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<h1>Python Enhancement Proposals</h1>
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<h1 class="page-title">PEP 645 – Allow writing optional types as <code class="docutils literal notranslate"><span class="pre">x?</span></code></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">Maggie Moss <maggiebmoss at gmail.com></dd>
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<dt class="field-even">Sponsor<span class="colon">:</span></dt>
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<dd class="field-even">Guido van Rossum <guido at python.org></dd>
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<dt class="field-odd">Status<span class="colon">:</span></dt>
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<dd class="field-odd"><abbr title="Removed from consideration by sponsor or authors">Withdrawn</abbr></dd>
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<dt class="field-even">Type<span class="colon">:</span></dt>
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<dd class="field-even"><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-odd">Created<span class="colon">:</span></dt>
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<dd class="field-odd">25-Aug-2020</dd>
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<dt class="field-even">Resolution<span class="colon">:</span></dt>
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<dd class="field-even"><a class="reference external" href="https://mail.python.org/archives/list/typing-sig@python.org/message/E75SPV6DDHLEEFSA5MBN5HUOQWDMUQJ2/">Typing-SIG message</a></dd>
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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="#pep-withdrawal">PEP Withdrawal</a></li>
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<li><a class="reference internal" href="#motivation">Motivation</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="#specification">Specification</a></li>
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<li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</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="#rejected-ideas">Rejected Ideas</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>This PEP proposes adding a <code class="docutils literal notranslate"><span class="pre">?</span></code> operator for types to allow writing <code class="docutils literal notranslate"><span class="pre">int?</span></code> in place of <code class="docutils literal notranslate"><span class="pre">Optional[int]</span></code>.</p>
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</section>
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<section id="pep-withdrawal">
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<h2><a class="toc-backref" href="#pep-withdrawal" role="doc-backlink">PEP Withdrawal</a></h2>
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<p>The notation <code class="docutils literal notranslate"><span class="pre">T|None</span></code> introduced by <a class="pep reference internal" href="../pep-0604/" title="PEP 604 – Allow writing union types as X | Y">PEP 604</a> to write <code class="docutils literal notranslate"><span class="pre">Optional[T]</span></code> is a
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fine alternative to <code class="docutils literal notranslate"><span class="pre">T?</span></code> and does not require new syntax.</p>
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<p>Using <code class="docutils literal notranslate"><span class="pre">T?</span></code> to mean <code class="docutils literal notranslate"><span class="pre">T|None</span></code> is also inconsistent with TypeScript
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where it roughly means <code class="docutils literal notranslate"><span class="pre">NotRequired[T]</span></code>.
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Such inconsistency would likely confuse folks coming from TypeScript to Python.</p>
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<p>The above represents the consensus of
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<a class="reference external" href="https://mail.python.org/archives/list/typing-sig@python.org/">typing-sig</a>
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and the sponsor of this PEP.</p>
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</section>
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<section id="motivation">
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<h2><a class="toc-backref" href="#motivation" role="doc-backlink">Motivation</a></h2>
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<p>Types have become a valuable and powerful part of the Python language. However, many type annotations are verbose and add
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considerable friction to using type annotations. By improving the typing syntax, adding types to Python code becomes simpler
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and improves the development experience for Python users.</p>
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<p>In a similar vein, a PEP to introduce short hand syntax for <a class="pep reference internal" href="../pep-0604/" title="PEP 604 – Allow writing union types as X | Y">Union types</a> has
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been approved and implemented.</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>Types in Python can be quite verbose, this can be a hindrance when working towards type adoption. Making types more ergonomic,
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as was done with the Union type in <a class="pep reference internal" href="../pep-0604/" title="PEP 604 – Allow writing union types as X | Y">PEP 604</a> (e.g., int | str), would reduce the effort needed to add types to new and existing Python code.
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The Optional annotation is used frequently in both partially and fully typed Python code bases. In a small sampling of <a class="reference external" href="https://gist.github.com/MaggieMoss/fd8dfe002b2702fae243dbf81a62624e">5 well-typed open
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source projects, on average 7% of annotations</a> included at least one optional type. This indicates
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that updating the syntax has the potential to make types more concise, reduce code length and improve readability.</p>
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<p>Simplifying the syntax for optionals has been <a class="reference external" href="https://github.com/python/typing/issues/429">discussed previously</a> within the typing community.
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The consensus during these conversations has been that <code class="docutils literal notranslate"><span class="pre">?</span></code> is the preferred operator. There is no native support for unary <code class="docutils literal notranslate"><span class="pre">?</span></code> in Python and this will
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need to be added to the runtime.</p>
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<p>Adding the ? sigil to the Python grammar has been proposed previously in <a class="pep reference internal" href="../pep-0505/" title="PEP 505 – None-aware operators">PEP 505</a>, which is currently in a deferred state.
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<a class="pep reference internal" href="../pep-0505/" title="PEP 505 – None-aware operators">PEP 505</a> proposes a:</p>
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<blockquote>
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<div><ul class="simple">
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<li>“None coalescing” binary operator <code class="docutils literal notranslate"><span class="pre">??</span></code></li>
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<li>“None-aware attribute access” operator <code class="docutils literal notranslate"><span class="pre">?.</span></code> (“maybe dot”)</li>
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<li>“None-aware indexing” operator <code class="docutils literal notranslate"><span class="pre">?[]</span></code> (“maybe subscript”)</li>
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</ul>
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</div></blockquote>
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<p>Should <a class="pep reference internal" href="../pep-0505/" title="PEP 505 – None-aware operators">PEP 505</a> be approved in the future, it would not interfere with the typing specific <code class="docutils literal notranslate"><span class="pre">?</span></code> proposed in this PEP. As well,
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since all uses of the <code class="docutils literal notranslate"><span class="pre">?</span></code> would be conceptually related, it would not be confusing in terms of learning Python or a hindrance to quick visual comprehension.</p>
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<p>The proposed syntax, with the postfix operator, mimics the optional syntax found in other typed languages, like C#, TypeScript and Swift.
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The widespread adoption and popularity of these languages means that Python developers are likely already familiar with this syntax.</p>
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<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>// Optional in Swift
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var example: String?
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// Optional in C#
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string? example;
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</pre></div>
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</div>
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<p>Adding this syntax would also follow the often used pattern of using builtin types as annotations. For example, <code class="docutils literal notranslate"><span class="pre">list</span></code>, <code class="docutils literal notranslate"><span class="pre">dict</span></code> and <code class="docutils literal notranslate"><span class="pre">None</span></code>. This would allow more annotations to be
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added to Python code without importing from <code class="docutils literal notranslate"><span class="pre">typing</span></code>.</p>
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</section>
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<section id="specification">
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<h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2>
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<p>The new optional syntax should be accepted for function, variable, attribute and parameter annotations.</p>
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<div class="highlight-text notranslate"><div class="highlight"><pre><span></span># instead of
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# def foo(x: Optional[int], y: Optional[str], z: Optional[list[int]): ...
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def foo(x: int?, y: str?, x: list[int]?): ...
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# def bar(x: list[typing.Optional[int]]): ...
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def bar(x: list[int?]): ...
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</pre></div>
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</div>
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<p>The new optional syntax should be equivalent to the existing typing.Optional syntax</p>
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<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>typing.Optional[int] == int?
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</pre></div>
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</div>
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<p>The new optional syntax should have the same identity as the existing typing.Optional syntax.</p>
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<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>typing.Optional[int] is int?
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</pre></div>
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</div>
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<p>It should also be equivalent to a Union with None.</p>
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<div class="highlight-text notranslate"><div class="highlight"><pre><span></span># old syntax
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int? == typing.Union[int, None]
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# new syntax
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int? == int | None
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</pre></div>
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</div>
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<p>Since the new Union syntax specified in <a class="pep reference internal" href="../pep-0604/" title="PEP 604 – Allow writing union types as X | Y">PEP 604</a> is supported in <code class="docutils literal notranslate"><span class="pre">isinstance</span></code> and <code class="docutils literal notranslate"><span class="pre">issubclass</span></code>, the new optional syntax should be supported in both <code class="docutils literal notranslate"><span class="pre">isinstance</span></code> and <code class="docutils literal notranslate"><span class="pre">issubclass</span></code>,</p>
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<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>isinstance(1, int?) # true
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issubclass(Child, Super?) # true
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</pre></div>
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</div>
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<p>A new dunder method will need to be implemented to allow the <code class="docutils literal notranslate"><span class="pre">?</span></code> operator to be overloaded for other functionality.</p>
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</section>
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<section id="backwards-compatibility">
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<h2><a class="toc-backref" href="#backwards-compatibility" role="doc-backlink">Backwards Compatibility</a></h2>
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<p><code class="docutils literal notranslate"><span class="pre">?</span></code> is currently unused in Python syntax, therefore this PEP is fully backwards compatible.</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 reference implementation can be found <a class="reference external" href="https://github.com/python/cpython/compare/main...MaggieMoss:new-optional-syntax-postfix">here</a>.</p>
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</section>
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<section id="rejected-ideas">
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<h2><a class="toc-backref" href="#rejected-ideas" role="doc-backlink">Rejected Ideas</a></h2>
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<p>Discussed alternatives were</p>
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<ul class="simple">
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<li>The <code class="docutils literal notranslate"><span class="pre">~</span></code> operator was considered in place of <code class="docutils literal notranslate"><span class="pre">?</span></code>.</li>
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<li>A prefix operator (<code class="docutils literal notranslate"><span class="pre">?int</span></code>).</li>
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</ul>
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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 is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.</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-0645.rst">https://github.com/python/peps/blob/main/peps/pep-0645.rst</a></p>
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<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0645.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="#pep-withdrawal">PEP Withdrawal</a></li>
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<li><a class="reference internal" href="#motivation">Motivation</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="#specification">Specification</a></li>
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<li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</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="#rejected-ideas">Rejected Ideas</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-0645.rst">Page Source (GitHub)</a>
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