nf">complex</span><span class="p">(</span><span class="n">real</span><span class="p">,</span> <span class="n">imag</span><span class="p">:</span> <span class="nb">float</span><span class="o">=</span><span class="mf">0.0</span><span class="p">):</span> <span class="k">return</span> <span class="n">Magic</span><span class="p">(</span><span class="n">r</span> <span class="o">=</span> <span class="n">real</span><span class="p">,</span> <span class="n">i</span> <span class="o">=</span> <span class="n">imag</span><span class="p">)</span>
</code></pre></div></div>
<p>Don’t use spaces to vertically align tokens on consecutive lines, since it
becomes a maintenance burden (applies to <code class="language-plaintext highlighter-rouge">:</code>, <code class="language-plaintext highlighter-rouge">#</code>, <code class="language-plaintext highlighter-rouge">=</code>, etc.):</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="n">foo</span> <span class="o">=</span> <span class="mi">1000</span> <span class="c1"># comment
</span> <span class="n">long_name</span> <span class="o">=</span> <span class="mi">2</span> <span class="c1"># comment that should not be aligned
</span>
<span class="n">dictionary</span> <span class="o">=</span> <span class="p">{</span>
<span class="s">'foo'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
<span class="s">'long_name'</span><span class="p">:</span> <span class="mi">2</span><span class="p">,</span>
<span class="p">}</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span>
<span class="n">foo</span> <span class="o">=</span> <span class="mi">1000</span> <span class="c1"># comment
</span> <span class="n">long_name</span> <span class="o">=</span> <span class="mi">2</span> <span class="c1"># comment that should not be aligned
</span>
<span class="n">dictionary</span> <span class="o">=</span> <span class="p">{</span>
<span class="s">'foo'</span> <span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
<span class="s">'long_name'</span><span class="p">:</span> <span class="mi">2</span><span class="p">,</span>
<span class="p">}</span>
</code></pre></div></div>
<p><a id="Python_Interpreter"></a>
<a id="s3.7-shebang-line"></a>
<a id="37-shebang-line"></a></p>
<p><a id="shebang-line"></a></p>
<h3 id="37-shebang-line">3.7 Shebang Line</h3>
<p>Most <code class="language-plaintext highlighter-rouge">.py</code> files do not need to start with a <code class="language-plaintext highlighter-rouge">#!</code> line. Start the main file of a
program with
<code class="language-plaintext highlighter-rouge">#!/usr/bin/env python3</code> (to support virtualenvs) or <code class="language-plaintext highlighter-rouge">#!/usr/bin/python3</code> per
<a href="https://peps.python.org/pep-0394/">PEP-394</a>.</p>
<p>This line is used by the kernel to find the Python interpreter, but is ignored by Python when importing modules. It is only necessary on a file intended to be executed directly.</p>
<p><a id="s3.8-comments-and-docstrings"></a>
<a id="s3.8-comments"></a>
<a id="38-comments-and-docstrings"></a></p>
<p><a id="documentation"></a></p>
<h3 id="38-comments-and-docstrings">3.8 Comments and Docstrings</h3>
<p>Be sure to use the right style for module, function, method docstrings and
inline comments.</p>
<p><a id="s3.8.1-comments-in-doc-strings"></a>
<a id="381-docstrings"></a>
<a id="comments-in-doc-strings"></a></p>
<p><a id="docstrings"></a></p>
<h4 id="381-docstrings">3.8.1 Docstrings</h4>
<p>Python uses <em>docstrings</em> to document code. A docstring is a string that is the
first statement in a package, module, class or function. These strings can be
extracted automatically through the <code class="language-plaintext highlighter-rouge">__doc__</code> member of the object and are used
by <code class="language-plaintext highlighter-rouge">pydoc</code>.
(Try running <code class="language-plaintext highlighter-rouge">pydoc</code> on your module to see how it looks.) Always use the
three-double-quote <code class="language-plaintext highlighter-rouge">"""</code> format for docstrings (per
<a href="https://peps.python.org/pep-0257/">PEP 257</a>). A docstring should be organized
as a summary line (one physical line not exceeding 80 characters) terminated by
a period, question mark, or exclamation point. When writing more (encouraged),
this must be followed by a blank line, followed by the rest of the docstring
starting at the same cursor position as the first quote of the first line. There
are more formatting guidelines for docstrings below.</p>
<p><a id="s3.8.2-comments-in-modules"></a>
<a id="382-modules"></a>
<a id="comments-in-modules"></a></p>
<p><a id="module-docs"></a></p>
<h4 id="382-modules">3.8.2 Modules</h4>
<p>Every file should contain license boilerplate. Choose the appropriate boilerplate for the license used by the project (for example, Apache 2.0, BSD, LGPL, GPL).</p>
<p>Files should start with a docstring describing the contents and usage of the
module.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="s">"""A one-line summary of the module or program, terminated by a period.
Leave one blank line. The rest of this docstring should contain an
overall description of the module or program. Optionally, it may also
contain a brief description of exported classes and functions and/or usage
examples.
Typical usage example:
foo = ClassFoo()
bar = foo.function_bar()
"""</span>
</code></pre></div></div>
<p><a id="s3.8.2.1-test-modules"></a></p>
<p><a id="test-docs"></a></p>
<h5 id="3821-test-modules">3.8.2.1 Test modules</h5>
<p>Module-level docstrings for test files are not required. They should be included
only when there is additional information that can be provided.</p>
<p>Examples include some specifics on how the test should be run, an explanation of
an unusual setup pattern, dependency on the external environment, and so on.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="s">"""This blaze test uses golden files.
You can update those files by running
`blaze run //foo/bar:foo_test -- --update_golden_files` from the `google3`
directory.
"""</span>
</code></pre></div></div>
<p>Docstrings that do not provide any new information should not be used.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="s">"""Tests for foo.bar."""</span>
</code></pre></div></div>
<p><a id="s3.8.3-functions-and-methods"></a>
<a id="383-functions-and-methods"></a>
<a id="functions-and-methods"></a></p>
<p><a id="function-docs"></a></p>
<h4 id="383-functions-and-methods">3.8.3 Functions and Methods</h4>
<p>In this section, “function” means a method, function, generator, or property.</p>
<p>A docstring is mandatory for every function that has one or more of the
following properties:</p>
<ul>
<li>being part of the public API</li>
<li>nontrivial size</li>
<li>non-obvious logic</li>
</ul>
<p>A docstring should give enough information to write a call to the function
without reading the function’s code. The docstring should describe the
function’s calling syntax and its semantics, but generally not its
implementation details, unless those details are relevant to how the function is
to be used. For example, a function that mutates one of its arguments as a side
effect should note that in its docstring. Otherwise, subtle but important
details of a function’s implementation that are not relevant to the caller are
better expressed as comments alongside the code than within the function’s
docstring.</p>
<p>The docstring may be descriptive-style (<code class="language-plaintext highlighter-rouge">"""Fetches rows from a Bigtable."""</code>)
or imperative-style (<code class="language-plaintext highlighter-rouge">"""Fetch rows from a Bigtable."""</code>), but the style should
be consistent within a file. The docstring for a <code class="language-plaintext highlighter-rouge">@property</code> data descriptor
should use the same style as the docstring for an attribute or a
<a href="#doc-function-args">function argument</a> (<code class="language-plaintext highlighter-rouge">"""The Bigtable path."""</code>,
rather than <code class="language-plaintext highlighter-rouge">"""Returns the Bigtable path."""</code>).</p>
<p>Certain aspects of a function should be documented in special sections, listed
below. Each section begins with a heading line, which ends with a colon. All
sections other than the heading should maintain a hanging indent of two or four
spaces (be consistent within a file). These sections can be omitted in cases
where the function’s name and signature are informative enough that it can be
aptly described using a one-line docstring.</p>
<dl>
<dt><a id="doc-function-args"></a></dt>
<dt><a href="#doc-function-args"><em>Args:</em></a></dt>
<dd>List each parameter by name. A description should follow the name, and be
separated by a colon followed by either a space or newline. If the
description is too long to fit on a single 80-character line, use a hanging
indent of 2 or 4 spaces more than the parameter name (be consistent with the
rest of the docstrings in the file). The description should include required
type(s) if the code does not contain a corresponding type annotation. If a
function accepts <code class="language-plaintext highlighter-rouge">*foo</code> (variable length argument lists) and/or <code class="language-plaintext highlighter-rouge">**bar</code>
(arbitrary keyword arguments), they should be listed as <code class="language-plaintext highlighter-rouge">*foo</code> and <code class="language-plaintext highlighter-rouge">**bar</code>.</dd>
<dt><a id="doc-function-returns"></a></dt>
<dt><a href="#doc-function-returns"><em>Returns:</em> (or <em>Yields:</em> for generators)</a></dt>
<dd>Describe the semantics of the return value, including any type information
that the type annotation does not provide. If the function only returns
None, this section is not required. It may also be omitted if the docstring
starts with “Return”, “Returns”, “Yield”, or “Yields” (e.g. <code class="language-plaintext highlighter-rouge">"""Returns row
from Bigtable as a tuple of strings."""</code>) <em>and</em> the opening sentence is
sufficient to describe the return value. Do not imitate older ‘NumPy style’
(<a href="https://numpy.org/doc/1.24/reference/generated/numpy.linalg.qr.html">example</a>),
which frequently documented a tuple return value as if it were multiple
return values with individual names (never mentioning the tuple). Instead,
describe such a return value as: “Returns: A tuple (mat_a, mat_b), where
mat_a is …, and …”. The auxiliary names in the docstring need not
necessarily correspond to any internal names used in the function body (as
those are not part of the API). If the function uses <code class="language-plaintext highlighter-rouge">yield</code> (is a
generator), the <code class="language-plaintext highlighter-rouge">Yields:</code> section should document the object returned by
<code class="language-plaintext highlighter-rouge">next()</code>, instead of the generator object itself that the call evaluates to.</dd>
<dt><a id="doc-function-raises"></a></dt>
<dt><a href="#doc-function-raises"><em>Raises:</em></a></dt>
<dd>List all exceptions that are relevant to the interface followed by a
description. Use a similar exception name + colon + space or newline and
hanging indent style as described in <em>Args:</em>. You should not document
exceptions that get raised if the API specified in the docstring is violated
(because this would paradoxically make behavior under violation of the API
part of the API).</dd>
</dl>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">def</span> <span class="nf">fetch_smalltable_rows</span><span class="p">(</span>
<span class="n">table_handle</span><span class="p">:</span> <span class="n">smalltable</span><span class="p">.</span><span class="n">Table</span><span class="p">,</span>
<span class="n">keys</span><span class="p">:</span> <span class="n">Sequence</span><span class="p">[</span><span class="nb">bytes</span> <span class="o">|</span> <span class="nb">str</span><span class="p">],</span>
<span class="n">require_all_keys</span><span class="p">:</span> <span class="nb">bool</span> <span class="o">=</span> <span class="bp">False</span><span class="p">,</span>
<span class="p">)</span> <span class="o">-&gt;</span> <span class="n">Mapping</span><span class="p">[</span><span class="nb">bytes</span><span class="p">,</span> <span class="nb">tuple</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="p">...]]:</span>
<span class="s">"""Fetches rows from a Smalltable.
Retrieves rows pertaining to the given keys from the Table instance
represented by table_handle. String keys will be UTF-8 encoded.
Args:
table_handle: An open smalltable.Table instance.
keys: A sequence of strings representing the key of each table
row to fetch. String keys will be UTF-8 encoded.
require_all_keys: If True only rows with values set for all keys will be
returned.
Returns:
A dict mapping keys to the corresponding table row data
fetched. Each row is represented as a tuple of strings. For
example:
{b'Serak': ('Rigel VII', 'Preparer'),
b'Zim': ('Irk', 'Invader'),
b'Lrrr': ('Omicron Persei 8', 'Emperor')}
Returned keys are always bytes. If a key from the keys argument is
missing from the dictionary, then that row was not found in the
table (and require_all_keys must have been False).
Raises:
IOError: An error occurred accessing the smalltable.
"""</span>
</code></pre></div></div>
<p>Similarly, this variation on <code class="language-plaintext highlighter-rouge">Args:</code> with a line break is also allowed:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">def</span> <span class="nf">fetch_smalltable_rows</span><span class="p">(</span>
<span class="n">table_handle</span><span class="p">:</span> <span class="n">smalltable</span><span class="p">.</span><span class="n">Table</span><span class="p">,</span>
<span class="n">keys</span><span class="p">:</span> <span class="n">Sequence</span><span class="p">[</span><span class="nb">bytes</span> <span class="o">|</span> <span class="nb">str</span><span class="p">],</span>
<span class="n">require_all_keys</span><span class="p">:</span> <span class="nb">bool</span> <span class="o">=</span> <span class="bp">False</span><span class="p">,</span>
<span class="p">)</span> <span class="o">-&gt;</span> <span class="n">Mapping</span><span class="p">[</span><span class="nb">bytes</span><span class="p">,</span> <span class="nb">tuple</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="p">...]]:</span>
<span class="s">"""Fetches rows from a Smalltable.
Retrieves rows pertaining to the given keys from the Table instance
represented by table_handle. String keys will be UTF-8 encoded.
Args:
table_handle:
An open smalltable.Table instance.
keys:
A sequence of strings representing the key of each table row to
fetch. String keys will be UTF-8 encoded.
require_all_keys:
If True only rows with values set for all keys will be returned.
Returns:
A dict mapping keys to the corresponding table row data
fetched. Each row is represented as a tuple of strings. For
example:
{b'Serak': ('Rigel VII', 'Preparer'),
b'Zim': ('Irk', 'Invader'),
b'Lrrr': ('Omicron Persei 8', 'Emperor')}
Returned keys are always bytes. If a key from the keys argument is
missing from the dictionary, then that row was not found in the
table (and require_all_keys must have been False).
Raises:
IOError: An error occurred accessing the smalltable.
"""</span>
</code></pre></div></div>
<p><a id="s3.8.3.1-overridden-methods"></a></p>
<p><a id="overridden-method-docs"></a></p>
<h5 id="3831-overridden-methods">3.8.3.1 Overridden Methods</h5>
<p>A method that overrides a method from a base class does not need a docstring if
it is explicitly decorated with
<a href="https://typing-extensions.readthedocs.io/en/latest/#override"><code class="language-plaintext highlighter-rouge">@override</code></a>
(from <code class="language-plaintext highlighter-rouge">typing_extensions</code> or <code class="language-plaintext highlighter-rouge">typing</code> modules), unless the overriding method’s
behavior materially refines the base method’s contract, or details need to be
provided (e.g., documenting additional side effects), in which case a docstring
with at least those differences is required on the overriding method.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">typing_extensions</span> <span class="kn">import</span> <span class="n">override</span>
<span class="k">class</span> <span class="nc">Parent</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">do_something</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="s">"""Parent method, includes docstring."""</span>
<span class="c1"># Child class, method annotated with override.
</span><span class="k">class</span> <span class="nc">Child</span><span class="p">(</span><span class="n">Parent</span><span class="p">):</span>
<span class="o">@</span><span class="n">override</span>
<span class="k">def</span> <span class="nf">do_something</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">pass</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># Child class, but without @override decorator, a docstring is required.
</span><span class="k">class</span> <span class="nc">Child</span><span class="p">(</span><span class="n">Parent</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">do_something</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">pass</span>
<span class="c1"># Docstring is trivial, @override is sufficient to indicate that docs can be
# found in the base class.
</span><span class="k">class</span> <span class="nc">Child</span><span class="p">(</span><span class="n">Parent</span><span class="p">):</span>
<span class="o">@</span><span class="n">override</span>
<span class="k">def</span> <span class="nf">do_something</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="s">"""See base class."""</span>
</code></pre></div></div>
<p><a id="s3.8.4-comments-in-classes"></a>
<a id="384-classes"></a>
<a id="comments-in-classes"></a></p>
<p><a id="class-docs"></a></p>
<h4 id="384-classes">3.8.4 Classes</h4>
<p>Classes should have a docstring below the class definition describing the class.
Public attributes, excluding <a href="#properties">properties</a>, should be documented
here in an <code class="language-plaintext highlighter-rouge">Attributes</code> section and follow the same formatting as a
<a href="#doc-function-args">function’s <code class="language-plaintext highlighter-rouge">Args</code></a> section.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">class</span> <span class="nc">SampleClass</span><span class="p">:</span>
<span class="s">"""Summary of class here.
Longer class information...
Longer class information...
Attributes:
likes_spam: A boolean indicating if we like SPAM or not.
eggs: An integer count of the eggs we have laid.
"""</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">likes_spam</span><span class="p">:</span> <span class="nb">bool</span> <span class="o">=</span> <span class="bp">False</span><span class="p">):</span>
<span class="s">"""Initializes the instance based on spam preference.
Args:
likes_spam: Defines if instance exhibits this preference.
"""</span>
<span class="bp">self</span><span class="p">.</span><span class="n">likes_spam</span> <span class="o">=</span> <span class="n">likes_spam</span>
<span class="bp">self</span><span class="p">.</span><span class="n">eggs</span> <span class="o">=</span> <span class="mi">0</span>
<span class="o">@</span><span class="nb">property</span>
<span class="k">def</span> <span class="nf">butter_sticks</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">int</span><span class="p">:</span>
<span class="s">"""The number of butter sticks we have."""</span>
</code></pre></div></div>
<p>All class docstrings should start with a one-line summary that describes what
the class instance represents. This implies that subclasses of <code class="language-plaintext highlighter-rouge">Exception</code>
should also describe what the exception represents, and not the context in which
it might occur. The class docstring should not repeat unnecessary information,
such as that the class is a class.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># Yes:
</span><span class="k">class</span> <span class="nc">CheeseShopAddress</span><span class="p">:</span>
<span class="s">"""The address of a cheese shop.
...
"""</span>
<span class="k">class</span> <span class="nc">OutOfCheeseError</span><span class="p">(</span><span class="nb">Exception</span><span class="p">):</span>
<span class="s">"""No more cheese is available."""</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># No:
</span><span class="k">class</span> <span class="nc">CheeseShopAddress</span><span class="p">:</span>
<span class="s">"""Class that describes the address of a cheese shop.
...
"""</span>
<span class="k">class</span> <span class="nc">OutOfCheeseError</span><span class="p">(</span><span class="nb">Exception</span><span class="p">):</span>
<span class="s">"""Raised when no more cheese is available."""</span>
</code></pre></div></div>
<p><a id="s3.8.5-block-and-inline-comments"></a>
<a id="comments-in-block-and-inline"></a>
<a id="s3.8.5-comments-in-block-and-inline"></a>
<a id="385-block-and-inline-comments"></a></p>
<p><a id="comments"></a></p>
<h4 id="385-block-and-inline-comments">3.8.5 Block and Inline Comments</h4>
<p>The final place to have comments is in tricky parts of the code. If you’re going
to have to explain it at the next <a href="http://en.wikipedia.org/wiki/Code_review">code review</a>,
you should comment it now. Complicated operations get a few lines of comments
before the operations commence. Non-obvious ones get comments at the end of the
line.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># We use a weighted dictionary search to find out where i is in
# the array. We extrapolate position based on the largest num
# in the array and the array size and then do binary search to
# get the exact number.
</span>
<span class="k">if</span> <span class="n">i</span> <span class="o">&amp;</span> <span class="p">(</span><span class="n">i</span><span class="o">-</span><span class="mi">1</span><span class="p">)</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span> <span class="c1"># True if i is 0 or a power of 2.
</span></code></pre></div></div>
<p>To improve legibility, these comments should start at least 2 spaces away from
the code with the comment character <code class="language-plaintext highlighter-rouge">#</code>, followed by at least one space before
the text of the comment itself.</p>
<p>On the other hand, never describe the code. Assume the person reading the code
knows Python (though not what you’re trying to do) better than you do.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># BAD COMMENT: Now go through the b array and make sure whenever i occurs
# the next element is i+1
</span></code></pre></div></div>
<!-- The next section is copied from the C++ style guide. -->
<p><a id="s3.8.6-punctuation-spelling-and-grammar"></a>
<a id="386-punctuation-spelling-and-grammar"></a>
<a id="spelling"></a>
<a id="punctuation"></a>
<a id="grammar"></a></p>
<p><a id="punctuation-spelling-grammar"></a></p>
<h4 id="386-punctuation-spelling-and-grammar">3.8.6 Punctuation, Spelling, and Grammar</h4>
<p>Pay attention to punctuation, spelling, and grammar; it is easier to read
well-written comments than badly written ones.</p>
<p>Comments should be as readable as narrative text, with proper capitalization and
punctuation. In many cases, complete sentences are more readable than sentence
fragments. Shorter comments, such as comments at the end of a line of code, can
sometimes be less formal, but you should be consistent with your style.</p>
<p>Although it can be frustrating to have a code reviewer point out that you are
using a comma when you should be using a semicolon, it is very important that
source code maintain a high level of clarity and readability. Proper
punctuation, spelling, and grammar help with that goal.</p>
<p><a id="s3.10-strings"></a>
<a id="310-strings"></a></p>
<p><a id="strings"></a></p>
<h3 id="310-strings">3.10 Strings</h3>
<p>Use an
<a href="https://docs.python.org/3/reference/lexical_analysis.html#f-strings">f-string</a>,
the <code class="language-plaintext highlighter-rouge">%</code> operator, or the <code class="language-plaintext highlighter-rouge">format</code> method for formatting strings, even when the
parameters are all strings. Use your best judgment to decide between string
formatting options. A single join with <code class="language-plaintext highlighter-rouge">+</code> is okay but do not format with <code class="language-plaintext highlighter-rouge">+</code>.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="n">x</span> <span class="o">=</span> <span class="sa">f</span><span class="s">'name: </span><span class="si">{</span><span class="n">name</span><span class="si">}</span><span class="s">; score: </span><span class="si">{</span><span class="n">n</span><span class="si">}</span><span class="s">'</span>
<span class="n">x</span> <span class="o">=</span> <span class="s">'%s, %s!'</span> <span class="o">%</span> <span class="p">(</span><span class="n">imperative</span><span class="p">,</span> <span class="n">expletive</span><span class="p">)</span>
<span class="n">x</span> <span class="o">=</span> <span class="s">'{}, {}'</span><span class="p">.</span><span class="nb">format</span><span class="p">(</span><span class="n">first</span><span class="p">,</span> <span class="n">second</span><span class="p">)</span>
<span class="n">x</span> <span class="o">=</span> <span class="s">'name: %s; score: %d'</span> <span class="o">%</span> <span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">n</span><span class="p">)</span>
<span class="n">x</span> <span class="o">=</span> <span class="s">'name: %(name)s; score: %(score)d'</span> <span class="o">%</span> <span class="p">{</span><span class="s">'name'</span><span class="p">:</span><span class="n">name</span><span class="p">,</span> <span class="s">'score'</span><span class="p">:</span><span class="n">n</span><span class="p">}</span>
<span class="n">x</span> <span class="o">=</span> <span class="s">'name: {}; score: {}'</span><span class="p">.</span><span class="nb">format</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">n</span><span class="p">)</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">a</span> <span class="o">+</span> <span class="n">b</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="n">x</span> <span class="o">=</span> <span class="n">first</span> <span class="o">+</span> <span class="s">', '</span> <span class="o">+</span> <span class="n">second</span>
<span class="n">x</span> <span class="o">=</span> <span class="s">'name: '</span> <span class="o">+</span> <span class="n">name</span> <span class="o">+</span> <span class="s">'; score: '</span> <span class="o">+</span> <span class="nb">str</span><span class="p">(</span><span class="n">n</span><span class="p">)</span>
</code></pre></div></div>
<p>Avoid using the <code class="language-plaintext highlighter-rouge">+</code> and <code class="language-plaintext highlighter-rouge">+=</code> operators to accumulate a string within a loop. In
some conditions, accumulating a string with addition can lead to quadratic
rather than linear running time. Although common accumulations of this sort may
be optimized on CPython, that is an implementation detail. The conditions under
which an optimization applies are not easy to predict and may change. Instead,
add each substring to a list and <code class="language-plaintext highlighter-rouge">''.join</code> the list after the loop terminates,
or write each substring to an <code class="language-plaintext highlighter-rouge">io.StringIO</code> buffer. These techniques
consistently have amortized-linear run-time complexity.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="n">items</span> <span class="o">=</span> <span class="p">[</span><span class="s">'&lt;table&gt;'</span><span class="p">]</span>
<span class="k">for</span> <span class="n">last_name</span><span class="p">,</span> <span class="n">first_name</span> <span class="ow">in</span> <span class="n">employee_list</span><span class="p">:</span>
<span class="n">items</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">'&lt;tr&gt;&lt;td&gt;%s, %s&lt;/td&gt;&lt;/tr&gt;'</span> <span class="o">%</span> <span class="p">(</span><span class="n">last_name</span><span class="p">,</span> <span class="n">first_name</span><span class="p">))</span>
<span class="n">items</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">'&lt;/table&gt;'</span><span class="p">)</span>
<span class="n">employee_table</span> <span class="o">=</span> <span class="s">''</span><span class="p">.</span><span class="n">join</span><span class="p">(</span><span class="n">items</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="n">employee_table</span> <span class="o">=</span> <span class="s">'&lt;table&gt;'</span>
<span class="k">for</span> <span class="n">last_name</span><span class="p">,</span> <span class="n">first_name</span> <span class="ow">in</span> <span class="n">employee_list</span><span class="p">:</span>
<span class="n">employee_table</span> <span class="o">+=</span> <span class="s">'&lt;tr&gt;&lt;td&gt;%s, %s&lt;/td&gt;&lt;/tr&gt;'</span> <span class="o">%</span> <span class="p">(</span><span class="n">last_name</span><span class="p">,</span> <span class="n">first_name</span><span class="p">)</span>
<span class="n">employee_table</span> <span class="o">+=</span> <span class="s">'&lt;/table&gt;'</span>
</code></pre></div></div>
<p>Be consistent with your choice of string quote character within a file. Pick <code class="language-plaintext highlighter-rouge">'</code>
or <code class="language-plaintext highlighter-rouge">"</code> and stick with it. It is okay to use the other quote character on a
string to avoid the need to backslash-escape quote characters within the string.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="n">Python</span><span class="p">(</span><span class="s">'Why are you hiding your eyes?'</span><span class="p">)</span>
<span class="n">Gollum</span><span class="p">(</span><span class="s">"I'm scared of lint errors."</span><span class="p">)</span>
<span class="n">Narrator</span><span class="p">(</span><span class="s">'"Good!" thought a happy Python reviewer.'</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span>
<span class="n">Python</span><span class="p">(</span><span class="s">"Why are you hiding your eyes?"</span><span class="p">)</span>
<span class="n">Gollum</span><span class="p">(</span><span class="s">'The lint. It burns. It burns us.'</span><span class="p">)</span>
<span class="n">Gollum</span><span class="p">(</span><span class="s">"Always the great lint. Watching. Watching."</span><span class="p">)</span>
</code></pre></div></div>
<p>Prefer <code class="language-plaintext highlighter-rouge">"""</code> for multi-line strings rather than <code class="language-plaintext highlighter-rouge">'''</code>. Projects may choose to
use <code class="language-plaintext highlighter-rouge">'''</code> for all non-docstring multi-line strings if and only if they also use
<code class="language-plaintext highlighter-rouge">'</code> for regular strings. Docstrings must use <code class="language-plaintext highlighter-rouge">"""</code> regardless.</p>
<p>Multi-line strings do not flow with the indentation of the rest of the program.
If you need to avoid embedding extra space in the string, use either
concatenated single-line strings or a multi-line string with
<a href="https://docs.python.org/3/library/textwrap.html#textwrap.dedent"><code class="language-plaintext highlighter-rouge">textwrap.dedent()</code></a>
to remove the initial space on each line:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="n">No</span><span class="p">:</span>
<span class="n">long_string</span> <span class="o">=</span> <span class="s">"""This is pretty ugly.
Don't do this.
"""</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="n">Yes</span><span class="p">:</span>
<span class="n">long_string</span> <span class="o">=</span> <span class="s">"""This is fine if your use case can accept
extraneous leading spaces."""</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="n">Yes</span><span class="p">:</span>
<span class="n">long_string</span> <span class="o">=</span> <span class="p">(</span><span class="s">"And this is fine if you cannot accept</span><span class="se">\n</span><span class="s">"</span> <span class="o">+</span>
<span class="s">"extraneous leading spaces."</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="n">Yes</span><span class="p">:</span>
<span class="n">long_string</span> <span class="o">=</span> <span class="p">(</span><span class="s">"And this too is fine if you cannot accept</span><span class="se">\n</span><span class="s">"</span>
<span class="s">"extraneous leading spaces."</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="n">Yes</span><span class="p">:</span>
<span class="kn">import</span> <span class="nn">textwrap</span>
<span class="n">long_string</span> <span class="o">=</span> <span class="n">textwrap</span><span class="p">.</span><span class="n">dedent</span><span class="p">(</span><span class="s">"""</span><span class="se">\
</span><span class="s"> This is also fine, because textwrap.dedent()
will collapse common leading spaces in each line."""</span><span class="p">)</span>
</code></pre></div></div>
<p>Note that using a backslash here does not violate the prohibition against
<a href="#line-length">explicit line continuation</a>; in this case, the backslash is
<a href="https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals">escaping a newline</a>
in a string literal.</p>
<p><a id="s3.10.1-logging"></a>
<a id="3101-logging"></a>
<a id="logging"></a></p>
<p><a id="logging"></a></p>
<h4 id="3101-logging">3.10.1 Logging</h4>
<p>For logging functions that expect a pattern-string (with %-placeholders) as
their first argument: Always call them with a string literal (not an f-string!)
as their first argument with pattern-parameters as subsequent arguments. Some
logging implementations collect the unexpanded pattern-string as a queryable
field. It also prevents spending time rendering a message that no logger is
configured to output.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="n">Yes</span><span class="p">:</span>
<span class="kn">import</span> <span class="nn">tensorflow</span> <span class="k">as</span> <span class="n">tf</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">tf</span><span class="p">.</span><span class="n">get_logger</span><span class="p">()</span>
<span class="n">logger</span><span class="p">.</span><span class="n">info</span><span class="p">(</span><span class="s">'TensorFlow Version is: %s'</span><span class="p">,</span> <span class="n">tf</span><span class="p">.</span><span class="n">__version__</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="n">Yes</span><span class="p">:</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="kn">from</span> <span class="nn">absl</span> <span class="kn">import</span> <span class="n">logging</span>
<span class="n">logging</span><span class="p">.</span><span class="n">info</span><span class="p">(</span><span class="s">'Current $PAGER is: %s'</span><span class="p">,</span> <span class="n">os</span><span class="p">.</span><span class="n">getenv</span><span class="p">(</span><span class="s">'PAGER'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="s">''</span><span class="p">))</span>
<span class="n">homedir</span> <span class="o">=</span> <span class="n">os</span><span class="p">.</span><span class="n">getenv</span><span class="p">(</span><span class="s">'HOME'</span><span class="p">)</span>
<span class="k">if</span> <span class="n">homedir</span> <span class="ow">is</span> <span class="bp">None</span> <span class="ow">or</span> <span class="ow">not</span> <span class="n">os</span><span class="p">.</span><span class="n">access</span><span class="p">(</span><span class="n">homedir</span><span class="p">,</span> <span class="n">os</span><span class="p">.</span><span class="n">W_OK</span><span class="p">):</span>
<span class="n">logging</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="s">'Cannot write to home directory, $HOME=%r'</span><span class="p">,</span> <span class="n">homedir</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="n">No</span><span class="p">:</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="kn">from</span> <span class="nn">absl</span> <span class="kn">import</span> <span class="n">logging</span>
<span class="n">logging</span><span class="p">.</span><span class="n">info</span><span class="p">(</span><span class="s">'Current $PAGER is:'</span><span class="p">)</span>
<span class="n">logging</span><span class="p">.</span><span class="n">info</span><span class="p">(</span><span class="n">os</span><span class="p">.</span><span class="n">getenv</span><span class="p">(</span><span class="s">'PAGER'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="s">''</span><span class="p">))</span>
<span class="n">homedir</span> <span class="o">=</span> <span class="n">os</span><span class="p">.</span><span class="n">getenv</span><span class="p">(</span><span class="s">'HOME'</span><span class="p">)</span>
<span class="k">if</span> <span class="n">homedir</span> <span class="ow">is</span> <span class="bp">None</span> <span class="ow">or</span> <span class="ow">not</span> <span class="n">os</span><span class="p">.</span><span class="n">access</span><span class="p">(</span><span class="n">homedir</span><span class="p">,</span> <span class="n">os</span><span class="p">.</span><span class="n">W_OK</span><span class="p">):</span>
<span class="n">logging</span><span class="p">.</span><span class="n">error</span><span class="p">(</span><span class="sa">f</span><span class="s">'Cannot write to home directory, $HOME=</span><span class="si">{</span><span class="n">homedir</span><span class="si">!r}</span><span class="s">'</span><span class="p">)</span>
</code></pre></div></div>
<p><a id="s3.10.2-error-messages"></a>
<a id="3102-error-messages"></a>
<a id="error-messages"></a></p>
<p><a id="error-messages"></a></p>
<h4 id="3102-error-messages">3.10.2 Error Messages</h4>
<p>Error messages (such as: message strings on exceptions like <code class="language-plaintext highlighter-rouge">ValueError</code>, or
messages shown to the user) should follow three guidelines:</p>
<ol>
<li>
<p>The message needs to precisely match the actual error condition.</p>
</li>
<li>
<p>Interpolated pieces need to always be clearly identifiable as such.</p>
</li>
<li>
<p>They should allow simple automated processing (e.g. grepping).</p>
</li>
</ol>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="n">Yes</span><span class="p">:</span>
<span class="k">if</span> <span class="ow">not</span> <span class="mi">0</span> <span class="o">&lt;=</span> <span class="n">p</span> <span class="o">&lt;=</span> <span class="mi">1</span><span class="p">:</span>
<span class="k">raise</span> <span class="nb">ValueError</span><span class="p">(</span><span class="sa">f</span><span class="s">'Not a probability: </span><span class="si">{</span><span class="n">p</span><span class="o">=</span><span class="si">}</span><span class="s">'</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">os</span><span class="p">.</span><span class="n">rmdir</span><span class="p">(</span><span class="n">workdir</span><span class="p">)</span>
<span class="k">except</span> <span class="nb">OSError</span> <span class="k">as</span> <span class="n">error</span><span class="p">:</span>
<span class="n">logging</span><span class="p">.</span><span class="n">warning</span><span class="p">(</span><span class="s">'Could not remove directory (reason: %r): %r'</span><span class="p">,</span>
<span class="n">error</span><span class="p">,</span> <span class="n">workdir</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="n">No</span><span class="p">:</span>
<span class="k">if</span> <span class="n">p</span> <span class="o">&lt;</span> <span class="mi">0</span> <span class="ow">or</span> <span class="n">p</span> <span class="o">&gt;</span> <span class="mi">1</span><span class="p">:</span> <span class="c1"># PROBLEM: also false for float('nan')!
</span> <span class="k">raise</span> <span class="nb">ValueError</span><span class="p">(</span><span class="sa">f</span><span class="s">'Not a probability: </span><span class="si">{</span><span class="n">p</span><span class="o">=</span><span class="si">}</span><span class="s">'</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">os</span><span class="p">.</span><span class="n">rmdir</span><span class="p">(</span><span class="n">workdir</span><span class="p">)</span>
<span class="k">except</span> <span class="nb">OSError</span><span class="p">:</span>
<span class="c1"># PROBLEM: Message makes an assumption that might not be true:
</span> <span class="c1"># Deletion might have failed for some other reason, misleading
</span> <span class="c1"># whoever has to debug this.
</span> <span class="n">logging</span><span class="p">.</span><span class="n">warning</span><span class="p">(</span><span class="s">'Directory already was deleted: %s'</span><span class="p">,</span> <span class="n">workdir</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">os</span><span class="p">.</span><span class="n">rmdir</span><span class="p">(</span><span class="n">workdir</span><span class="p">)</span>
<span class="k">except</span> <span class="nb">OSError</span><span class="p">:</span>
<span class="c1"># PROBLEM: The message is harder to grep for than necessary, and
</span> <span class="c1"># not universally non-confusing for all possible values of `workdir`.
</span> <span class="c1"># Imagine someone calling a library function with such code
</span> <span class="c1"># using a name such as workdir = 'deleted'. The warning would read:
</span> <span class="c1"># "The deleted directory could not be deleted."
</span> <span class="n">logging</span><span class="p">.</span><span class="n">warning</span><span class="p">(</span><span class="s">'The %s directory could not be deleted.'</span><span class="p">,</span> <span class="n">workdir</span><span class="p">)</span>
</code></pre></div></div>
<p><a id="s3.11-files-sockets-closeables"></a>
<a id="s3.11-files-and-sockets"></a>
<a id="311-files-and-sockets"></a>
<a id="files-and-sockets"></a></p>
<p><a id="files"></a></p>
<h3 id="311-files-sockets-and-similar-stateful-resources">3.11 Files, Sockets, and similar Stateful Resources</h3>
<p>Explicitly close files and sockets when done with them. This rule naturally
extends to closeable resources that internally use sockets, such as database
connections, and also other resources that need to be closed down in a similar
fashion. To name only a few examples, this also includes
<a href="https://docs.python.org/3/library/mmap.html">mmap</a> mappings,
<a href="https://docs.h5py.org/en/stable/high/file.html">h5py File objects</a>, and
<a href="https://matplotlib.org/2.1.0/api/_as_gen/matplotlib.pyplot.close.html">matplotlib.pyplot figure windows</a>.</p>
<p>Leaving files, sockets or other such stateful objects open unnecessarily has
many downsides:</p>
<ul>
<li>They may consume limited system resources, such as file descriptors. Code
that deals with many such objects may exhaust those resources unnecessarily
if they’re not returned to the system promptly after use.</li>
<li>Holding files open may prevent other actions such as moving or deleting
them, or unmounting a filesystem.</li>
<li>Files and sockets that are shared throughout a program may inadvertently be
read from or written to after logically being closed. If they are actually
closed, attempts to read or write from them will raise exceptions, making
the problem known sooner.</li>
</ul>
<p>Furthermore, while files and sockets (and some similarly behaving resources) are
automatically closed when the object is destructed, coupling the lifetime of the
object to the state of the resource is poor practice:</p>
<ul>
<li>There are no guarantees as to when the runtime will actually invoke the
<code class="language-plaintext highlighter-rouge">__del__</code> method. Different Python implementations use different memory
management techniques, such as delayed garbage collection, which may
increase the object’s lifetime arbitrarily and indefinitely.</li>
<li>Unexpected references to the file, e.g. in globals or exception tracebacks,
may keep it around longer than intended.</li>
</ul>
<p>Relying on finalizers to do automatic cleanup that has observable side effects
has been rediscovered over and over again to lead to major problems, across many
decades and multiple languages (see e.g.
<a href="https://wiki.sei.cmu.edu/confluence/display/java/MET12-J.+Do+not+use+finalizers">this article</a>
for Java).</p>
<p>The preferred way to manage files and similar resources is using the
<a href="http://docs.python.org/reference/compound_stmts.html#the-with-statement"><code class="language-plaintext highlighter-rouge">with</code> statement</a>:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s">"hello.txt"</span><span class="p">)</span> <span class="k">as</span> <span class="n">hello_file</span><span class="p">:</span>
<span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">hello_file</span><span class="p">:</span>
<span class="k">print</span><span class="p">(</span><span class="n">line</span><span class="p">)</span>
</code></pre></div></div>
<p>For file-like objects that do not support the <code class="language-plaintext highlighter-rouge">with</code> statement, use
<code class="language-plaintext highlighter-rouge">contextlib.closing()</code>:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">import</span> <span class="nn">contextlib</span>
<span class="k">with</span> <span class="n">contextlib</span><span class="p">.</span><span class="n">closing</span><span class="p">(</span><span class="n">urllib</span><span class="p">.</span><span class="n">urlopen</span><span class="p">(</span><span class="s">"http://www.python.org/"</span><span class="p">))</span> <span class="k">as</span> <span class="n">front_page</span><span class="p">:</span>
<span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">front_page</span><span class="p">:</span>
<span class="k">print</span><span class="p">(</span><span class="n">line</span><span class="p">)</span>
</code></pre></div></div>
<p>In rare cases where context-based resource management is infeasible, code
documentation must explain clearly how resource lifetime is managed.</p>
<p><a id="s3.12-todo-comments"></a>
<a id="312-todo-comments"></a></p>
<p><a id="todo"></a></p>
<h3 id="312-todo-comments">3.12 TODO Comments</h3>
<p>Use <code class="language-plaintext highlighter-rouge">TODO</code> comments for code that is temporary, a short-term solution, or
good-enough but not perfect.</p>
<p>A <code class="language-plaintext highlighter-rouge">TODO</code> comment begins with the word <code class="language-plaintext highlighter-rouge">TODO</code> in all caps, a following colon, and
a link to a resource that contains the context, ideally a bug reference. A bug
reference is preferable because bugs are tracked and have follow-up comments.
Follow this piece of context with an explanatory string introduced with a hyphen
<code class="language-plaintext highlighter-rouge">-</code>.
The purpose is to have a consistent <code class="language-plaintext highlighter-rouge">TODO</code> format that can be searched to find
out how to get more details.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># TODO: crbug.com/192795 - Investigate cpufreq optimizations.
</span></code></pre></div></div>
<p>Old style, formerly recommended, but discouraged for use in new code:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># TODO(crbug.com/192795): Investigate cpufreq optimizations.
# TODO(yourusername): Use a "\*" here for concatenation operator.
</span></code></pre></div></div>
<p>Avoid adding TODOs that refer to an individual or team as the context:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># TODO: @yourusername - File an issue and use a '*' for repetition.
</span></code></pre></div></div>
<p>If your <code class="language-plaintext highlighter-rouge">TODO</code> is of the form “At a future date do something” make sure that you
either include a very specific date (“Fix by November 2009”) or a very specific
event (“Remove this code when all clients can handle XML responses.”) that
future code maintainers will comprehend. Issues are ideal for tracking this.</p>
<p><a id="s3.13-imports-formatting"></a>
<a id="313-imports-formatting"></a></p>
<p><a id="imports-formatting"></a></p>
<h3 id="313-imports-formatting">3.13 Imports formatting</h3>
<p>Imports should be on separate lines; there are
<a href="#typing-imports">exceptions for <code class="language-plaintext highlighter-rouge">typing</code> and <code class="language-plaintext highlighter-rouge">collections.abc</code> imports</a>.</p>
<p>E.g.:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span> <span class="kn">from</span> <span class="nn">collections.abc</span> <span class="kn">import</span> <span class="n">Mapping</span><span class="p">,</span> <span class="n">Sequence</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="kn">from</span> <span class="nn">typing</span> <span class="kn">import</span> <span class="n">Any</span><span class="p">,</span> <span class="n">NewType</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span> <span class="kn">import</span> <span class="nn">os</span><span class="p">,</span> <span class="n">sys</span>
</code></pre></div></div>
<p>Imports are always put at the top of the file, just after any module comments
and docstrings and before module globals and constants. Imports should be
grouped from most generic to least generic:</p>
<ol>
<li>
<p>Python future import statements. For example:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">__future__</span> <span class="kn">import</span> <span class="n">annotations</span>
</code></pre></div> </div>
<p>See <a href="#from-future-imports">above</a> for more information about those.</p>
</li>
<li>
<p>Python standard library imports. For example:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">import</span> <span class="nn">sys</span>
</code></pre></div> </div>
</li>
<li>
<p><a href="https://pypi.org/">third-party</a> module
or package imports. For example:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">import</span> <span class="nn">tensorflow</span> <span class="k">as</span> <span class="n">tf</span>
</code></pre></div> </div>
</li>
<li>
<p>Code repository
sub-package imports. For example:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">otherproject.ai</span> <span class="kn">import</span> <span class="n">mind</span>
</code></pre></div> </div>
</li>
<li>
<p><strong>Deprecated:</strong> application-specific imports that are part of the same
top-level
sub-package as this file. For example:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">myproject.backend.hgwells</span> <span class="kn">import</span> <span class="n">time_machine</span>
</code></pre></div> </div>
<p>You may find older Google Python Style code doing this, but it is no longer
required. <strong>New code is encouraged not to bother with this.</strong> Simply treat
application-specific sub-package imports the same as other sub-package
imports.</p>
</li>
</ol>
<p>Within each grouping, imports should be sorted lexicographically, ignoring case,
according to each module’s full package path (the <code class="language-plaintext highlighter-rouge">path</code> in <code class="language-plaintext highlighter-rouge">from path import
...</code>). Code may optionally place a blank line between import sections.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">import</span> <span class="nn">collections</span>
<span class="kn">import</span> <span class="nn">queue</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="kn">from</span> <span class="nn">absl</span> <span class="kn">import</span> <span class="n">app</span>
<span class="kn">from</span> <span class="nn">absl</span> <span class="kn">import</span> <span class="n">flags</span>
<span class="kn">import</span> <span class="nn">bs4</span>
<span class="kn">import</span> <span class="nn">cryptography</span>
<span class="kn">import</span> <span class="nn">tensorflow</span> <span class="k">as</span> <span class="n">tf</span>
<span class="kn">from</span> <span class="nn">book.genres</span> <span class="kn">import</span> <span class="n">scifi</span>
<span class="kn">from</span> <span class="nn">myproject.backend</span> <span class="kn">import</span> <span class="n">huxley</span>
<span class="kn">from</span> <span class="nn">myproject.backend.hgwells</span> <span class="kn">import</span> <span class="n">time_machine</span>
<span class="kn">from</span> <span class="nn">myproject.backend.state_machine</span> <span class="kn">import</span> <span class="n">main_loop</span>
<span class="kn">from</span> <span class="nn">otherproject.ai</span> <span class="kn">import</span> <span class="n">body</span>
<span class="kn">from</span> <span class="nn">otherproject.ai</span> <span class="kn">import</span> <span class="n">mind</span>
<span class="kn">from</span> <span class="nn">otherproject.ai</span> <span class="kn">import</span> <span class="n">soul</span>
<span class="c1"># Older style code may have these imports down here instead:
#from myproject.backend.hgwells import time_machine
#from myproject.backend.state_machine import main_loop
</span></code></pre></div></div>
<p><a id="s3.14-statements"></a>
<a id="314-statements"></a></p>
<p><a id="statements"></a></p>
<h3 id="314-statements">3.14 Statements</h3>
<p>Generally only one statement per line.</p>
<p>However, you may put the result of a test on the same line as the test only if
the entire statement fits on one line. In particular, you can never do so with
<code class="language-plaintext highlighter-rouge">try</code>/<code class="language-plaintext highlighter-rouge">except</code> since the <code class="language-plaintext highlighter-rouge">try</code> and <code class="language-plaintext highlighter-rouge">except</code> can’t both fit on the same line, and
you can only do so with an <code class="language-plaintext highlighter-rouge">if</code> if there is no <code class="language-plaintext highlighter-rouge">else</code>.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="k">if</span> <span class="n">foo</span><span class="p">:</span> <span class="n">bar</span><span class="p">(</span><span class="n">foo</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span>
<span class="k">if</span> <span class="n">foo</span><span class="p">:</span> <span class="n">bar</span><span class="p">(</span><span class="n">foo</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span> <span class="n">baz</span><span class="p">(</span><span class="n">foo</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span> <span class="n">bar</span><span class="p">(</span><span class="n">foo</span><span class="p">)</span>
<span class="k">except</span> <span class="nb">ValueError</span><span class="p">:</span> <span class="n">baz</span><span class="p">(</span><span class="n">foo</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">bar</span><span class="p">(</span><span class="n">foo</span><span class="p">)</span>
<span class="k">except</span> <span class="nb">ValueError</span><span class="p">:</span> <span class="n">baz</span><span class="p">(</span><span class="n">foo</span><span class="p">)</span>
</code></pre></div></div>
<p><a id="s3.15-accessors"></a>
<a id="s3.15-access-control"></a>
<a id="315-access-control"></a>
<a id="access-control"></a>
<a id="accessors"></a></p>
<p><a id="getters-and-setters"></a></p>
<h3 id="315-getters-and-setters">3.15 Getters and Setters</h3>
<p>Getter and setter functions (also called accessors and mutators) should be used
when they provide a meaningful role or behavior for getting or setting a
variable’s value.</p>
<p>In particular, they should be used when getting or setting the variable is
complex or the cost is significant, either currently or in a reasonable future.</p>
<p>If, for example, a pair of getters/setters simply read and write an internal
attribute, the internal attribute should be made public instead. By comparison,
if setting a variable means some state is invalidated or rebuilt, it should be a
setter function. The function invocation hints that a potentially non-trivial
operation is occurring. Alternatively, <a href="#properties">properties</a> may be an
option when simple logic is needed, or refactoring to no longer need getters and
setters.</p>
<p>Getters and setters should follow the <a href="#s3.16-naming">Naming</a> guidelines, such
as <code class="language-plaintext highlighter-rouge">get_foo()</code> and <code class="language-plaintext highlighter-rouge">set_foo()</code>.</p>
<p>If the past behavior allowed access through a property, do not bind the new
getter/setter functions to the property. Any code still attempting to access the
variable by the old method should break visibly so they are made aware of the
change in complexity.</p>
<p><a id="s3.16-naming"></a>
<a id="316-naming"></a></p>
<p><a id="naming"></a></p>
<h3 id="316-naming">3.16 Naming</h3>
<p><code class="language-plaintext highlighter-rouge">module_name</code>, <code class="language-plaintext highlighter-rouge">package_name</code>, <code class="language-plaintext highlighter-rouge">ClassName</code>, <code class="language-plaintext highlighter-rouge">method_name</code>, <code class="language-plaintext highlighter-rouge">ExceptionName</code>,
<code class="language-plaintext highlighter-rouge">function_name</code>, <code class="language-plaintext highlighter-rouge">GLOBAL_CONSTANT_NAME</code>, <code class="language-plaintext highlighter-rouge">global_var_name</code>, <code class="language-plaintext highlighter-rouge">instance_var_name</code>,
<code class="language-plaintext highlighter-rouge">function_parameter_name</code>, <code class="language-plaintext highlighter-rouge">local_var_name</code>, <code class="language-plaintext highlighter-rouge">query_proper_noun_for_thing</code>,
<code class="language-plaintext highlighter-rouge">send_acronym_via_https</code>.</p>
<p>Names should be descriptive. This includes functions, classes, variables,
attributes, files and any other type of named entities.</p>
<p>Avoid abbreviation. In particular, do not use abbreviations that are ambiguous
or unfamiliar to readers outside your project, and do not abbreviate by deleting
letters within a word.</p>
<p>Always use a <code class="language-plaintext highlighter-rouge">.py</code> filename extension. Never use dashes.</p>
<p><a id="s3.16.1-names-to-avoid"></a>
<a id="3161-names-to-avoid"></a></p>
<p><a id="names-to-avoid"></a></p>
<h4 id="3161-names-to-avoid">3.16.1 Names to Avoid</h4>
<ul>
<li>
<p>single character names, except for specifically allowed cases:</p>
<ul>
<li>counters or iterators (e.g. <code class="language-plaintext highlighter-rouge">i</code>, <code class="language-plaintext highlighter-rouge">j</code>, <code class="language-plaintext highlighter-rouge">k</code>, <code class="language-plaintext highlighter-rouge">v</code>, et al.)</li>
<li><code class="language-plaintext highlighter-rouge">e</code> as an exception identifier in <code class="language-plaintext highlighter-rouge">try/except</code> statements.</li>
<li><code class="language-plaintext highlighter-rouge">f</code> as a file handle in <code class="language-plaintext highlighter-rouge">with</code> statements</li>
<li>private <a href="#typing-type-var">type variables</a> with no constraints (e.g.
<code class="language-plaintext highlighter-rouge">_T = TypeVar("_T")</code>, <code class="language-plaintext highlighter-rouge">_P = ParamSpec("_P")</code>)</li>
<li>names that match established notation in a reference paper or algorithm
(see <a href="#math-notation">Mathematical Notation</a>)</li>
</ul>
<p>Please be mindful not to abuse single-character naming. Generally speaking,
descriptiveness should be proportional to the name’s scope of visibility.
For example, <code class="language-plaintext highlighter-rouge">i</code> might be a fine name for 5-line code block but within
multiple nested scopes, it is likely too vague.</p>
</li>
<li>
<p>dashes (<code class="language-plaintext highlighter-rouge">-</code>) in any package/module name</p>
</li>
<li>
<p><code class="language-plaintext highlighter-rouge">__double_leading_and_trailing_underscore__</code> names (reserved by Python)</p>
</li>
<li>
<p>offensive terms</p>
</li>
<li>
<p>names that needlessly include the type of the variable (for example:
<code class="language-plaintext highlighter-rouge">id_to_name_dict</code>)</p>
</li>
</ul>
<p><a id="s3.16.2-naming-conventions"></a>
<a id="3162-naming-convention"></a></p>
<p><a id="naming-conventions"></a></p>
<h4 id="3162-naming-conventions">3.16.2 Naming Conventions</h4>
<ul>
<li>
<p>“Internal” means internal to a module, or protected or private within a
class.</p>
</li>
<li>
<p>Prepending a single underscore (<code class="language-plaintext highlighter-rouge">_</code>) has some support for protecting module
variables and functions (linters will flag protected member access). Note
that it is okay for unit tests to access protected constants from the
modules under test.</p>
</li>
<li>
<p>Prepending a double underscore (<code class="language-plaintext highlighter-rouge">__</code> aka “dunder”) to an instance variable
or method effectively makes the variable or method private to its class
(using name mangling); we discourage its use as it impacts readability and
testability, and isn’t <em>really</em> private. Prefer a single underscore.</p>
</li>
<li>
<p>Place related classes and top-level functions together in a
module.
Unlike Java, there is no need to limit yourself to one class per module.</p>
</li>
<li>
<p>Use CapWords for class names, but lower_with_under.py for module names.
Although there are some old modules named CapWords.py, this is now
discouraged because it’s confusing when the module happens to be named after
a class. (“wait – did I write <code class="language-plaintext highlighter-rouge">import StringIO</code> or <code class="language-plaintext highlighter-rouge">from StringIO import
StringIO</code>?”)</p>
</li>
<li>
<p>New <em>unit test</em> files follow PEP 8 compliant lower_with_under method
names, for example, <code class="language-plaintext highlighter-rouge">test_&lt;method_under_test&gt;_&lt;state&gt;</code>. For consistency(*)
with legacy modules that follow CapWords function names, underscores may
appear in method names starting with <code class="language-plaintext highlighter-rouge">test</code> to separate logical components
of the name. One possible pattern is <code class="language-plaintext highlighter-rouge">test&lt;MethodUnderTest&gt;_&lt;state&gt;</code>.</p>
</li>
</ul>
<p><a id="s3.16.3-file-naming"></a>
<a id="3163-file-naming"></a></p>
<p><a id="file-naming"></a></p>
<h4 id="3163-file-naming">3.16.3 File Naming</h4>
<p>Python filenames must have a <code class="language-plaintext highlighter-rouge">.py</code> extension and must not contain dashes (<code class="language-plaintext highlighter-rouge">-</code>).
This allows them to be imported and unittested. If you want an executable to be
accessible without the extension, use a symbolic link or a simple bash wrapper
containing <code class="language-plaintext highlighter-rouge">exec "$0.py" "$@"</code>.</p>
<p><a id="s3.16.4-guidelines-derived-from-guidos-recommendations"></a>
<a id="3164-guidelines-derived-from-guidos-recommendations"></a></p>
<p><a id="guidelines-derived-from-guidos-recommendations"></a></p>
<h4 id="3164-guidelines-derived-from-guidos-recommendations">3.16.4 Guidelines derived from <a href="https://en.wikipedia.org/wiki/Guido_van_Rossum">Guido</a>’s Recommendations</h4>
<table rules="all" border="1" summary="Guidelines from Guido's Recommendations" cellspacing="2" cellpadding="2">
<tr>
<th>Type</th>
<th>Public</th>
<th>Internal</th>
</tr>
<tr>
<td>Packages</td>
<td><code>lower_with_under</code></td>
<td></td>
</tr>
<tr>
<td>Modules</td>
<td><code>lower_with_under</code></td>
<td><code>_lower_with_under</code></td>
</tr>
<tr>
<td>Classes</td>
<td><code>CapWords</code></td>
<td><code>_CapWords</code></td>
</tr>
<tr>
<td>Exceptions</td>
<td><code>CapWords</code></td>
<td></td>
</tr>
<tr>
<td>Functions</td>
<td><code>lower_with_under()</code></td>
<td><code>_lower_with_under()</code></td>
</tr>
<tr>
<td>Global/Class Constants</td>
<td><code>CAPS_WITH_UNDER</code></td>
<td><code>_CAPS_WITH_UNDER</code></td>
</tr>
<tr>
<td>Global/Class Variables</td>
<td><code>lower_with_under</code></td>
<td><code>_lower_with_under</code></td>
</tr>
<tr>
<td>Instance Variables</td>
<td><code>lower_with_under</code></td>
<td><code>_lower_with_under</code> (protected)</td>
</tr>
<tr>
<td>Method Names</td>
<td><code>lower_with_under()</code></td>
<td><code>_lower_with_under()</code> (protected)</td>
</tr>
<tr>
<td>Function/Method Parameters</td>
<td><code>lower_with_under</code></td>
<td></td>
</tr>
<tr>
<td>Local Variables</td>
<td><code>lower_with_under</code></td>
<td></td>
</tr>
</table>
<p><a id="s3.17-main"></a>
<a id="317-main"></a></p>
<p><a id="math-notation"></a></p>
<h4 id="3165-mathematical-notation">3.16.5 Mathematical Notation</h4>
<p>For mathematically-heavy code, short variable names that would otherwise violate
the style guide are preferred when they match established notation in a
reference paper or algorithm.</p>
<p>When using names based on established notation:</p>
<ol>
<li>Cite the source of all naming conventions, preferably with a hyperlink to
academic resource itself, in a comment or docstring. If the source is not
accessible, clearly document the naming conventions.</li>
<li>Prefer PEP8-compliant <code class="language-plaintext highlighter-rouge">descriptive_names</code> for public APIs, which are much
more likely to be encountered out of context.</li>
<li>Use a narrowly-scoped <code class="language-plaintext highlighter-rouge">pylint: disable=invalid-name</code> directive to silence
warnings. For just a few variables, use the directive as an endline comment
for each one; for more, apply the directive at the beginning of a block.</li>
</ol>
<p><a id="main"></a></p>
<h3 id="317-main">3.17 Main</h3>
<p>In Python, <code class="language-plaintext highlighter-rouge">pydoc</code> as well as unit tests require modules to be importable. If a
file is meant to be used as an executable, its main functionality should be in a
<code class="language-plaintext highlighter-rouge">main()</code> function, and your code should always check <code class="language-plaintext highlighter-rouge">if __name__ == '__main__'</code>
before executing your main program, so that it is not executed when the module
is imported.</p>
<p>When using <a href="https://github.com/abseil/abseil-py">absl</a>, use <code class="language-plaintext highlighter-rouge">app.run</code>:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">absl</span> <span class="kn">import</span> <span class="n">app</span>
<span class="p">...</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">(</span><span class="n">argv</span><span class="p">:</span> <span class="n">Sequence</span><span class="p">[</span><span class="nb">str</span><span class="p">]):</span>
<span class="c1"># process non-flag arguments
</span> <span class="p">...</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s">'__main__'</span><span class="p">:</span>
<span class="n">app</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">)</span>
</code></pre></div></div>
<p>Otherwise, use:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="p">...</span>
<span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s">'__main__'</span><span class="p">:</span>
<span class="n">main</span><span class="p">()</span>
</code></pre></div></div>
<p>All code at the top level will be executed when the module is imported. Be
careful not to call functions, create objects, or perform other operations that
should not be executed when the file is being <code class="language-plaintext highlighter-rouge">pydoc</code>ed.</p>
<p><a id="s3.18-function-length"></a>
<a id="318-function-length"></a></p>
<p><a id="function-length"></a></p>
<h3 id="318-function-length">3.18 Function length</h3>
<p>Prefer small and focused functions.</p>
<p>We recognize that long functions are sometimes appropriate, so no hard limit is
placed on function length. If a function exceeds about 40 lines, think about
whether it can be broken up without harming the structure of the program.</p>
<p>Even if your long function works perfectly now, someone modifying it in a few
months may add new behavior. This could result in bugs that are hard to find.
Keeping your functions short and simple makes it easier for other people to read
and modify your code.</p>
<p>You could find long and complicated functions when working with
some
code. Do not be intimidated by modifying existing code: if working with such a
function proves to be difficult, you find that errors are hard to debug, or you
want to use a piece of it in several different contexts, consider breaking up
the function into smaller and more manageable pieces.</p>
<p><a id="s3.19-type-annotations"></a>
<a id="319-type-annotations"></a></p>
<p><a id="type-annotations"></a></p>
<h3 id="319-type-annotations">3.19 Type Annotations</h3>
<p><a id="s3.19.1-general-rules"></a>
<a id="s3.19.1-general"></a>
<a id="3191-general-rules"></a></p>
<p><a id="typing-general"></a></p>
<h4 id="3191-general-rules">3.19.1 General Rules</h4>
<ul>
<li>
<p>Familiarize yourself with
<a href="https://docs.python.org/3/library/typing.html">type hints</a>.</p>
</li>
<li>
<p>Annotating <code class="language-plaintext highlighter-rouge">self</code> or <code class="language-plaintext highlighter-rouge">cls</code> is generally not necessary.
<a href="https://docs.python.org/3/library/typing.html#typing.Self"><code class="language-plaintext highlighter-rouge">Self</code></a> can be
used if it is necessary for proper type information, e.g.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">typing</span> <span class="kn">import</span> <span class="n">Self</span>
<span class="k">class</span> <span class="nc">BaseClass</span><span class="p">:</span>
<span class="o">@</span><span class="nb">classmethod</span>
<span class="k">def</span> <span class="nf">create</span><span class="p">(</span><span class="n">cls</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">Self</span><span class="p">:</span>
<span class="p">...</span>
<span class="k">def</span> <span class="nf">difference</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">:</span> <span class="n">Self</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">float</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div> </div>
</li>
<li>
<p>Similarly, don’t feel compelled to annotate the return value of <code class="language-plaintext highlighter-rouge">__init__</code>
(where <code class="language-plaintext highlighter-rouge">None</code> is the only valid option).</p>
</li>
<li>
<p>If any other variable or a returned type should not be expressed, use <code class="language-plaintext highlighter-rouge">Any</code>.</p>
</li>
<li>
<p>You are not required to annotate all the functions in a module.</p>
<ul>
<li>At least annotate your public APIs.</li>
<li>Use judgment to get to a good balance between safety and clarity on the
one hand, and flexibility on the other.</li>
<li>Annotate code that is prone to type-related errors (previous bugs or
complexity).</li>
<li>Annotate code that is hard to understand.</li>
<li>Annotate code as it becomes stable from a types perspective. In many
cases, you can annotate all the functions in mature code without losing
too much flexibility.</li>
</ul>
</li>
</ul>
<p><a id="s3.19.2-line-breaking"></a>
<a id="3192-line-breaking"></a></p>
<p><a id="typing-line-breaking"></a></p>
<h4 id="3192-line-breaking">3.19.2 Line Breaking</h4>
<p>Try to follow the existing <a href="#indentation">indentation</a> rules.</p>
<p>After annotating, many function signatures will become “one parameter per line”.
To ensure the return type is also given its own line, a comma can be placed
after the last parameter.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">def</span> <span class="nf">my_method</span><span class="p">(</span>
<span class="bp">self</span><span class="p">,</span>
<span class="n">first_var</span><span class="p">:</span> <span class="nb">int</span><span class="p">,</span>
<span class="n">second_var</span><span class="p">:</span> <span class="n">Foo</span><span class="p">,</span>
<span class="n">third_var</span><span class="p">:</span> <span class="n">Bar</span> <span class="o">|</span> <span class="bp">None</span><span class="p">,</span>
<span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">int</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div></div>
<p>Always prefer breaking between variables, and not, for example, between variable
names and type annotations. However, if everything fits on the same line, go for
it.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">def</span> <span class="nf">my_method</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">first_var</span><span class="p">:</span> <span class="nb">int</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">int</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div></div>
<p>If the combination of the function name, the last parameter, and the return type
is too long, indent by 4 in a new line. When using line breaks, prefer putting
each parameter and the return type on their own lines and aligning the closing
parenthesis with the <code class="language-plaintext highlighter-rouge">def</code>:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">my_method</span><span class="p">(</span>
<span class="bp">self</span><span class="p">,</span>
<span class="n">other_arg</span><span class="p">:</span> <span class="n">MyLongType</span> <span class="o">|</span> <span class="bp">None</span><span class="p">,</span>
<span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">tuple</span><span class="p">[</span><span class="n">MyLongType1</span><span class="p">,</span> <span class="n">MyLongType1</span><span class="p">]:</span>
<span class="p">...</span>
</code></pre></div></div>
<p>Optionally, the return type may be put on the same line as the last parameter:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Okay</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">my_method</span><span class="p">(</span>
<span class="bp">self</span><span class="p">,</span>
<span class="n">first_var</span><span class="p">:</span> <span class="nb">int</span><span class="p">,</span>
<span class="n">second_var</span><span class="p">:</span> <span class="nb">int</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">dict</span><span class="p">[</span><span class="n">OtherLongType</span><span class="p">,</span> <span class="n">MyLongType</span><span class="p">]:</span>
<span class="p">...</span>
</code></pre></div></div>
<p><code class="language-plaintext highlighter-rouge">pylint</code>
allows you to move the closing parenthesis to a new line and align with the
opening one, but this is less readable.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">my_method</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span>
<span class="n">other_arg</span><span class="p">:</span> <span class="n">MyLongType</span> <span class="o">|</span> <span class="bp">None</span><span class="p">,</span>
<span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">dict</span><span class="p">[</span><span class="n">OtherLongType</span><span class="p">,</span> <span class="n">MyLongType</span><span class="p">]:</span>
<span class="p">...</span>
</code></pre></div></div>
<p>As in the examples above, prefer not to break types. However, sometimes they are
too long to be on a single line (try to keep sub-types unbroken).</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">def</span> <span class="nf">my_method</span><span class="p">(</span>
<span class="bp">self</span><span class="p">,</span>
<span class="n">first_var</span><span class="p">:</span> <span class="nb">tuple</span><span class="p">[</span><span class="nb">list</span><span class="p">[</span><span class="n">MyLongType1</span><span class="p">],</span>
<span class="nb">list</span><span class="p">[</span><span class="n">MyLongType2</span><span class="p">]],</span>
<span class="n">second_var</span><span class="p">:</span> <span class="nb">list</span><span class="p">[</span><span class="nb">dict</span><span class="p">[</span>
<span class="n">MyLongType3</span><span class="p">,</span> <span class="n">MyLongType4</span><span class="p">]],</span>
<span class="p">)</span> <span class="o">-&gt;</span> <span class="bp">None</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div></div>
<p>If a single name and type is too long, consider using an
<a href="#typing-aliases">alias</a> for the type. The last resort is to break after the
colon and indent by 4.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">my_function</span><span class="p">(</span>
<span class="n">long_variable_name</span><span class="p">:</span>
<span class="n">long_module_name</span><span class="p">.</span><span class="n">LongTypeName</span><span class="p">,</span>
<span class="p">)</span> <span class="o">-&gt;</span> <span class="bp">None</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">my_function</span><span class="p">(</span>
<span class="n">long_variable_name</span><span class="p">:</span> <span class="n">long_module_name</span><span class="p">.</span>
<span class="n">LongTypeName</span><span class="p">,</span>
<span class="p">)</span> <span class="o">-&gt;</span> <span class="bp">None</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div></div>
<p><a id="s3.19.3-forward-declarations"></a>
<a id="3193-forward-declarations"></a></p>
<p><a id="forward-declarations"></a></p>
<h4 id="3193-forward-declarations">3.19.3 Forward Declarations</h4>
<p>If you need to use a class name (from the same module) that is not yet
defined – for example, if you need the class name inside the declaration of
that class, or if you use a class that is defined later in the code – either
use <code class="language-plaintext highlighter-rouge">from __future__ import annotations</code> or use a string for the class name.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="kn">from</span> <span class="nn">__future__</span> <span class="kn">import</span> <span class="n">annotations</span>
<span class="k">class</span> <span class="nc">MyClass</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">stack</span><span class="p">:</span> <span class="n">Sequence</span><span class="p">[</span><span class="n">MyClass</span><span class="p">],</span> <span class="n">item</span><span class="p">:</span> <span class="n">OtherClass</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="bp">None</span><span class="p">:</span>
<span class="k">class</span> <span class="nc">OtherClass</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="k">class</span> <span class="nc">MyClass</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">stack</span><span class="p">:</span> <span class="n">Sequence</span><span class="p">[</span><span class="s">'MyClass'</span><span class="p">],</span> <span class="n">item</span><span class="p">:</span> <span class="s">'OtherClass'</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="bp">None</span><span class="p">:</span>
<span class="k">class</span> <span class="nc">OtherClass</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div></div>
<p><a id="s3.19.4-default-values"></a>
<a id="3194-default-values"></a></p>
<p><a id="typing-default-values"></a></p>
<h4 id="3194-default-values">3.19.4 Default Values</h4>
<p>As per <a href="https://peps.python.org/pep-0008/#other-recommendations">PEP-008</a>, use
spaces around the <code class="language-plaintext highlighter-rouge">=</code> <em>only</em> for arguments that have both a type annotation and
a default value.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="n">a</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">0</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">int</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="n">a</span><span class="p">:</span><span class="nb">int</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">int</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div></div>
<p><a id="s3.19.5-nonetype"></a>
<a id="s3.19.5-none-type"></a>
<a id="3195-nonetype"></a></p>
<p><a id="none-type"></a></p>
<h4 id="3195-nonetype">3.19.5 NoneType</h4>
<p>In the Python type system, <code class="language-plaintext highlighter-rouge">NoneType</code> is a “first class” type, and for typing
purposes, <code class="language-plaintext highlighter-rouge">None</code> is an alias for <code class="language-plaintext highlighter-rouge">NoneType</code>. If an argument can be <code class="language-plaintext highlighter-rouge">None</code>, it
has to be declared! You can use <code class="language-plaintext highlighter-rouge">|</code> union type expressions (recommended in new
Python 3.10+ code), or the older <code class="language-plaintext highlighter-rouge">Optional</code> and <code class="language-plaintext highlighter-rouge">Union</code> syntaxes.</p>
<p>Use explicit <code class="language-plaintext highlighter-rouge">X | None</code> instead of implicit. Earlier versions of type checkers
allowed <code class="language-plaintext highlighter-rouge">a: str = None</code> to be interpreted as <code class="language-plaintext highlighter-rouge">a: str | None = None</code>, but that is
no longer the preferred behavior.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">modern_or_union</span><span class="p">(</span><span class="n">a</span><span class="p">:</span> <span class="nb">str</span> <span class="o">|</span> <span class="nb">int</span> <span class="o">|</span> <span class="bp">None</span><span class="p">,</span> <span class="n">b</span><span class="p">:</span> <span class="nb">str</span> <span class="o">|</span> <span class="bp">None</span> <span class="o">=</span> <span class="bp">None</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">str</span><span class="p">:</span>
<span class="p">...</span>
<span class="k">def</span> <span class="nf">union_optional</span><span class="p">(</span><span class="n">a</span><span class="p">:</span> <span class="n">Union</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="nb">int</span><span class="p">,</span> <span class="bp">None</span><span class="p">],</span> <span class="n">b</span><span class="p">:</span> <span class="n">Optional</span><span class="p">[</span><span class="nb">str</span><span class="p">]</span> <span class="o">=</span> <span class="bp">None</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">str</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">nullable_union</span><span class="p">(</span><span class="n">a</span><span class="p">:</span> <span class="n">Union</span><span class="p">[</span><span class="bp">None</span><span class="p">,</span> <span class="nb">str</span><span class="p">])</span> <span class="o">-&gt;</span> <span class="nb">str</span><span class="p">:</span>
<span class="p">...</span>
<span class="k">def</span> <span class="nf">implicit_optional</span><span class="p">(</span><span class="n">a</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="bp">None</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">str</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div></div>
<p><a id="s3.19.6-type-aliases"></a>
<a id="s3.19.6-aliases"></a>
<a id="3196-type-aliases"></a>
<a id="typing-aliases"></a></p>
<p><a id="type-aliases"></a></p>
<h4 id="3196-type-aliases">3.19.6 Type Aliases</h4>
<p>You can declare aliases of complex types. The name of an alias should be
CapWorded. If the alias is used only in this module, it should be _Private.</p>
<p>Note that the <code class="language-plaintext highlighter-rouge">: TypeAlias</code> annotation is only supported in versions 3.10+.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">typing</span> <span class="kn">import</span> <span class="n">TypeAlias</span>
<span class="n">_LossAndGradient</span><span class="p">:</span> <span class="n">TypeAlias</span> <span class="o">=</span> <span class="nb">tuple</span><span class="p">[</span><span class="n">tf</span><span class="p">.</span><span class="n">Tensor</span><span class="p">,</span> <span class="n">tf</span><span class="p">.</span><span class="n">Tensor</span><span class="p">]</span>
<span class="n">ComplexTFMap</span><span class="p">:</span> <span class="n">TypeAlias</span> <span class="o">=</span> <span class="n">Mapping</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="n">_LossAndGradient</span><span class="p">]</span>
</code></pre></div></div>
<p><a id="s3.19.7-ignoring-types"></a>
<a id="s3.19.7-ignore"></a>
<a id="3197-ignoring-types"></a></p>
<p><a id="typing-ignore"></a></p>
<h4 id="3197-ignoring-types">3.19.7 Ignoring Types</h4>
<p>You can disable type checking on a line with the special comment <code class="language-plaintext highlighter-rouge"># type:
ignore</code>.</p>
<p><code class="language-plaintext highlighter-rouge">pytype</code> has a disable option for specific errors (similar to lint):</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># pytype: disable=attribute-error
</span></code></pre></div></div>
<p><a id="s3.19.8-typing-variables"></a>
<a id="s3.19.8-comments"></a>
<a id="3198-typing-internal-variables"></a></p>
<p><a id="typing-variables"></a></p>
<h4 id="3198-typing-variables">3.19.8 Typing Variables</h4>
<dl>
<dt><a id="annotated-assignments"></a></dt>
<dt><a href="#annotated-assignments"><em>Annotated Assignments</em></a></dt>
<dd>If an internal variable has a type that is hard or impossible to infer,
specify its type with an annotated assignment - use a colon and type between
the variable name and value (the same as is done with function arguments
that have a default value):
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">a</span><span class="p">:</span> <span class="n">Foo</span> <span class="o">=</span> <span class="n">SomeUndecoratedFunction</span><span class="p">()</span>
</code></pre></div> </div>
</dd>
<dt><a id="type-comments"></a></dt>
<dt><a href="#type-comments"><em>Type Comments</em></a></dt>
<dd>Though you may see them remaining in the codebase (they were necessary
before Python 3.6), do not add any more uses of a <code class="language-plaintext highlighter-rouge"># type: &lt;type name&gt;</code>
comment on the end of the line:
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">a</span> <span class="o">=</span> <span class="n">SomeUndecoratedFunction</span><span class="p">()</span> <span class="c1"># type: Foo
</span></code></pre></div> </div>
</dd>
</dl>
<p><a id="s3.19.9-tuples-vs-lists"></a>
<a id="s3.19.9-tuples"></a>
<a id="3199-tuples-vs-lists"></a></p>
<p><a id="typing-tuples"></a></p>
<h4 id="3199-tuples-vs-lists">3.19.9 Tuples vs Lists</h4>
<p>Typed lists can only contain objects of a single type. Typed tuples can either
have a single repeated type or a set number of elements with different types.
The latter is commonly used as the return type from a function.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">a</span><span class="p">:</span> <span class="nb">list</span><span class="p">[</span><span class="nb">int</span><span class="p">]</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span>
<span class="n">b</span><span class="p">:</span> <span class="nb">tuple</span><span class="p">[</span><span class="nb">int</span><span class="p">,</span> <span class="p">...]</span> <span class="o">=</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>
<span class="n">c</span><span class="p">:</span> <span class="nb">tuple</span><span class="p">[</span><span class="nb">int</span><span class="p">,</span> <span class="nb">str</span><span class="p">,</span> <span class="nb">float</span><span class="p">]</span> <span class="o">=</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="s">"2"</span><span class="p">,</span> <span class="mf">3.5</span><span class="p">)</span>
</code></pre></div></div>
<p><a id="s3.19.10-typevars"></a>
<a id="s3.19.10-type-var"></a>
<a id="31910-typevar"></a>
<a id="typing-type-var"></a></p>
<p><a id="typevars"></a></p>
<h4 id="31910-type-variables">3.19.10 Type variables</h4>
<p>The Python type system has
<a href="https://docs.python.org/3/library/typing.html#generics">generics</a>. A type
variable, such as <code class="language-plaintext highlighter-rouge">TypeVar</code> and <code class="language-plaintext highlighter-rouge">ParamSpec</code>, is a common way to use them.</p>
<p>Example:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">collections.abc</span> <span class="kn">import</span> <span class="n">Callable</span>
<span class="kn">from</span> <span class="nn">typing</span> <span class="kn">import</span> <span class="n">ParamSpec</span><span class="p">,</span> <span class="n">TypeVar</span>
<span class="n">_P</span> <span class="o">=</span> <span class="n">ParamSpec</span><span class="p">(</span><span class="s">"_P"</span><span class="p">)</span>
<span class="n">_T</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s">"_T"</span><span class="p">)</span>
<span class="p">...</span>
<span class="k">def</span> <span class="nf">next</span><span class="p">(</span><span class="n">l</span><span class="p">:</span> <span class="nb">list</span><span class="p">[</span><span class="n">_T</span><span class="p">])</span> <span class="o">-&gt;</span> <span class="n">_T</span><span class="p">:</span>
<span class="k">return</span> <span class="n">l</span><span class="p">.</span><span class="n">pop</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">print_when_called</span><span class="p">(</span><span class="n">f</span><span class="p">:</span> <span class="n">Callable</span><span class="p">[</span><span class="n">_P</span><span class="p">,</span> <span class="n">_T</span><span class="p">])</span> <span class="o">-&gt;</span> <span class="n">Callable</span><span class="p">[</span><span class="n">_P</span><span class="p">,</span> <span class="n">_T</span><span class="p">]:</span>
<span class="k">def</span> <span class="nf">inner</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">:</span> <span class="n">_P</span><span class="p">.</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">:</span> <span class="n">_P</span><span class="p">.</span><span class="n">kwargs</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">_T</span><span class="p">:</span>
<span class="k">print</span><span class="p">(</span><span class="s">"Function was called"</span><span class="p">)</span>
<span class="k">return</span> <span class="n">f</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
<span class="k">return</span> <span class="n">inner</span>
</code></pre></div></div>
<p>A <code class="language-plaintext highlighter-rouge">TypeVar</code> can be constrained:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">AddableType</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s">"AddableType"</span><span class="p">,</span> <span class="nb">int</span><span class="p">,</span> <span class="nb">float</span><span class="p">,</span> <span class="nb">str</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">add</span><span class="p">(</span><span class="n">a</span><span class="p">:</span> <span class="n">AddableType</span><span class="p">,</span> <span class="n">b</span><span class="p">:</span> <span class="n">AddableType</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">AddableType</span><span class="p">:</span>
<span class="k">return</span> <span class="n">a</span> <span class="o">+</span> <span class="n">b</span>
</code></pre></div></div>
<p>A common predefined type variable in the <code class="language-plaintext highlighter-rouge">typing</code> module is <code class="language-plaintext highlighter-rouge">AnyStr</code>. Use it for
multiple annotations that can be <code class="language-plaintext highlighter-rouge">bytes</code> or <code class="language-plaintext highlighter-rouge">str</code> and must all be the same type.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">typing</span> <span class="kn">import</span> <span class="n">AnyStr</span>
<span class="k">def</span> <span class="nf">check_length</span><span class="p">(</span><span class="n">x</span><span class="p">:</span> <span class="n">AnyStr</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">AnyStr</span><span class="p">:</span>
<span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">&lt;=</span> <span class="mi">42</span><span class="p">:</span>
<span class="k">return</span> <span class="n">x</span>
<span class="k">raise</span> <span class="nb">ValueError</span><span class="p">()</span>
</code></pre></div></div>
<p>A type variable must have a descriptive name, unless it meets all of the
following criteria:</p>
<ul>
<li>not externally visible</li>
<li>not constrained</li>
</ul>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">Yes</span><span class="p">:</span>
<span class="n">_T</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s">"_T"</span><span class="p">)</span>
<span class="n">_P</span> <span class="o">=</span> <span class="n">ParamSpec</span><span class="p">(</span><span class="s">"_P"</span><span class="p">)</span>
<span class="n">AddableType</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s">"AddableType"</span><span class="p">,</span> <span class="nb">int</span><span class="p">,</span> <span class="nb">float</span><span class="p">,</span> <span class="nb">str</span><span class="p">)</span>
<span class="n">AnyFunction</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s">"AnyFunction"</span><span class="p">,</span> <span class="n">bound</span><span class="o">=</span><span class="n">Callable</span><span class="p">)</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">No</span><span class="p">:</span>
<span class="n">T</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s">"T"</span><span class="p">)</span>
<span class="n">P</span> <span class="o">=</span> <span class="n">ParamSpec</span><span class="p">(</span><span class="s">"P"</span><span class="p">)</span>
<span class="n">_T</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s">"_T"</span><span class="p">,</span> <span class="nb">int</span><span class="p">,</span> <span class="nb">float</span><span class="p">,</span> <span class="nb">str</span><span class="p">)</span>
<span class="n">_F</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s">"_F"</span><span class="p">,</span> <span class="n">bound</span><span class="o">=</span><span class="n">Callable</span><span class="p">)</span>
</code></pre></div></div>
<p><a id="s3.19.11-string-types"></a>
<a id="s3.19.11-strings"></a>
<a id="31911-string-types"></a></p>
<p><a id="typing-strings"></a></p>
<h4 id="31911-string-types">3.19.11 String types</h4>
<blockquote>
<p>Do not use <code class="language-plaintext highlighter-rouge">typing.Text</code> in new code. It’s only for Python 2/3 compatibility.</p>
</blockquote>
<p>Use <code class="language-plaintext highlighter-rouge">str</code> for string/text data. For code that deals with binary data, use
<code class="language-plaintext highlighter-rouge">bytes</code>.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">def</span> <span class="nf">deals_with_text_data</span><span class="p">(</span><span class="n">x</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">str</span><span class="p">:</span>
<span class="p">...</span>
<span class="k">def</span> <span class="nf">deals_with_binary_data</span><span class="p">(</span><span class="n">x</span><span class="p">:</span> <span class="nb">bytes</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">bytes</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div></div>
<p>If all the string types of a function are always the same, for example if the
return type is the same as the argument type in the code above, use
<a href="#typing-type-var">AnyStr</a>.</p>
<p><a id="s3.19.12-imports-for-typing"></a>
<a id="s3.19.12-imports"></a>
<a id="31912-imports-for-typing"></a></p>
<p><a id="typing-imports"></a></p>
<h4 id="31912-imports-for-typing">3.19.12 Imports For Typing</h4>
<p>For symbols (including types, functions, and constants) from the <code class="language-plaintext highlighter-rouge">typing</code> or
<code class="language-plaintext highlighter-rouge">collections.abc</code> modules used to support static analysis and type checking,
always import the symbol itself. This keeps common annotations more concise and
matches typing practices used around the world. You are explicitly allowed to
import multiple specific symbols on one line from the <code class="language-plaintext highlighter-rouge">typing</code> and
<code class="language-plaintext highlighter-rouge">collections.abc</code> modules. For example:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">collections.abc</span> <span class="kn">import</span> <span class="n">Mapping</span><span class="p">,</span> <span class="n">Sequence</span>
<span class="kn">from</span> <span class="nn">typing</span> <span class="kn">import</span> <span class="n">Any</span><span class="p">,</span> <span class="n">Generic</span><span class="p">,</span> <span class="n">cast</span><span class="p">,</span> <span class="n">TYPE_CHECKING</span>
</code></pre></div></div>
<p>Given that this way of importing adds items to the local namespace, names in
<code class="language-plaintext highlighter-rouge">typing</code> or <code class="language-plaintext highlighter-rouge">collections.abc</code> should be treated similarly to keywords, and not
be defined in your Python code, typed or not. If there is a collision between a
type and an existing name in a module, import it using <code class="language-plaintext highlighter-rouge">import x as y</code>.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">typing</span> <span class="kn">import</span> <span class="n">Any</span> <span class="k">as</span> <span class="n">AnyType</span>
</code></pre></div></div>
<p>When annotating function signatures, prefer abstract container types like
<code class="language-plaintext highlighter-rouge">collections.abc.Sequence</code> over concrete types like <code class="language-plaintext highlighter-rouge">list</code>. If you need to use a
concrete type (for example, a <code class="language-plaintext highlighter-rouge">tuple</code> of typed elements), prefer built-in types
like <code class="language-plaintext highlighter-rouge">tuple</code> over the parametric type aliases from the <code class="language-plaintext highlighter-rouge">typing</code> module (e.g.,
<code class="language-plaintext highlighter-rouge">typing.Tuple</code>).</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">typing</span> <span class="kn">import</span> <span class="n">List</span><span class="p">,</span> <span class="n">Tuple</span>
<span class="k">def</span> <span class="nf">transform_coordinates</span><span class="p">(</span><span class="n">original</span><span class="p">:</span> <span class="n">List</span><span class="p">[</span><span class="n">Tuple</span><span class="p">[</span><span class="nb">float</span><span class="p">,</span> <span class="nb">float</span><span class="p">]])</span> <span class="o">-&gt;</span>
<span class="n">List</span><span class="p">[</span><span class="n">Tuple</span><span class="p">[</span><span class="nb">float</span><span class="p">,</span> <span class="nb">float</span><span class="p">]]:</span>
<span class="p">...</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">collections.abc</span> <span class="kn">import</span> <span class="n">Sequence</span>
<span class="k">def</span> <span class="nf">transform_coordinates</span><span class="p">(</span><span class="n">original</span><span class="p">:</span> <span class="n">Sequence</span><span class="p">[</span><span class="nb">tuple</span><span class="p">[</span><span class="nb">float</span><span class="p">,</span> <span class="nb">float</span><span class="p">]])</span> <span class="o">-&gt;</span>
<span class="n">Sequence</span><span class="p">[</span><span class="nb">tuple</span><span class="p">[</span><span class="nb">float</span><span class="p">,</span> <span class="nb">float</span><span class="p">]]:</span>
<span class="p">...</span>
</code></pre></div></div>
<p><a id="s3.19.13-conditional-imports"></a>
<a id="31913-conditional-imports"></a></p>
<p><a id="typing-conditional-imports"></a></p>
<h4 id="31913-conditional-imports">3.19.13 Conditional Imports</h4>
<p>Use conditional imports only in exceptional cases where the additional imports
needed for type checking must be avoided at runtime. This pattern is
discouraged; alternatives such as refactoring the code to allow top-level
imports should be preferred.</p>
<p>Imports that are needed only for type annotations can be placed within an <code class="language-plaintext highlighter-rouge">if
TYPE_CHECKING:</code> block.</p>
<ul>
<li>Conditionally imported types need to be referenced as strings, to be forward
compatible with Python 3.6 where the annotation expressions are actually
evaluated.</li>
<li>Only entities that are used solely for typing should be defined here; this
includes aliases. Otherwise it will be a runtime error, as the module will
not be imported at runtime.</li>
<li>The block should be right after all the normal imports.</li>
<li>There should be no empty lines in the typing imports list.</li>
<li>Sort this list as if it were a regular imports list.
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">import</span> <span class="nn">typing</span>
<span class="k">if</span> <span class="n">typing</span><span class="p">.</span><span class="n">TYPE_CHECKING</span><span class="p">:</span>
<span class="kn">import</span> <span class="nn">sketch</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">:</span> <span class="s">"sketch.Sketch"</span><span class="p">):</span> <span class="p">...</span>
</code></pre></div> </div>
</li>
</ul>
<p><a id="s3.19.14-circular-dependencies"></a>
<a id="s3.19.14-circular-deps"></a>
<a id="31914-circular-dependencies"></a></p>
<p><a id="typing-circular-deps"></a></p>
<h4 id="31914-circular-dependencies">3.19.14 Circular Dependencies</h4>
<p>Circular dependencies that are caused by typing are code smells. Such code is a
good candidate for refactoring. Although technically it is possible to keep
circular dependencies, various build systems will not let you do so
because each module has to depend on the other.</p>
<p>Replace modules that create circular dependency imports with <code class="language-plaintext highlighter-rouge">Any</code>. Set an
<a href="#typing-aliases">alias</a> with a meaningful name, and use the real type name from
this module (any attribute of <code class="language-plaintext highlighter-rouge">Any</code> is <code class="language-plaintext highlighter-rouge">Any</code>). Alias definitions should be
separated from the last import by one line.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">from</span> <span class="nn">typing</span> <span class="kn">import</span> <span class="n">Any</span>
<span class="n">some_mod</span> <span class="o">=</span> <span class="n">Any</span> <span class="c1"># some_mod.py imports this module.
</span><span class="p">...</span>
<span class="k">def</span> <span class="nf">my_method</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">var</span><span class="p">:</span> <span class="s">"some_mod.SomeType"</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="bp">None</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div></div>
<p><a id="typing-generics"></a>
<a id="s3.19.15-generics"></a>
<a id="31915-generics"></a></p>
<p><a id="generics"></a></p>
<h4 id="31915-generics">3.19.15 Generics</h4>
<p>When annotating, prefer to specify type parameters for
<a href="https://docs.python.org/3/library/typing.html#generics">generic</a> types in a
parameter list; otherwise, the generics’ parameters will be assumed to be
<a href="https://docs.python.org/3/library/typing.html#the-any-type"><code class="language-plaintext highlighter-rouge">Any</code></a>.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># Yes:
</span><span class="k">def</span> <span class="nf">get_names</span><span class="p">(</span><span class="n">employee_ids</span><span class="p">:</span> <span class="n">Sequence</span><span class="p">[</span><span class="nb">int</span><span class="p">])</span> <span class="o">-&gt;</span> <span class="n">Mapping</span><span class="p">[</span><span class="nb">int</span><span class="p">,</span> <span class="nb">str</span><span class="p">]:</span>
<span class="p">...</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># No:
# This is interpreted as get_names(employee_ids: Sequence[Any]) -&gt; Mapping[Any, Any]
</span><span class="k">def</span> <span class="nf">get_names</span><span class="p">(</span><span class="n">employee_ids</span><span class="p">:</span> <span class="n">Sequence</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">Mapping</span><span class="p">:</span>
<span class="p">...</span>
</code></pre></div></div>
<p>If the best type parameter for a generic is <code class="language-plaintext highlighter-rouge">Any</code>, make it explicit, but
remember that in many cases <a href="#typing-type-var"><code class="language-plaintext highlighter-rouge">TypeVar</code></a> might be more
appropriate:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># No:
</span><span class="k">def</span> <span class="nf">get_names</span><span class="p">(</span><span class="n">employee_ids</span><span class="p">:</span> <span class="n">Sequence</span><span class="p">[</span><span class="n">Any</span><span class="p">])</span> <span class="o">-&gt;</span> <span class="n">Mapping</span><span class="p">[</span><span class="n">Any</span><span class="p">,</span> <span class="nb">str</span><span class="p">]:</span>
<span class="s">"""Returns a mapping from employee ID to employee name for given IDs."""</span>
</code></pre></div></div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># Yes:
</span><span class="n">_T</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s">'_T'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">get_names</span><span class="p">(</span><span class="n">employee_ids</span><span class="p">:</span> <span class="n">Sequence</span><span class="p">[</span><span class="n">_T</span><span class="p">])</span> <span class="o">-&gt;</span> <span class="n">Mapping</span><span class="p">[</span><span class="n">_T</span><span class="p">,</span> <span class="nb">str</span><span class="p">]:</span>
<span class="s">"""Returns a mapping from employee ID to employee name for given IDs."""</span>
</code></pre></div></div>
<p><a id="4-parting-words"></a></p>
<p><a id="consistency"></a></p>
<h2 id="4-parting-words">4 Parting Words</h2>
<p><em>BE CONSISTENT</em>.</p>
<p>If you’re editing code, take a few minutes to look at the code around you and
determine its style. If they use <code class="language-plaintext highlighter-rouge">_idx</code> suffixes in index variable names, you
should too. If their comments have little boxes of hash marks around them, make
your comments have little boxes of hash marks around them too.</p>
<p>The point of having style guidelines is to have a common vocabulary of coding so
people can concentrate on what you’re saying rather than on how you’re saying
it. We present global style rules here so people know the vocabulary, but local
style is also important. If code you add to a file looks drastically different
from the existing code around it, it throws readers out of their rhythm when
they go to read it.</p>
<p>However, there are limits to consistency. It applies more heavily locally and on
choices unspecified by the global style. Consistency should not generally be
used as a justification to do things in an old style without considering the
benefits of the new style, or the tendency of the codebase to converge on newer
styles over time.</p>
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