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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 274 – Dict Comprehensions | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0274/"> <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 274 – Dict Comprehensions | peps.python.org'> <meta property="og:description" content="PEP 202 introduces a syntactical extension to Python called the “list comprehension”. 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You can use dict comprehensions in ways ve..."> <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 0a7.5 7.5 0 0 0 7.92 12.446a9 9 0 1 1 -8.313 -12.454z"></path> </svg> </symbol> <symbol id="svg-sun" viewBox="0 0 24 24" pointer-events="all"> <title>Selected light 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="5"></circle> <line x1="12" y1="1" x2="12" y2="3"></line> <line x1="12" y1="21" x2="12" y2="23"></line> <line x1="4.22" y1="4.22" x2="5.64" y2="5.64"></line> <line x1="18.36" y1="18.36" x2="19.78" y2="19.78"></line> <line x1="1" y1="12" x2="3" y2="12"></line> <line x1="21" y1="12" x2="23" y2="12"></line> <line x1="4.22" y1="19.78" x2="5.64" y2="18.36"></line> <line x1="18.36" y1="5.64" x2="19.78" y2="4.22"></line> </svg> </symbol> </svg> <script> document.documentElement.dataset.colour_scheme = localStorage.getItem("colour_scheme") || "auto" </script> <section id="pep-page-section"> <header> <h1>Python Enhancement 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 274</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> <span class="visually-hidden">Toggle light / dark / auto colour theme</span> </button> </header> <article> <section id="pep-content"> <h1 class="page-title">PEP 274 – Dict Comprehensions</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author<span class="colon">:</span></dt> <dd class="field-odd">Barry Warsaw <barry at python.org></dd> <dt class="field-even">Status<span class="colon">:</span></dt> <dd class="field-even"><abbr title="Accepted and implementation complete, or no longer active">Final</abbr></dd> <dt class="field-odd">Type<span class="colon">:</span></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<span class="colon">:</span></dt> <dd class="field-even">25-Oct-2001</dd> <dt class="field-odd">Python-Version<span class="colon">:</span></dt> <dd class="field-odd">2.7, 3.0</dd> <dt class="field-even">Post-History<span class="colon">:</span></dt> <dd class="field-even">29-Oct-2001</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="#resolution">Resolution</a></li> <li><a class="reference internal" href="#proposed-solution">Proposed Solution</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#semantics">Semantics</a></li> <li><a class="reference internal" href="#examples">Examples</a></li> <li><a class="reference internal" href="#implementation">Implementation</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> <p><a class="pep reference internal" href="../pep-0202/" title="PEP 202 – List Comprehensions">PEP 202</a> introduces a syntactical extension to Python called the “list comprehension”. This PEP proposes a similar syntactical extension called the “dictionary comprehension” or “dict comprehension” for short. You can use dict comprehensions in ways very similar to list comprehensions, except that they produce Python dictionary objects instead of list objects.</p> </section> <section id="resolution"> <h2><a class="toc-backref" href="#resolution" role="doc-backlink">Resolution</a></h2> <p>This PEP was originally written for inclusion in Python 2.3. It was withdrawn after observation that substantially all of its benefits were subsumed by generator expressions coupled with the <code class="docutils literal notranslate"><span class="pre">dict()</span></code> constructor.</p> <p>However, Python 2.7 and 3.0 introduces this exact feature, as well as the closely related set comprehensions. On 2012-04-09, the PEP was changed to reflect this reality by updating its Status to Accepted, and updating the Python-Version field. The Open Questions section was also removed since these have been long resolved by the current implementation.</p> </section> <section id="proposed-solution"> <h2><a class="toc-backref" href="#proposed-solution" role="doc-backlink">Proposed Solution</a></h2> <p>Dict comprehensions are just like list comprehensions, except that you group the expression using curly braces instead of square braces. Also, the left part before the <code class="docutils literal notranslate"><span class="pre">for</span></code> keyword expresses both a key and a value, separated by a colon. The notation is specifically designed to remind you of list comprehensions as applied to dictionaries.</p> </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> <p>There are times when you have some data arranged as a sequences of length-2 sequences, and you want to turn that into a dictionary. In Python 2.2, the <code class="docutils literal notranslate"><span class="pre">dict()</span></code> constructor accepts an argument that is a sequence of length-2 sequences, used as (key, value) pairs to initialize a new dictionary object.</p> <p>However, the act of turning some data into a sequence of length-2 sequences can be inconvenient or inefficient from a memory or performance standpoint. Also, for some common operations, such as turning a list of things into a set of things for quick duplicate removal or set inclusion tests, a better syntax can help code clarity.</p> <p>As with list comprehensions, an explicit for loop can always be used (and in fact was the only way to do it in earlier versions of Python). But as with list comprehensions, dict comprehensions can provide a more syntactically succinct idiom that the traditional for loop.</p> </section> <section id="semantics"> <h2><a class="toc-backref" href="#semantics" role="doc-backlink">Semantics</a></h2> <p>The semantics of dict comprehensions can actually be demonstrated in stock Python 2.2, by passing a list comprehension to the built-in dictionary constructor:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">dict</span><span class="p">([(</span><span class="n">i</span><span class="p">,</span> <span class="nb">chr</span><span class="p">(</span><span class="mi">65</span><span class="o">+</span><span class="n">i</span><span class="p">))</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">)])</span> </pre></div> </div> <p>is semantically equivalent to:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="p">{</span><span class="n">i</span> <span class="p">:</span> <span class="nb">chr</span><span class="p">(</span><span class="mi">65</span><span class="o">+</span><span class="n">i</span><span class="p">)</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">)}</span> </pre></div> </div> <p>The dictionary constructor approach has two distinct disadvantages from the proposed syntax though. First, it isn’t as legible as a dict comprehension. Second, it forces the programmer to create an in-core list object first, which could be expensive.</p> </section> <section id="examples"> <h2><a class="toc-backref" href="#examples" role="doc-backlink">Examples</a></h2> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span> <span class="p">{</span><span class="n">i</span> <span class="p">:</span> <span class="nb">chr</span><span class="p">(</span><span class="mi">65</span><span class="o">+</span><span class="n">i</span><span class="p">)</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">)}</span> <span class="go">{0 : 'A', 1 : 'B', 2 : 'C', 3 : 'D'}</span> </pre></div> </div> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span> <span class="p">{</span><span class="n">k</span> <span class="p">:</span> <span class="n">v</span> <span class="k">for</span> <span class="n">k</span><span class="p">,</span> <span class="n">v</span> <span class="ow">in</span> <span class="n">someDict</span><span class="o">.</span><span class="n">iteritems</span><span class="p">()}</span> <span class="o">==</span> <span class="n">someDict</span><span class="o">.</span><span class="n">copy</span><span class="p">()</span> <span class="go">1</span> </pre></div> </div> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span> <span class="p">{</span><span class="n">x</span><span class="o">.</span><span class="n">lower</span><span class="p">()</span> <span class="p">:</span> <span class="mi">1</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">list_of_email_addrs</span><span class="p">}</span> <span class="go">{'barry@zope.com' : 1, 'barry@python.org' : 1, 'guido@python.org' : 1}</span> </pre></div> </div> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span><span class="w"> </span><span class="nf">invert</span><span class="p">(</span><span class="n">d</span><span class="p">):</span> <span class="gp">... </span> <span class="k">return</span> <span class="p">{</span><span class="n">v</span> <span class="p">:</span> <span class="n">k</span> <span class="k">for</span> <span class="n">k</span><span class="p">,</span> <span class="n">v</span> <span class="ow">in</span> <span class="n">d</span><span class="o">.</span><span class="n">iteritems</span><span class="p">()}</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="mi">0</span> <span class="p">:</span> <span class="s1">'A'</span><span class="p">,</span> <span class="mi">1</span> <span class="p">:</span> <span class="s1">'B'</span><span class="p">,</span> <span class="mi">2</span> <span class="p">:</span> <span class="s1">'C'</span><span class="p">,</span> <span class="mi">3</span> <span class="p">:</span> <span class="s1">'D'</span><span class="p">}</span> <span class="gp">>>> </span><span class="nb">print</span> <span class="n">invert</span><span class="p">(</span><span class="n">d</span><span class="p">)</span> <span class="go">{'A' : 0, 'B' : 1, 'C' : 2, 'D' : 3}</span> </pre></div> </div> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="p">{(</span><span class="n">k</span><span class="p">,</span> <span class="n">v</span><span class="p">):</span> <span class="n">k</span><span class="o">+</span><span class="n">v</span> <span class="k">for</span> <span class="n">k</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span> <span class="k">for</span> <span class="n">v</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">)}</span> <span class="gp">... </span><span class="p">{(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">3</span><span class="p">):</span> <span class="mi">6</span><span class="p">,</span> <span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">2</span><span class="p">):</span> <span class="mi">5</span><span class="p">,</span> <span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">1</span><span class="p">):</span> <span class="mi">4</span><span class="p">,</span> <span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">):</span> <span class="mi">1</span><span class="p">,</span> <span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">1</span><span class="p">):</span> <span class="mi">3</span><span class="p">,</span> <span class="go"> (0, 2): 2, (3, 0): 3, (0, 3): 3, (1, 1): 2, (1, 0): 1,</span> <span class="go"> (0, 0): 0, (1, 2): 3, (2, 0): 2, (1, 3): 4, (2, 2): 4, (</span> <span class="go"> 2, 3): 5}</span> </pre></div> </div> </section> <section id="implementation"> <h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2> <p>All implementation details were resolved in the Python 2.7 and 3.0 time-frame.</p> </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-0274.rst">https://github.com/python/peps/blob/main/peps/pep-0274.rst</a></p> <p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0274.rst">2025-02-01 08:59:27 GMT</a></p> </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="#resolution">Resolution</a></li> <li><a class="reference internal" href="#proposed-solution">Proposed Solution</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#semantics">Semantics</a></li> <li><a class="reference internal" href="#examples">Examples</a></li> <li><a class="reference internal" href="#implementation">Implementation</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-0274.rst">Page Source (GitHub)</a> </nav> </section> <script src="../_static/colour_scheme.js"></script> <script 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