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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 542 – Dot Notation Assignment In Function Header | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0542/"> <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 542 – Dot Notation Assignment In Function Header | peps.python.org'> <meta 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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 542 – Dot Notation Assignment In Function Header</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author:</dt> <dd class="field-odd">Markus Meskanen <markusmeskanen at gmail.com></dd> <dt class="field-even">Status:</dt> <dd class="field-even"><abbr title="Formally declined and will not be accepted">Rejected</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">10-Feb-2017</dd> <dt class="field-odd">Resolution:</dt> <dd class="field-odd"><a class="reference external" href="https://mail.python.org/pipermail/python-dev/2019-March/156695.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="#implementation">Implementation</a></li> <li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</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> Function definitions only allow simple function names to be used, even though functions are assignable first class objects. This PEP proposes adding support for assigning a function to a class or instance attribute directly in the function definition’s header by using the dot notation to separate the object from the function’s name. Although a similar feature, this PEP does not address general assignment to anything that supports assignment, such as dict keys and list indexes. </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> Currently if a function needs to be assigned to a class or instance attribute, it requires an additional assignment statement to be made: <div class="highlight-default notranslate"><div class="highlight"><pre>class MyClass: ... my_instance = MyClass() def my_function(self): ... # Assign to class attribute MyClass.my_function = my_function # Or assign to instance attribute my_instance.my_function = my_function </pre></div> </div> While this isn’t usually an inconvenience, using dot notation to assign directly in the function’s header would greatly simplify this: <div class="highlight-default notranslate"><div class="highlight"><pre>class MyClass: ... my_instance = MyClass() # Assign to class attribute def MyClass.my_function(self): ... # Or assign to instance attribute def my_instance.my_function(self): ... </pre></div> </div> There are multiple reasons to use this functionality over a standard class method, for example when the class is referenced inside the function’s header (such as with decorators and typing). This is also useful when an instance requires a callback attribute: <div class="highlight-default notranslate"><div class="highlight"><pre>class Menu: def __init__(self, items=None, select_callback=None): self.items = items if items is not None else [] self.select_callback = select_callback my_menu = Menu([item1, item2]) def my_menu.select_callback(item_index, menu): print(menu.items[item_index]) </pre></div> </div> As opposed to: <div class="highlight-default notranslate"><div class="highlight"><pre>my_menu = Menu([item1, item2]) def select_callback(item_index, menu): print(menu.items[item_index]) my_menu.select_callback = select_callback </pre></div> </div> Or defining them in an “unnatural” order: <div class="highlight-default notranslate"><div class="highlight"><pre>def select_callback(item_index, menu): print(menu.items[item_index]) my_menu = Menu([item1, item2], select_callback) </pre></div> </div> It reads better than the “unnatural” way, since you already know at the time of the function definition what it’s goig to be used for. It also saves one line of code while removing visual complexity. The feature would also avoid leaving the function’s name into the global namespace: <div class="highlight-default notranslate"><div class="highlight"><pre>eggs = 'something' def Spam.eggs(self): ... def Cheese.eggs(self): ... assert eggs == 'something' </pre></div> </div> Ideally this would be just syntastic sugar: <div class="highlight-default notranslate"><div class="highlight"><pre>def x.y(): ... # Equals to def y(): ... x.y = y </pre></div> </div> Similar to how decorators are syntastic sugar: <div class="highlight-default notranslate"><div class="highlight"><pre>@decorate def f(): ... # Equals to def f(): ... f = decorate(f) </pre></div> </div> </section> <section id="implementation"> <h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2> The <code class="docutils literal notranslate">__name__</code> would follow the principles of a normal function: <div class="highlight-default notranslate"><div class="highlight"><pre>class MyClass: def my_function1(self): ... def MyClass.my_function2(self): ... assert my_function1.__name__ == 'my_function1' assert my_function2.__name__ == 'my_function2' </pre></div> </div> The grammar would use <code class="docutils literal notranslate">dotted_name</code> to support chaining of attributes: <div class="highlight-default notranslate"><div class="highlight"><pre>def Person.name.fset(self, value): self._name = value </pre></div> </div> </section> <section id="backwards-compatibility"> <h2><a class="toc-backref" href="#backwards-compatibility" role="doc-backlink">Backwards Compatibility</a></h2> This PEP is fully backwards compatible. </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-0542.rst">https://github.com/python/peps/blob/main/peps/pep-0542.rst</a> Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0542.rst">2023-09-09 17:39:29 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="#implementation">Implementation</a></li> <li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</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-0542.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>