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<section id="pep-content">
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<h1 class="page-title">PEP 3155 – Qualified name for classes and functions</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">Antoine Pitrou <solipsis at pitrou.net></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abbr title="Accepted and implementation complete, or no longer active"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">29-Oct-2011</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">3.3</dd>
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<dt class="field-even">Post-History<span class="colon">:</span></dt>
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<dd class="field-even"><p></p></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/2011-November/114545.html">Python-Dev message</a></dd>
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</dl>
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<hr class="docutils" />
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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="#rationale">Rationale</a></li>
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<li><a class="reference internal" href="#proposal">Proposal</a><ul>
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<li><a class="reference internal" href="#example-with-nested-classes">Example with nested classes</a></li>
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<li><a class="reference internal" href="#example-with-nested-functions">Example with nested functions</a></li>
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</ul>
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</li>
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<li><a class="reference internal" href="#limitations">Limitations</a></li>
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<li><a class="reference internal" href="#discussion">Discussion</a><ul>
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<li><a class="reference internal" href="#excluding-the-module-name">Excluding the module name</a></li>
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<li><a class="reference internal" href="#reviving-unbound-methods">Reviving unbound methods</a></li>
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</ul>
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</li>
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<li><a class="reference internal" href="#naming-choice">Naming choice</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="rationale">
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<h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2>
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<p>Python’s introspection facilities have long had poor support for
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nested classes. Given a class object, it is impossible to know
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||
whether it was defined inside another class or at module top-level;
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||
and, if the former, it is also impossible to know in which class it
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||
was defined. While use of nested classes is often considered poor
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style, the only reason for them to have second class introspection
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support is a lousy pun.</p>
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<p>Python 3 adds insult to injury by dropping what was formerly known as
|
||
unbound methods. In Python 2, given the following definition:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
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<span class="k">def</span> <span class="nf">f</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>you can then walk up from the <code class="docutils literal notranslate"><span class="pre">C.f</span></code> object to its defining class:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">C</span><span class="o">.</span><span class="n">f</span><span class="o">.</span><span class="n">im_class</span>
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<span class="go"><class '__main__.C'></span>
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</pre></div>
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</div>
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<p>This possibility is gone in Python 3:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">C</span><span class="o">.</span><span class="n">f</span><span class="o">.</span><span class="n">im_class</span>
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<span class="gt">Traceback (most recent call last):</span>
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File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
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||
<span class="gr">AttributeError</span>: <span class="n">'function' object has no attribute 'im_class'</span>
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<span class="gp">>>> </span><span class="nb">dir</span><span class="p">(</span><span class="n">C</span><span class="o">.</span><span class="n">f</span><span class="p">)</span>
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<span class="go">['__annotations__', '__call__', '__class__', '__closure__', '__code__',</span>
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||
<span class="go">'__defaults__', '__delattr__', '__dict__', '__dir__', '__doc__',</span>
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<span class="go">'__eq__', '__format__', '__ge__', '__get__', '__getattribute__',</span>
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||
<span class="go">'__globals__', '__gt__', '__hash__', '__init__', '__kwdefaults__',</span>
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<span class="go">'__le__', '__lt__', '__module__', '__name__', '__ne__', '__new__',</span>
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<span class="go">'__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__',</span>
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<span class="go">'__str__', '__subclasshook__']</span>
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||
</pre></div>
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</div>
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<p>This limits again the introspection capabilities available to the
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||
user. It can produce actual issues when porting software to Python 3,
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||
for example Twisted Core where the issue of introspecting method
|
||
objects came up several times. It also limits pickling support <a class="footnote-reference brackets" href="#id3" id="id1">[1]</a>.</p>
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</section>
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||
<section id="proposal">
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||
<h2><a class="toc-backref" href="#proposal" role="doc-backlink">Proposal</a></h2>
|
||
<p>This PEP proposes the addition of a <code class="docutils literal notranslate"><span class="pre">__qualname__</span></code> attribute to
|
||
functions and classes. For top-level functions and classes, the
|
||
<code class="docutils literal notranslate"><span class="pre">__qualname__</span></code> attribute is equal to the <code class="docutils literal notranslate"><span class="pre">__name__</span></code> attribute. For
|
||
nested classes, methods, and nested functions, the <code class="docutils literal notranslate"><span class="pre">__qualname__</span></code>
|
||
attribute contains a dotted path leading to the object from the module
|
||
top-level. A function’s local namespace is represented in that dotted
|
||
path by a component named <code class="docutils literal notranslate"><span class="pre"><locals></span></code>.</p>
|
||
<p>The repr() and str() of functions and classes is modified to use
|
||
<code class="docutils literal notranslate"><span class="pre">__qualname__</span></code> rather than <code class="docutils literal notranslate"><span class="pre">__name__</span></code>.</p>
|
||
<section id="example-with-nested-classes">
|
||
<h3><a class="toc-backref" href="#example-with-nested-classes" role="doc-backlink">Example with nested classes</a></h3>
|
||
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="k">def</span> <span class="nf">f</span><span class="p">():</span> <span class="k">pass</span>
|
||
<span class="gp">... </span> <span class="k">class</span> <span class="nc">D</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="k">def</span> <span class="nf">g</span><span class="p">():</span> <span class="k">pass</span>
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<span class="gp">...</span>
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<span class="gp">>>> </span><span class="n">C</span><span class="o">.</span><span class="vm">__qualname__</span>
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<span class="go">'C'</span>
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<span class="gp">>>> </span><span class="n">C</span><span class="o">.</span><span class="n">f</span><span class="o">.</span><span class="vm">__qualname__</span>
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<span class="go">'C.f'</span>
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<span class="gp">>>> </span><span class="n">C</span><span class="o">.</span><span class="n">D</span><span class="o">.</span><span class="vm">__qualname__</span>
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<span class="go">'C.D'</span>
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<span class="gp">>>> </span><span class="n">C</span><span class="o">.</span><span class="n">D</span><span class="o">.</span><span class="n">g</span><span class="o">.</span><span class="vm">__qualname__</span>
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<span class="go">'C.D.g'</span>
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||
</pre></div>
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||
</div>
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||
</section>
|
||
<section id="example-with-nested-functions">
|
||
<h3><a class="toc-backref" href="#example-with-nested-functions" role="doc-backlink">Example with nested functions</a></h3>
|
||
<div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">f</span><span class="p">():</span>
|
||
<span class="gp">... </span> <span class="k">def</span> <span class="nf">g</span><span class="p">():</span> <span class="k">pass</span>
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||
<span class="gp">... </span> <span class="k">return</span> <span class="n">g</span>
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||
<span class="gp">...</span>
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||
<span class="gp">>>> </span><span class="n">f</span><span class="o">.</span><span class="vm">__qualname__</span>
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||
<span class="go">'f'</span>
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<span class="gp">>>> </span><span class="n">f</span><span class="p">()</span><span class="o">.</span><span class="vm">__qualname__</span>
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<span class="go">'f.<locals>.g'</span>
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</pre></div>
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</div>
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</section>
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||
</section>
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||
<section id="limitations">
|
||
<h2><a class="toc-backref" href="#limitations" role="doc-backlink">Limitations</a></h2>
|
||
<p>With nested functions (and classes defined inside functions), the
|
||
dotted path will not be walkable programmatically as a function’s
|
||
namespace is not available from the outside. It will still be more
|
||
helpful to the human reader than the bare <code class="docutils literal notranslate"><span class="pre">__name__</span></code>.</p>
|
||
<p>As the <code class="docutils literal notranslate"><span class="pre">__name__</span></code> attribute, the <code class="docutils literal notranslate"><span class="pre">__qualname__</span></code> attribute is computed
|
||
statically and it will not automatically follow rebinding.</p>
|
||
</section>
|
||
<section id="discussion">
|
||
<h2><a class="toc-backref" href="#discussion" role="doc-backlink">Discussion</a></h2>
|
||
<section id="excluding-the-module-name">
|
||
<h3><a class="toc-backref" href="#excluding-the-module-name" role="doc-backlink">Excluding the module name</a></h3>
|
||
<p>As <code class="docutils literal notranslate"><span class="pre">__name__</span></code>, <code class="docutils literal notranslate"><span class="pre">__qualname__</span></code> doesn’t include the module name. This
|
||
makes it independent of module aliasing and rebinding, and also allows to
|
||
compute it at compile time.</p>
|
||
</section>
|
||
<section id="reviving-unbound-methods">
|
||
<h3><a class="toc-backref" href="#reviving-unbound-methods" role="doc-backlink">Reviving unbound methods</a></h3>
|
||
<p>Reviving unbound methods would only solve a fraction of the problems this
|
||
PEP solves, at a higher price (an additional object type and an additional
|
||
indirection, rather than an additional attribute).</p>
|
||
</section>
|
||
</section>
|
||
<section id="naming-choice">
|
||
<h2><a class="toc-backref" href="#naming-choice" role="doc-backlink">Naming choice</a></h2>
|
||
<p>“Qualified name” is the best approximation, as a short phrase, of what the
|
||
additional attribute is about. It is not a “full name” or “fully qualified
|
||
name” since it (deliberately) does not include the module name. Calling
|
||
it a “path” would risk confusion with filesystem paths and the <code class="docutils literal notranslate"><span class="pre">__file__</span></code>
|
||
attribute.</p>
|
||
<p>The first proposal for the attribute name was to call it <code class="docutils literal notranslate"><span class="pre">__qname__</span></code> but
|
||
many people (who are not aware of previous use of such jargon in e.g. the
|
||
XML specification <a class="footnote-reference brackets" href="#id4" id="id2">[2]</a>) found it obscure and non-obvious, which is why the
|
||
slightly less short and more explicit <code class="docutils literal notranslate"><span class="pre">__qualname__</span></code> was finally chosen.</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="id3" role="doc-footnote">
|
||
<dt class="label" id="id3">[<a href="#id1">1</a>]</dt>
|
||
<dd>“pickle should support methods”:
|
||
<a class="reference external" href="http://bugs.python.org/issue9276">http://bugs.python.org/issue9276</a></aside>
|
||
<aside class="footnote brackets" id="id4" role="doc-footnote">
|
||
<dt class="label" id="id4">[<a href="#id2">2</a>]</dt>
|
||
<dd>“QName” entry in Wikipedia:
|
||
<a class="reference external" href="http://en.wikipedia.org/wiki/QName">http://en.wikipedia.org/wiki/QName</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-3155.rst">https://github.com/python/peps/blob/main/peps/pep-3155.rst</a></p>
|
||
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-3155.rst">2023-09-09 17:39:29 GMT</a></p>
|
||
|
||
</article>
|
||
<nav id="pep-sidebar">
|
||
<h2>Contents</h2>
|
||
<ul>
|
||
<li><a class="reference internal" href="#rationale">Rationale</a></li>
|
||
<li><a class="reference internal" href="#proposal">Proposal</a><ul>
|
||
<li><a class="reference internal" href="#example-with-nested-classes">Example with nested classes</a></li>
|
||
<li><a class="reference internal" href="#example-with-nested-functions">Example with nested functions</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#limitations">Limitations</a></li>
|
||
<li><a class="reference internal" href="#discussion">Discussion</a><ul>
|
||
<li><a class="reference internal" href="#excluding-the-module-name">Excluding the module name</a></li>
|
||
<li><a class="reference internal" href="#reviving-unbound-methods">Reviving unbound methods</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#naming-choice">Naming choice</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-3155.rst">Page Source (GitHub)</a>
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