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<h1>Python Enhancement Proposals</h1>
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<h1 class="page-title">PEP 486 – Make the Python Launcher aware of virtual environments</h1>
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<dl class="rfc2822 field-list simple">
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<dt class="field-odd">Author<span class="colon">:</span></dt>
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<dd class="field-odd">Paul Moore <p.f.moore at gmail.com></dd>
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<dt class="field-even">Status<span class="colon">:</span></dt>
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<dd class="field-even"><abbr title="Accepted and implementation complete, or no longer active">Final</abbr></dd>
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<dt class="field-odd">Type<span class="colon">:</span></dt>
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<dd class="field-odd"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd>
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<dt class="field-even">Created<span class="colon">:</span></dt>
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<dd class="field-even">12-Feb-2015</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">3.5</dd>
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<dt class="field-even">Post-History<span class="colon">:</span></dt>
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<dd class="field-even">12-Feb-2015</dd>
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<dt class="field-odd">Resolution<span class="colon">:</span></dt>
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<dd class="field-odd"><a class="reference external" href="https://mail.python.org/pipermail/python-dev/2015-February/138579.html">Python-Dev message</a></dd>
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<section id="contents">
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<details><summary>Table of Contents</summary><ul class="simple">
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<li><a class="reference internal" href="#abstract">Abstract</a></li>
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<li><a class="reference internal" href="#rationale">Rationale</a></li>
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<li><a class="reference internal" href="#implementation">Implementation</a></li>
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<li><a class="reference internal" href="#impact-on-script-launching">Impact on Script Launching</a></li>
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<li><a class="reference internal" href="#exclusions">Exclusions</a></li>
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<li><a class="reference internal" href="#reference-implementation">Reference Implementation</a></li>
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<li><a class="reference internal" href="#references">References</a></li>
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<li><a class="reference internal" href="#copyright">Copyright</a></li>
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</ul>
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</details></section>
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<section id="abstract">
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<h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2>
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<p>The Windows installers for Python include a launcher that locates the
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correct Python interpreter to run (see <a class="pep reference internal" href="../pep-0397/" title="PEP 397 – Python launcher for Windows">PEP 397</a>). However, the
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launcher is not aware of virtual environments (virtualenv <a class="footnote-reference brackets" href="#id2" id="id1">[1]</a> or PEP
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405 based), and so cannot be used to run commands from the active
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virtualenv.</p>
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<p>This PEP proposes making the launcher “virtualenv aware”. This means
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that when run without specifying an explicit Python interpreter to
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use, the launcher will use the currently active virtualenv, if any,
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before falling back to the configured default Python.</p>
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</section>
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<section id="rationale">
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<h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2>
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<p>Windows users with multiple copies of Python installed need a means of
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selecting which one to use. The Python launcher provides this
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facility by means of a <code class="docutils literal notranslate"><span class="pre">py</span></code> command that can be used to run either a
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configured “default” Python or a specific interpreter, by means of
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command line arguments. So typical usage would be:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># Run the Python interactive interpreter</span>
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<span class="n">py</span>
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<span class="c1"># Execute an installed module</span>
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<span class="n">py</span> <span class="o">-</span><span class="n">m</span> <span class="n">pip</span> <span class="n">install</span> <span class="n">pytest</span>
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<span class="n">py</span> <span class="o">-</span><span class="n">m</span> <span class="n">pytest</span>
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</pre></div>
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</div>
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<p>When using virtual environments, the <code class="docutils literal notranslate"><span class="pre">py</span></code> launcher is unaware that a
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virtualenv is active, and will continue to use the system Python. So
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different command invocations are needed to run the same commands in a
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virtualenv:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># Run the Python interactive interpreter</span>
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<span class="n">python</span>
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<span class="c1"># Execute an installed module (these could use python -m,</span>
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<span class="c1"># which is longer to type but is a little more similar to the</span>
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<span class="c1"># launcher approach)</span>
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<span class="n">pip</span> <span class="n">install</span> <span class="n">pytest</span>
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<span class="n">py</span><span class="o">.</span><span class="n">test</span>
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</pre></div>
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</div>
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<p>Having to use different commands is error-prone, and in many cases
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the error is difficult to spot immediately. The PEP proposes making
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the <code class="docutils literal notranslate"><span class="pre">py</span></code> command usable with virtual environments, so that the first
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form of command can be used in all cases.</p>
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</section>
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<section id="implementation">
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<h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2>
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<p>Both <code class="docutils literal notranslate"><span class="pre">virtualenv</span></code> and the core <code class="docutils literal notranslate"><span class="pre">venv</span></code> module set an environment
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variable <code class="docutils literal notranslate"><span class="pre">VIRTUAL_ENV</span></code> when activating a virtualenv. This PEP
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proposes that the launcher checks for the <code class="docutils literal notranslate"><span class="pre">VIRTUAL_ENV</span></code> environment
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variable whenever it would run the “default” Python interpreter for
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the system (i.e., when no specific version flags such as <code class="docutils literal notranslate"><span class="pre">py</span> <span class="pre">-2.7</span></code>
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are used) and if present, run the Python interpreter for the
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virtualenv rather than the default system Python.</p>
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<p>The “default” Python interpreter referred to above is (as per <a class="pep reference internal" href="../pep-0397/" title="PEP 397 – Python launcher for Windows">PEP 397</a>)
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either the latest version of Python installed on the system, or
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a version configured via the <code class="docutils literal notranslate"><span class="pre">py.ini</span></code> configuration file. When the
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user specifies an explicit Python version on the command line, this
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will always be used (as at present).</p>
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</section>
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<section id="impact-on-script-launching">
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<h2><a class="toc-backref" href="#impact-on-script-launching" role="doc-backlink">Impact on Script Launching</a></h2>
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<p>As well as interactive use, the launcher is used as the Windows file
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||
association for Python scripts. In that case, a “shebang” (<code class="docutils literal notranslate"><span class="pre">#!</span></code>)
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line at the start of the script is used to identify the interpreter to
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||
run. A fully-qualified path can be used, or a version-specific Python
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||
(<code class="docutils literal notranslate"><span class="pre">python3</span></code> or <code class="docutils literal notranslate"><span class="pre">python2</span></code>, or even <code class="docutils literal notranslate"><span class="pre">python3.5</span></code>), or the generic
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<code class="docutils literal notranslate"><span class="pre">python</span></code>, which means to use the default interpreter.</p>
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<p>The launcher also looks for the specific shebang line
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<code class="docutils literal notranslate"><span class="pre">#!/usr/bin/env</span> <span class="pre">python</span></code>. On Unix, the <code class="docutils literal notranslate"><span class="pre">env</span></code> program searches for a
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command on <code class="docutils literal notranslate"><span class="pre">$PATH</span></code> and runs the command so located. Similarly, with
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this shebang line, the launcher will look for a copy of <code class="docutils literal notranslate"><span class="pre">python.exe</span></code>
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on the user’s current <code class="docutils literal notranslate"><span class="pre">%PATH%</span></code> and will run that copy.</p>
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<p>As activating a virtualenv means that it is added to <code class="docutils literal notranslate"><span class="pre">PATH</span></code>, no
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special handling is needed to run scripts with the active virtualenv -
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||
they just need to use the <code class="docutils literal notranslate"><span class="pre">#!/usr/bin/env</span> <span class="pre">python</span></code> shebang line,
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||
exactly as on Unix. (If there is no activated virtualenv, and no
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||
<code class="docutils literal notranslate"><span class="pre">python.exe</span></code> on <code class="docutils literal notranslate"><span class="pre">PATH</span></code>, the launcher will look for a default
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Python exactly as if the shebang line had said <code class="docutils literal notranslate"><span class="pre">#!python</span></code>).</p>
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</section>
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<section id="exclusions">
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<h2><a class="toc-backref" href="#exclusions" role="doc-backlink">Exclusions</a></h2>
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<p>The PEP makes no attempt to promote the use of the launcher for
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running Python on Windows. Most existing documentation assumes the
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||
user of <code class="docutils literal notranslate"><span class="pre">python</span></code> as the command to run Python, and (for example)
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||
<code class="docutils literal notranslate"><span class="pre">pip</span></code> to run an installed Python command. This documentation is not
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||
expected to change, and users who choose to manage their <code class="docutils literal notranslate"><span class="pre">PATH</span></code>
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environment variable can continue to use this form. The focus of this
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PEP is purely on allowing users who prefer to use the launcher when
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dealing with their system Python installations, to be able to continue
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to do so when using virtual environments.</p>
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</section>
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<section id="reference-implementation">
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<h2><a class="toc-backref" href="#reference-implementation" role="doc-backlink">Reference Implementation</a></h2>
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<p>A patch implementing the proposed behaviour is available at
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<a class="reference external" href="http://bugs.python.org/issue23465">http://bugs.python.org/issue23465</a></p>
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</section>
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<section id="references">
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<h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2>
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<aside class="footnote-list brackets">
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<aside class="footnote brackets" id="id2" role="doc-footnote">
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<dt class="label" id="id2">[<a href="#id1">1</a>]</dt>
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<dd><a class="reference external" href="https://virtualenv.pypa.io/">https://virtualenv.pypa.io/</a></aside>
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</aside>
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</section>
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<section id="copyright">
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<h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2>
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<p>This document has been placed in the public domain.</p>
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</section>
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</section>
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<hr class="docutils" />
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<p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0486.rst">https://github.com/python/peps/blob/main/peps/pep-0486.rst</a></p>
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<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0486.rst">2023-09-09 17:39:29 GMT</a></p>
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<h2>Contents</h2>
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<ul>
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<li><a class="reference internal" href="#abstract">Abstract</a></li>
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<li><a class="reference internal" href="#rationale">Rationale</a></li>
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<li><a class="reference internal" href="#implementation">Implementation</a></li>
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<li><a class="reference internal" href="#impact-on-script-launching">Impact on Script Launching</a></li>
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<li><a class="reference internal" href="#exclusions">Exclusions</a></li>
|
||
<li><a class="reference internal" href="#reference-implementation">Reference Implementation</a></li>
|
||
<li><a class="reference internal" href="#references">References</a></li>
|
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<li><a class="reference internal" href="#copyright">Copyright</a></li>
|
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</ul>
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<br>
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<a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0486.rst">Page Source (GitHub)</a>
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