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<h1 class="page-title">PEP 3129 – Class Decorators</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">Collin Winter <collinwinter at google.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abbr title="Accepted and implementation complete, or no longer active"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">01-May-2007</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">3.0</dd>
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<dt class="field-even">Post-History<span class="colon">:</span></dt>
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<dd class="field-even">07-May-2007</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="#rationale">Rationale</a></li>
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<li><a class="reference internal" href="#semantics">Semantics</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="#acceptance">Acceptance</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>This PEP proposes class decorators, an extension to the function
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and method decorators introduced in <a class="pep reference internal" href="../pep-0318/" title="PEP 318 – Decorators for Functions and Methods">PEP 318</a>.</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>When function decorators were originally debated for inclusion in
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Python 2.4, class decorators were seen as
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<a class="pep reference internal" href="../pep-0318/#motivation" title="PEP 318 – Decorators for Functions and Methods § Motivation">obscure and unnecessary</a>
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thanks to metaclasses. After several years’ experience
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with the Python 2.4.x series of releases and an increasing
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familiarity with function decorators and their uses, the BDFL and
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the community re-evaluated class decorators and recommended their
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inclusion in Python 3.0 <a class="footnote-reference brackets" href="#approval" id="id1">[1]</a>.</p>
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<p>The motivating use-case was to make certain constructs more easily
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expressed and less reliant on implementation details of the CPython
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interpreter. While it is possible to express class decorator-like
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functionality using metaclasses, the results are generally
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unpleasant and the implementation highly fragile <a class="footnote-reference brackets" href="#motivation" id="id2">[2]</a>. In
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addition, metaclasses are inherited, whereas class decorators are not,
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making metaclasses unsuitable for some, single class-specific uses of
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class decorators. The fact that large-scale Python projects like Zope
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were going through these wild contortions to achieve something like
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class decorators won over the BDFL.</p>
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</section>
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<section id="semantics">
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<h2><a class="toc-backref" href="#semantics" role="doc-backlink">Semantics</a></h2>
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<p>The semantics and design goals of class decorators are the same as
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for function decorators (<a class="pep reference internal" href="../pep-0318/#current-syntax" title="PEP 318 – Decorators for Functions and Methods § Current Syntax">PEP 318</a>, <a class="pep reference internal" href="../pep-0318/#design-goals" title="PEP 318 – Decorators for Functions and Methods § Design Goals">PEP 318</a>);
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the only
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difference is that you’re decorating a class instead of a function.
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The following two snippets are semantically identical:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">A</span><span class="p">:</span>
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<span class="k">pass</span>
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<span class="n">A</span> <span class="o">=</span> <span class="n">foo</span><span class="p">(</span><span class="n">bar</span><span class="p">(</span><span class="n">A</span><span class="p">))</span>
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<span class="nd">@foo</span>
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<span class="nd">@bar</span>
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<span class="k">class</span> <span class="nc">A</span><span class="p">:</span>
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<span class="k">pass</span>
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</pre></div>
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</div>
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<p>For a detailed examination of decorators, please refer to <a class="pep reference internal" href="../pep-0318/" title="PEP 318 – Decorators for Functions and Methods">PEP 318</a>.</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>Adapting Python’s grammar to support class decorators requires
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modifying two rules and adding a new rule:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">funcdef</span><span class="p">:</span> <span class="p">[</span><span class="n">decorators</span><span class="p">]</span> <span class="s1">'def'</span> <span class="n">NAME</span> <span class="n">parameters</span> <span class="p">[</span><span class="s1">'->'</span> <span class="n">test</span><span class="p">]</span> <span class="s1">':'</span> <span class="n">suite</span>
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<span class="n">compound_stmt</span><span class="p">:</span> <span class="n">if_stmt</span> <span class="o">|</span> <span class="n">while_stmt</span> <span class="o">|</span> <span class="n">for_stmt</span> <span class="o">|</span> <span class="n">try_stmt</span> <span class="o">|</span>
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<span class="n">with_stmt</span> <span class="o">|</span> <span class="n">funcdef</span> <span class="o">|</span> <span class="n">classdef</span>
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</pre></div>
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</div>
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<p>need to be changed to</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">decorated</span><span class="p">:</span> <span class="n">decorators</span> <span class="p">(</span><span class="n">classdef</span> <span class="o">|</span> <span class="n">funcdef</span><span class="p">)</span>
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<span class="n">funcdef</span><span class="p">:</span> <span class="s1">'def'</span> <span class="n">NAME</span> <span class="n">parameters</span> <span class="p">[</span><span class="s1">'->'</span> <span class="n">test</span><span class="p">]</span> <span class="s1">':'</span> <span class="n">suite</span>
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||
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||
<span class="n">compound_stmt</span><span class="p">:</span> <span class="n">if_stmt</span> <span class="o">|</span> <span class="n">while_stmt</span> <span class="o">|</span> <span class="n">for_stmt</span> <span class="o">|</span> <span class="n">try_stmt</span> <span class="o">|</span>
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<span class="n">with_stmt</span> <span class="o">|</span> <span class="n">funcdef</span> <span class="o">|</span> <span class="n">classdef</span> <span class="o">|</span> <span class="n">decorated</span>
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</pre></div>
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</div>
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<p>Adding <code class="docutils literal notranslate"><span class="pre">decorated</span></code> is necessary to avoid an ambiguity in the
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grammar.</p>
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<p>The Python AST and bytecode must be modified accordingly.</p>
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<p>A reference implementation <a class="footnote-reference brackets" href="#id4" id="id3">[3]</a> has been provided by
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Jack Diederich.</p>
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</section>
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<section id="acceptance">
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<h2><a class="toc-backref" href="#acceptance" role="doc-backlink">Acceptance</a></h2>
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<p>There was virtually no discussion following the posting of this PEP,
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meaning that everyone agreed it should be accepted.</p>
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<p>The patch was committed to Subversion as revision 55430.</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="approval" role="doc-footnote">
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<dt class="label" id="approval">[<a href="#id1">1</a>]</dt>
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<dd><a class="reference external" href="https://mail.python.org/pipermail/python-dev/2006-March/062942.html">https://mail.python.org/pipermail/python-dev/2006-March/062942.html</a></aside>
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<aside class="footnote brackets" id="motivation" role="doc-footnote">
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<dt class="label" id="motivation">[<a href="#id2">2</a>]</dt>
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<dd><a class="reference external" href="https://mail.python.org/pipermail/python-dev/2006-March/062888.html">https://mail.python.org/pipermail/python-dev/2006-March/062888.html</a></aside>
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<aside class="footnote brackets" id="id4" role="doc-footnote">
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<dt class="label" id="id4">[<a href="#id3">3</a>]</dt>
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<dd><a class="reference external" href="https://bugs.python.org/issue1671208">https://bugs.python.org/issue1671208</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-3129.rst">https://github.com/python/peps/blob/main/peps/pep-3129.rst</a></p>
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<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-3129.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="#semantics">Semantics</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="#acceptance">Acceptance</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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