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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 379 – Adding an Assignment Expression | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0379/"> <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 379 – Adding an Assignment Expression | peps.python.org'> <meta property="og:description" content="This PEP adds a new assignment expression to the Python language to make it possible to assign the result of an expression in almost any place. 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The new expression will allow the assignment of the result of an expression at first use (in a comparison for example). </section> <section id="motivation-and-summary"> <h2><a class="toc-backref" href="#motivation-and-summary" role="doc-backlink">Motivation and Summary</a></h2> Issue1714448 “if something as x:” <a class="footnote-reference brackets" href="#id2" id="id1">[1]</a> describes a feature to allow assignment of the result of an expression in an if statement to a name. It supposed that the <code class="docutils literal notranslate">as</code> syntax could be borrowed for this purpose. Many times it is not the expression itself that is interesting, rather one of the terms that make up the expression. To be clear, something like this: <div class="highlight-default notranslate"><div class="highlight"><pre>if (f_result() == [1, 2, 3]) as res: </pre></div> </div> seems awfully limited, when this: <div class="highlight-default notranslate"><div class="highlight"><pre>if (f_result() as res) == [1, 2, 3]: </pre></div> </div> is probably the desired result. </section> <section id="use-cases"> <h2><a class="toc-backref" href="#use-cases" role="doc-backlink">Use Cases</a></h2> See the Examples section near the end. </section> <section id="specification"> <h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2> A new expression is proposed with the (nominal) syntax: <div class="highlight-default notranslate"><div class="highlight"><pre>EXPR -> VAR </pre></div> </div> This single expression does the following: <ul class="simple"> <li>Evaluate the value of <code class="docutils literal notranslate">EXPR</code>, an arbitrary expression;</li> <li>Assign the result to <code class="docutils literal notranslate">VAR</code>, a single assignment target; and</li> <li>Leave the result of <code class="docutils literal notranslate">EXPR</code> on the Top of Stack (TOS)</li> </ul> Here <code class="docutils literal notranslate">-></code> or (<code class="docutils literal notranslate">RARROW</code>) has been used to illustrate the concept that the result of <code class="docutils literal notranslate">EXPR</code> is assigned to <code class="docutils literal notranslate">VAR</code>. The translation of the proposed syntax is: <div class="highlight-default notranslate"><div class="highlight"><pre>VAR = (EXPR) (EXPR) </pre></div> </div> The assignment target can be either an attribute, a subscript or name: <div class="highlight-default notranslate"><div class="highlight"><pre>f() -> name[0] # where 'name' exists previously. f() -> name.attr # again 'name' exists prior to this expression. f() -> name </pre></div> </div> This expression should be available anywhere that an expression is currently accepted. All exceptions that are currently raised during invalid assignments will continue to be raised when using the assignment expression. For example, a <code class="docutils literal notranslate">NameError</code> will be raised when in example 1 and 2 above if <code class="docutils literal notranslate">name</code> is not previously defined, or an <code class="docutils literal notranslate">IndexError</code> if index 0 was out of range. </section> <section id="examples-from-the-standard-library"> <h2><a class="toc-backref" href="#examples-from-the-standard-library" role="doc-backlink">Examples from the Standard Library</a></h2> The following two examples were chosen after a brief search through the standard library, specifically both are from ast.py which happened to be open at the time of the search. Original: <div class="highlight-default notranslate"><div class="highlight"><pre>def walk(node): from collections import deque todo = deque([node]) while todo: node = todo.popleft() todo.extend(iter_child_nodes(node)) yield node </pre></div> </div> Using assignment expression: <div class="highlight-default notranslate"><div class="highlight"><pre>def walk(node): from collections import deque todo = deque([node]) while todo: todo.extend(iter_child_nodes(todo.popleft() -> node)) yield node </pre></div> </div> Original: <div class="highlight-default notranslate"><div class="highlight"><pre>def get_docstring(node, clean=True): if not isinstance(node, (FunctionDef, ClassDef, Module)): raise TypeError("%r can't have docstrings" % node.__class__.__name__) if node.body and isinstance(node.body[0], Expr) and \ isinstance(node.body[0].value, Str): if clean: import inspect return inspect.cleandoc(node.body[0].value.s) return node.body[0].value.s </pre></div> </div> Using assignment expression: <div class="highlight-default notranslate"><div class="highlight"><pre>def get_docstring(node, clean=True): if not isinstance(node, (FunctionDef, ClassDef, Module)): raise TypeError("%r can't have docstrings" % node.__class__.__name__) if node.body -> body and isinstance(body[0] -> elem, Expr) and \ isinstance(elem.value -> value, Str): if clean: import inspect return inspect.cleandoc(value.s) return value.s </pre></div> </div> </section> <section id="examples"> <h2><a class="toc-backref" href="#examples" role="doc-backlink">Examples</a></h2> The examples shown below highlight some of the desirable features of the assignment expression, and some of the possible corner cases. <ol class="arabic"> <li>Assignment in an if statement for use later:<div class="highlight-default notranslate"><div class="highlight"><pre>def expensive(): import time; time.sleep(1) return 'spam' if expensive() -> res in ('spam', 'eggs'): dosomething(res) </pre></div> </div> </li> <li>Assignment in a while loop clause:<div class="highlight-default notranslate"><div class="highlight"><pre>while len(expensive() -> res) == 4: dosomething(res) </pre></div> </div> </li> <li>Keep the iterator object from the for loop:<div class="highlight-default notranslate"><div class="highlight"><pre>for ch in expensive() -> res: sell_on_internet(res) </pre></div> </div> </li> <li>Corner case:<div class="highlight-default notranslate"><div class="highlight"><pre>for ch -> please_dont in expensive(): pass # who would want to do this? Not I. </pre></div> </div> </li> </ol> </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="id2" role="doc-footnote"> <dt class="label" id="id2">[<a href="#id1">1</a>]</dt> <dd>Issue1714448 “if something as x:”, k0wax <a class="reference external" href="http://bugs.python.org/issue1714448">http://bugs.python.org/issue1714448</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-0379.rst">https://github.com/python/peps/blob/main/peps/pep-0379.rst</a> Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0379.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="#motivation-and-summary">Motivation and Summary</a></li> <li><a class="reference internal" href="#use-cases">Use Cases</a></li> <li><a class="reference internal" href="#specification">Specification</a></li> <li><a class="reference internal" href="#examples-from-the-standard-library">Examples from the Standard Library</a></li> <li><a class="reference internal" href="#examples">Examples</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-0379.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>