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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 562 – Module __getattr__ and __dir__ | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0562/"> <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 562 – Module __getattr__ and __dir__ | peps.python.org'> <meta property="og:description" 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href="#svg-sun-half"></use></svg> <svg aria-hidden="true" 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> Toggle light / dark / auto colour theme </button> </header> <article> <section id="pep-content"> <h1 class="page-title">PEP 562 – Module __getattr__ and __dir__</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author:</dt> <dd class="field-odd">Ivan Levkivskyi <levkivskyi at gmail.com></dd> <dt class="field-even">Status:</dt> <dd class="field-even"><abbr title="Accepted and implementation complete, or no longer active">Final</abbr></dd> <dt class="field-odd">Type:</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:</dt> <dd class="field-even">09-Sep-2017</dd> <dt class="field-odd">Python-Version:</dt> <dd class="field-odd">3.7</dd> <dt class="field-even">Post-History:</dt> <dd class="field-even">09-Sep-2017</dd> <dt class="field-odd">Resolution:</dt> <dd class="field-odd"><a class="reference external" href="https://mail.python.org/pipermail/python-dev/2017-December/151033.html">Python-Dev message</a></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="#specification">Specification</a></li> <li><a class="reference internal" href="#backwards-compatibility-and-impact-on-performance">Backwards compatibility and impact on performance</a></li> <li><a class="reference internal" href="#discussion">Discussion</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> <div class="pep-banner canonical-doc sticky-banner admonition important"> Important This PEP is a historical document. The up-to-date, canonical documentation can now be found at <a class="reference external" href="https://docs.python.org/3/reference/datamodel.html#customizing-module-attribute-access">Customizing Module Attribute Access</a>. × See <a class="pep reference internal" href="../pep-0001/" title="PEP 1 – PEP Purpose and Guidelines">PEP 1</a> for how to propose changes. </div> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> It is proposed to support <code class="docutils literal notranslate">__getattr__</code> and <code class="docutils literal notranslate">__dir__</code> function defined on modules to provide basic customization of module attribute access. </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> It is sometimes convenient to customize or otherwise have control over access to module attributes. A typical example is managing deprecation warnings. Typical workarounds are assigning <code class="docutils literal notranslate">__class__</code> of a module object to a custom subclass of <code class="docutils literal notranslate">types.ModuleType</code> or replacing the <code class="docutils literal notranslate">sys.modules</code> item with a custom wrapper instance. It would be convenient to simplify this procedure by recognizing <code class="docutils literal notranslate">__getattr__</code> defined directly in a module that would act like a normal <code class="docutils literal notranslate">__getattr__</code> method, except that it will be defined on module instances. For example: <div class="highlight-default notranslate"><div class="highlight"><pre># lib.py from warnings import warn deprecated_names = ["old_function", ...] def _deprecated_old_function(arg, other): ... def __getattr__(name): if name in deprecated_names: warn(f"{name} is deprecated", DeprecationWarning) return globals()[f"_deprecated_{name}"] raise AttributeError(f"module {__name__!r} has no attribute {name!r}") # main.py from lib import old_function # Works, but emits the warning </pre></div> </div> Another widespread use case for <code class="docutils literal notranslate">__getattr__</code> would be lazy submodule imports. Consider a simple example: <div class="highlight-default notranslate"><div class="highlight"><pre># lib/__init__.py import importlib __all__ = ['submod', ...] def __getattr__(name): if name in __all__: return importlib.import_module("." + name, __name__) raise AttributeError(f"module {__name__!r} has no attribute {name!r}") # lib/submod.py print("Submodule loaded") class HeavyClass: ... # main.py import lib lib.submod.HeavyClass # prints "Submodule loaded" </pre></div> </div> There is a related proposal <a class="pep reference internal" href="../pep-0549/" title="PEP 549 – Instance Descriptors">PEP 549</a> that proposes to support instance properties for a similar functionality. The difference is this PEP proposes a faster and simpler mechanism, but provides more basic customization. An additional motivation for this proposal is that <a class="pep reference internal" href="../pep-0484/" title="PEP 484 – Type Hints">PEP 484</a> already defines the use of module <code class="docutils literal notranslate">__getattr__</code> for this purpose in Python stub files, see <a class="pep reference internal" href="../pep-0484/#stub-files" title="PEP 484 – Type Hints § Stub Files">PEP 484</a>. In addition, to allow modifying result of a <code class="docutils literal notranslate">dir()</code> call on a module to show deprecated and other dynamically generated attributes, it is proposed to support module level <code class="docutils literal notranslate">__dir__</code> function. For example: <div class="highlight-default notranslate"><div class="highlight"><pre># lib.py deprecated_names = ["old_function", ...] __all__ = ["new_function_one", "new_function_two", ...] def new_function_one(arg, other): ... def new_function_two(arg, other): ... def __dir__(): return sorted(__all__ + deprecated_names) # main.py import lib dir(lib) # prints ["new_function_one", "new_function_two", "old_function", ...] </pre></div> </div> </section> <section id="specification"> <h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2> The <code class="docutils literal notranslate">__getattr__</code> function at the module level should accept one argument which is the name of an attribute and return the computed value or raise an <code class="docutils literal notranslate">AttributeError</code>: <div class="highlight-default notranslate"><div class="highlight"><pre>def __getattr__(name: str) -> Any: ... </pre></div> </div> If an attribute is not found on a module object through the normal lookup (i.e. <code class="docutils literal notranslate">object.__getattribute__</code>), then <code class="docutils literal notranslate">__getattr__</code> is searched in the module <code class="docutils literal notranslate">__dict__</code> before raising an <code class="docutils literal notranslate">AttributeError</code>. If found, it is called with the attribute name and the result is returned. Looking up a name as a module global will bypass module <code class="docutils literal notranslate">__getattr__</code>. This is intentional, otherwise calling <code class="docutils literal notranslate">__getattr__</code> for builtins will significantly harm performance. The <code class="docutils literal notranslate">__dir__</code> function should accept no arguments, and return a list of strings that represents the names accessible on module: <div class="highlight-default notranslate"><div class="highlight"><pre>def __dir__() -> List[str]: ... </pre></div> </div> If present, this function overrides the standard <code class="docutils literal notranslate">dir()</code> search on a module. The reference implementation for this PEP can be found in <a class="footnote-reference brackets" href="#id3" id="id1">[2]</a>. </section> <section id="backwards-compatibility-and-impact-on-performance"> <h2><a class="toc-backref" href="#backwards-compatibility-and-impact-on-performance" role="doc-backlink">Backwards compatibility and impact on performance</a></h2> This PEP may break code that uses module level (global) names <code class="docutils literal notranslate">__getattr__</code> and <code class="docutils literal notranslate">__dir__</code>. (But the language reference explicitly reserves all undocumented dunder names, and allows “breakage without warning”; see <a class="footnote-reference brackets" href="#id4" id="id2">[3]</a>.) The performance implications of this PEP are minimal, since <code class="docutils literal notranslate">__getattr__</code> is called only for missing attributes. Some tools that perform module attributes discovery might not expect <code class="docutils literal notranslate">__getattr__</code>. This problem is not new however, since it is already possible to replace a module with a module subclass with overridden <code class="docutils literal notranslate">__getattr__</code> and <code class="docutils literal notranslate">__dir__</code>, but with this PEP such problems can occur more often. </section> <section id="discussion"> <h2><a class="toc-backref" href="#discussion" role="doc-backlink">Discussion</a></h2> Note that the use of module <code class="docutils literal notranslate">__getattr__</code> requires care to keep the referred objects pickleable. For example, the <code class="docutils literal notranslate">__name__</code> attribute of a function should correspond to the name with which it is accessible via <code class="docutils literal notranslate">__getattr__</code>: <div class="highlight-default notranslate"><div class="highlight"><pre>def keep_pickleable(func): func.__name__ = func.__name__.replace('_deprecated_', '') func.__qualname__ = func.__qualname__.replace('_deprecated_', '') return func @keep_pickleable def _deprecated_old_function(arg, other): ... </pre></div> </div> One should be also careful to avoid recursion as one would do with a class level <code class="docutils literal notranslate">__getattr__</code>. To use a module global with triggering <code class="docutils literal notranslate">__getattr__</code> (for example if one wants to use a lazy loaded submodule) one can access it as: <div class="highlight-default notranslate"><div class="highlight"><pre>sys.modules[__name__].some_global </pre></div> </div> or as: <div class="highlight-default notranslate"><div class="highlight"><pre>from . import some_global </pre></div> </div> Note that the latter sets the module attribute, thus <code class="docutils literal notranslate">__getattr__</code> will be called only once. </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">2</a>]</dt> <dd>The reference implementation (<a class="reference external" href="https://github.com/ilevkivskyi/cpython/pull/3/files">https://github.com/ilevkivskyi/cpython/pull/3/files</a>)</aside> <aside class="footnote brackets" id="id4" role="doc-footnote"> <dt class="label" id="id4">[<a href="#id2">3</a>]</dt> <dd>Reserved classes of identifiers (<a class="reference external" href="https://docs.python.org/3/reference/lexical_analysis.html#reserved-classes-of-identifiers">https://docs.python.org/3/reference/lexical_analysis.html#reserved-classes-of-identifiers</a>)</aside> </aside> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> This document has been placed in the public domain. </section> </section> <hr class="docutils" /> Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0562.rst">https://github.com/python/peps/blob/main/peps/pep-0562.rst</a> Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0562.rst">2025-02-01 08:55:40 GMT</a> </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="#specification">Specification</a></li> <li><a class="reference internal" href="#backwards-compatibility-and-impact-on-performance">Backwards compatibility and impact on performance</a></li> <li><a class="reference internal" href="#discussion">Discussion</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> <a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0562.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>