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<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta name="color-scheme" content="light dark"> <title>PEP 3129 – Class Decorators | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-3129/"> <link rel="stylesheet" href="../_static/style.css" type="text/css"> <link rel="stylesheet" href="../_static/mq.css" type="text/css"> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" media="(prefers-color-scheme: light)" id="pyg-light"> <link rel="stylesheet" href="../_static/pygments_dark.css" type="text/css" media="(prefers-color-scheme: dark)" id="pyg-dark"> <link rel="alternate" type="application/rss+xml" title="Latest PEPs" href="https://peps.python.org/peps.rss"> <meta property="og:title" content='PEP 3129 – Class Decorators | peps.python.org'> <meta property="og:description" content="This PEP proposes class 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class="colour-scheme-icon-when-dark"><use href="#svg-moon"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-light"><use href="#svg-sun"></use></svg> <span class="visually-hidden">Toggle light / dark / auto colour theme</span> </button> </header> <article> <section id="pep-content"> <h1 class="page-title">PEP 3129 – Class Decorators</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author<span class="colon">:</span></dt> <dd class="field-odd">Collin Winter <collinwinter at google.com></dd> <dt class="field-even">Status<span class="colon">:</span></dt> <dd class="field-even"><abbr title="Accepted and implementation complete, or no longer active">Final</abbr></dd> <dt class="field-odd">Type<span class="colon">:</span></dt> <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> <dt class="field-even">Created<span class="colon">:</span></dt> <dd class="field-even">01-May-2007</dd> <dt class="field-odd">Python-Version<span class="colon">:</span></dt> <dd class="field-odd">3.0</dd> <dt class="field-even">Post-History<span class="colon">:</span></dt> <dd class="field-even">07-May-2007</dd> </dl> <hr class="docutils" /> <section id="contents"> <details><summary>Table of Contents</summary><ul class="simple"> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#semantics">Semantics</a></li> <li><a class="reference internal" href="#implementation">Implementation</a></li> <li><a class="reference internal" href="#acceptance">Acceptance</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> </details></section> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> <p>This PEP proposes class decorators, an extension to the function 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> </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> <p>When function decorators were originally debated for inclusion in Python 2.4, class decorators were seen as <a class="pep reference internal" href="../pep-0318/#motivation" title="PEP 318 – Decorators for Functions and Methods § Motivation">obscure and unnecessary</a> thanks to metaclasses. After several years’ experience with the Python 2.4.x series of releases and an increasing familiarity with function decorators and their uses, the BDFL and the community re-evaluated class decorators and recommended their inclusion in Python 3.0 <a class="footnote-reference brackets" href="#approval" id="id1">[1]</a>.</p> <p>The motivating use-case was to make certain constructs more easily expressed and less reliant on implementation details of the CPython interpreter. While it is possible to express class decorator-like functionality using metaclasses, the results are generally unpleasant and the implementation highly fragile <a class="footnote-reference brackets" href="#motivation" id="id2">[2]</a>. In addition, metaclasses are inherited, whereas class decorators are not, making metaclasses unsuitable for some, single class-specific uses of class decorators. The fact that large-scale Python projects like Zope were going through these wild contortions to achieve something like class decorators won over the BDFL.</p> </section> <section id="semantics"> <h2><a class="toc-backref" href="#semantics" role="doc-backlink">Semantics</a></h2> <p>The semantics and design goals of class decorators are the same as 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>); the only difference is that you’re decorating a class instead of a function. The following two snippets are semantically identical:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">A</span><span class="p">:</span> <span class="k">pass</span> <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> <span class="nd">@foo</span> <span class="nd">@bar</span> <span class="k">class</span><span class="w"> </span><span class="nc">A</span><span class="p">:</span> <span class="k">pass</span> </pre></div> </div> <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> </section> <section id="implementation"> <h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2> <p>Adapting Python’s grammar to support class decorators requires modifying two rules and adding a new rule:</p> <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> <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> <span class="n">with_stmt</span> <span class="o">|</span> <span class="n">funcdef</span> <span class="o">|</span> <span class="n">classdef</span> </pre></div> </div> <p>need to be changed to</p> <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> <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> <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> <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> </pre></div> </div> <p>Adding <code class="docutils literal notranslate"><span class="pre">decorated</span></code> is necessary to avoid an ambiguity in the grammar.</p> <p>The Python AST and bytecode must be modified accordingly.</p> <p>A reference implementation <a class="footnote-reference brackets" href="#id4" id="id3">[3]</a> has been provided by Jack Diederich.</p> </section> <section id="acceptance"> <h2><a class="toc-backref" href="#acceptance" role="doc-backlink">Acceptance</a></h2> <p>There was virtually no discussion following the posting of this PEP, meaning that everyone agreed it should be accepted.</p> <p>The patch was committed to Subversion as revision 55430.</p> </section> <section id="references"> <h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2> <aside class="footnote-list brackets"> <aside class="footnote brackets" id="approval" role="doc-footnote"> <dt class="label" id="approval">[<a href="#id1">1</a>]</dt> <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> <aside class="footnote brackets" id="motivation" role="doc-footnote"> <dt class="label" id="motivation">[<a href="#id2">2</a>]</dt> <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> <aside class="footnote brackets" id="id4" role="doc-footnote"> <dt class="label" id="id4">[<a href="#id3">3</a>]</dt> <dd><a class="reference external" href="https://bugs.python.org/issue1671208">https://bugs.python.org/issue1671208</a></aside> </aside> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> <p>This document has been placed in the public domain.</p> </section> </section> <hr class="docutils" /> <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> <p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-3129.rst">2025-02-01 08:59:27 GMT</a></p> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#semantics">Semantics</a></li> <li><a class="reference internal" href="#implementation">Implementation</a></li> <li><a class="reference internal" href="#acceptance">Acceptance</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> <br> <a id="source" href="https://github.com/python/peps/blob/main/peps/pep-3129.rst">Page Source (GitHub)</a> </nav> </section> <script src="../_static/colour_scheme.js"></script> <script src="../_static/wrap_tables.js"></script> <script src="../_static/sticky_banner.js"></script> </body> </html>