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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 530 – Asynchronous Comprehensions | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0530/"> <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 530 – Asynchronous Comprehensions | peps.python.org'> <meta property="og:description" content="PEP 492 and PEP 525 introduce support for native coroutines and asynchronous generators using async / await syntax. 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This PEP proposes to add asynchronous versions of list, set, dict comprehensions and generator expressions."> <meta name="theme-color" content="#3776ab"> </head> <body> <svg xmlns="http://www.w3.org/2000/svg" style="display: none;"> <symbol id="svg-sun-half" viewBox="0 0 24 24" pointer-events="all"> <title>Following system colour scheme</title> <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"> <circle cx="12" cy="12" r="9"></circle> <path d="M12 3v18m0-12l4.65-4.65M12 14.3l7.37-7.37M12 19.6l8.85-8.85"></path> </svg> </symbol> <symbol id="svg-moon" viewBox="0 0 24 24" pointer-events="all"> <title>Selected dark colour scheme</title> <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"> <path stroke="none" d="M0 0h24v24H0z" fill="none"></path> <path d="M12 3c.132 0 .263 0 .393 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Proposals</h1> <ul class="breadcrumbs"> <li><a href="https://www.python.org/" title="The Python Programming Language">Python</a> » </li> <li><a href="../pep-0000/">PEP Index</a> » </li> <li>PEP 530</li> </ul> <button id="colour-scheme-cycler" onClick="setColourScheme(nextColourScheme())"> <svg aria-hidden="true" class="colour-scheme-icon-when-auto"><use 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 530 – Asynchronous Comprehensions</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author:</dt> <dd class="field-odd">Yury Selivanov <yury at edgedb.com></dd> <dt class="field-even">Discussions-To:</dt> <dd class="field-even"><a class="reference external" href="https://mail.python.org/archives/list/python-dev@python.org/">Python-Dev list</a></dd> <dt class="field-odd">Status:</dt> <dd class="field-odd"><abbr title="Accepted and implementation complete, or no longer active">Final</abbr></dd> <dt class="field-even">Type:</dt> <dd class="field-even"><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-odd">Created:</dt> <dd class="field-odd">03-Sep-2016</dd> <dt class="field-even">Python-Version:</dt> <dd class="field-even">3.6</dd> <dt class="field-odd">Post-History:</dt> <dd class="field-odd">03-Sep-2016</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-and-goals">Rationale and Goals</a></li> <li><a class="reference internal" href="#specification">Specification</a><ul> <li><a class="reference internal" href="#asynchronous-comprehensions">Asynchronous Comprehensions</a></li> <li><a class="reference internal" href="#await-in-comprehensions"><code class="docutils literal notranslate">await</code> in Comprehensions</a></li> <li><a class="reference internal" href="#grammar-updates">Grammar Updates</a></li> <li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</a></li> </ul> </li> <li><a class="reference internal" href="#acceptance">Acceptance</a></li> <li><a class="reference internal" href="#implementation">Implementation</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#acknowledgments">Acknowledgments</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> <a class="pep reference internal" href="../pep-0492/" title="PEP 492 – Coroutines with async and await syntax">PEP 492</a> and <a class="pep reference internal" href="../pep-0525/" title="PEP 525 – Asynchronous Generators">PEP 525</a> introduce support for native coroutines and asynchronous generators using <code class="docutils literal notranslate">async</code> / <code class="docutils literal notranslate">await</code> syntax. This PEP proposes to add asynchronous versions of list, set, dict comprehensions and generator expressions. </section> <section id="rationale-and-goals"> <h2><a class="toc-backref" href="#rationale-and-goals" role="doc-backlink">Rationale and Goals</a></h2> Python has extensive support for synchronous comprehensions, allowing to produce lists, dicts, and sets with a simple and concise syntax. We propose implementing similar syntactic constructions for the asynchronous code. To illustrate the readability improvement, consider the following example: <div class="highlight-default notranslate"><div class="highlight"><pre>result = [] async for i in aiter(): if i % 2: result.append(i) </pre></div> </div> With the proposed asynchronous comprehensions syntax, the above code becomes as short as: <div class="highlight-default notranslate"><div class="highlight"><pre>result = [i async for i in aiter() if i % 2] </pre></div> </div> The PEP also makes it possible to use the <code class="docutils literal notranslate">await</code> expressions in all kinds of comprehensions: <div class="highlight-default notranslate"><div class="highlight"><pre>result = [await fun() for fun in funcs] </pre></div> </div> </section> <section id="specification"> <h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2> <section id="asynchronous-comprehensions"> <h3><a class="toc-backref" href="#asynchronous-comprehensions" role="doc-backlink">Asynchronous Comprehensions</a></h3> We propose to allow using <code class="docutils literal notranslate">async for</code> inside list, set and dict comprehensions. Pending <a class="pep reference internal" href="../pep-0525/" title="PEP 525 – Asynchronous Generators">PEP 525</a> approval, we can also allow creation of asynchronous generator expressions. Examples: <ul class="simple"> <li>set comprehension: <code class="docutils literal notranslate">{i async for i in agen()}</code>;</li> <li>list comprehension: <code class="docutils literal notranslate">[i async for i in agen()]</code>;</li> <li>dict comprehension: <code class="docutils literal notranslate">{i: i ** 2 async for i in agen()}</code>;</li> <li>generator expression: <code class="docutils literal notranslate">(i ** 2 async for i in agen())</code>.</li> </ul> It is allowed to use <code class="docutils literal notranslate">async for</code> along with <code class="docutils literal notranslate">if</code> and <code class="docutils literal notranslate">for</code> clauses in asynchronous comprehensions and generator expressions: <div class="highlight-default notranslate"><div class="highlight"><pre>dataset = {data for line in aiter() async for data in line if check(data)} </pre></div> </div> Asynchronous comprehensions are only allowed inside an <code class="docutils literal notranslate">async def</code> function. In principle, asynchronous generator expressions are allowed in any context. However, in Python 3.6, due to <code class="docutils literal notranslate">async</code> and <code class="docutils literal notranslate">await</code> soft-keyword status, asynchronous generator expressions are only allowed in an <code class="docutils literal notranslate">async def</code> function. Once <code class="docutils literal notranslate">async</code> and <code class="docutils literal notranslate">await</code> become reserved keywords in Python 3.7, this restriction will be removed. </section> <section id="await-in-comprehensions"> <h3><a class="toc-backref" href="#await-in-comprehensions" role="doc-backlink"><code class="docutils literal notranslate">await</code> in Comprehensions</a></h3> We propose to allow the use of <code class="docutils literal notranslate">await</code> expressions in both asynchronous and synchronous comprehensions: <div class="highlight-default notranslate"><div class="highlight"><pre>result = [await fun() for fun in funcs] result = {await fun() for fun in funcs} result = {fun: await fun() for fun in funcs} result = [await fun() for fun in funcs if await smth] result = {await fun() for fun in funcs if await smth} result = {fun: await fun() for fun in funcs if await smth} result = [await fun() async for fun in funcs] result = {await fun() async for fun in funcs} result = {fun: await fun() async for fun in funcs} result = [await fun() async for fun in funcs if await smth] result = {await fun() async for fun in funcs if await smth} result = {fun: await fun() async for fun in funcs if await smth} </pre></div> </div> This is only valid in <code class="docutils literal notranslate">async def</code> function body. </section> <section id="grammar-updates"> <h3><a class="toc-backref" href="#grammar-updates" role="doc-backlink">Grammar Updates</a></h3> The proposal requires one change on the grammar level: adding the optional “async” keyword to <code class="docutils literal notranslate">comp_for</code>: <div class="highlight-default notranslate"><div class="highlight"><pre>comp_for: [ASYNC] 'for' exprlist 'in' or_test [comp_iter] </pre></div> </div> The <code class="docutils literal notranslate">comprehension</code> AST node will have the new <code class="docutils literal notranslate">is_async</code> argument. </section> <section id="backwards-compatibility"> <h3><a class="toc-backref" href="#backwards-compatibility" role="doc-backlink">Backwards Compatibility</a></h3> The proposal is fully backwards compatible. </section> </section> <section id="acceptance"> <h2><a class="toc-backref" href="#acceptance" role="doc-backlink">Acceptance</a></h2> <a class="pep reference internal" href="../pep-0530/" title="PEP 530 – Asynchronous Comprehensions">PEP 530</a> was accepted by Guido, September 6, 2016 <a class="footnote-reference brackets" href="#id4" id="id1">[1]</a>. </section> <section id="implementation"> <h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2> The implementation is tracked in issue 28008 <a class="footnote-reference brackets" href="#id6" id="id2">[3]</a>. The reference implementation git repository is available at <a class="footnote-reference brackets" href="#id5" id="id3">[2]</a>. </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="id4" role="doc-footnote"> <dt class="label" id="id4">[<a href="#id1">1</a>]</dt> <dd><a class="reference external" href="https://mail.python.org/pipermail/python-ideas/2016-September/042141.html">https://mail.python.org/pipermail/python-ideas/2016-September/042141.html</a></aside> <aside class="footnote brackets" id="id5" role="doc-footnote"> <dt class="label" id="id5">[<a href="#id3">2</a>]</dt> <dd><a class="reference external" href="https://github.com/1st1/cpython/tree/asyncomp">https://github.com/1st1/cpython/tree/asyncomp</a></aside> <aside class="footnote brackets" id="id6" role="doc-footnote"> <dt class="label" id="id6">[<a href="#id2">3</a>]</dt> <dd><a class="reference external" href="http://bugs.python.org/issue28008">http://bugs.python.org/issue28008</a></aside> </aside> </section> <section id="acknowledgments"> <h2><a class="toc-backref" href="#acknowledgments" role="doc-backlink">Acknowledgments</a></h2> I thank Guido van Rossum, Victor Stinner and Elvis Pranskevichus for their feedback, code reviews, and discussions around this PEP. </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-0530.rst">https://github.com/python/peps/blob/main/peps/pep-0530.rst</a> Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0530.rst">2025-02-01 08:59:27 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-and-goals">Rationale and Goals</a></li> <li><a class="reference internal" href="#specification">Specification</a><ul> <li><a class="reference internal" href="#asynchronous-comprehensions">Asynchronous Comprehensions</a></li> <li><a class="reference internal" href="#await-in-comprehensions"><code class="docutils literal notranslate">await</code> in Comprehensions</a></li> <li><a class="reference internal" href="#grammar-updates">Grammar Updates</a></li> <li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</a></li> </ul> </li> <li><a class="reference internal" href="#acceptance">Acceptance</a></li> <li><a class="reference internal" href="#implementation">Implementation</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#acknowledgments">Acknowledgments</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-0530.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>