PEP 424 – A method for exposing a length hint

<!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 424 – A method for exposing a length hint | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0424/"> <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 424 – A method for exposing a length hint | peps.python.org'> <meta property="og:description" content="CPython currently defines a __length_hint__ method on several types, such as various iterators. 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This method is then used by various other functions (such as <code class="docutils literal notranslate"><span class="pre">list</span></code>) to presize lists based on the estimate returned by <code class="docutils literal notranslate"><span class="pre">__length_hint__</span></code>. Types which are not sized, and thus should not define <code class="docutils literal notranslate"><span class="pre">__len__</span></code>, can then define <code class="docutils literal notranslate"><span class="pre">__length_hint__</span></code>, to allow estimating or computing a size (such as many iterators).</p> </section> <section id="specification"> <h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2> <p>This PEP formally documents <code class="docutils literal notranslate"><span class="pre">__length_hint__</span></code> for other interpreters and non-standard-library Python modules to implement.</p> <p><code class="docutils literal notranslate"><span class="pre">__length_hint__</span></code> must return an integer (else a <code class="docutils literal notranslate"><span class="pre">TypeError</span></code> is raised) or <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code>, and is not required to be accurate. It may return a value that is either larger or smaller than the actual size of the container. A return value of <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code> indicates that there is no finite length estimate. It may not return a negative value (else a ValueError is raised).</p> <p>In addition, a new function <code class="docutils literal notranslate"><span class="pre">operator.length_hint</span></code> hint is added, with the following semantics (which define how <code class="docutils literal notranslate"><span class="pre">__length_hint__</span></code> should be used):</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">length_hint</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">0</span><span class="p">):</span> <span class="w"> </span><span class="sd">"""Return an estimate of the number of items in obj.</span> <span class="sd"> This is useful for presizing containers when building from an</span> <span class="sd"> iterable.</span> <span class="sd"> If the object supports len(), the result will be</span> <span class="sd"> exact. Otherwise, it may over- or under-estimate by an</span> <span class="sd"> arbitrary amount. The result will be an integer >= 0.</span> <span class="sd"> """</span> <span class="k">try</span><span class="p">:</span> <span class="k">return</span> <span class="nb">len</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span> <span class="k">except</span> <span class="ne">TypeError</span><span class="p">:</span> <span class="k">try</span><span class="p">:</span> <span class="n">get_hint</span> <span class="o">=</span> <span class="nb">type</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span><span class="o">.</span><span class="fm">__length_hint__</span> <span class="k">except</span> <span class="ne">AttributeError</span><span class="p">:</span> <span class="k">return</span> <span class="n">default</span> <span class="k">try</span><span class="p">:</span> <span class="n">hint</span> <span class="o">=</span> <span class="n">get_hint</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span> <span class="k">except</span> <span class="ne">TypeError</span><span class="p">:</span> <span class="k">return</span> <span class="n">default</span> <span class="k">if</span> <span class="n">hint</span> <span class="ow">is</span> <span class="bp">NotImplemented</span><span class="p">:</span> <span class="k">return</span> <span class="n">default</span> <span class="k">if</span> <span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">hint</span><span class="p">,</span> <span class="nb">int</span><span class="p">):</span> <span class="k">raise</span> <span class="ne">TypeError</span><span class="p">(</span><span class="s2">"Length hint must be an integer, not </span><span class="si">%r</span><span class="s2">"</span> <span class="o">%</span> <span class="nb">type</span><span class="p">(</span><span class="n">hint</span><span class="p">))</span> <span class="k">if</span> <span class="n">hint</span> <span class="o"><</span> <span class="mi">0</span><span class="p">:</span> <span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s2">"__length_hint__() should return >= 0"</span><span class="p">)</span> <span class="k">return</span> <span class="n">hint</span> </pre></div> </div> </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> <p>Being able to pre-allocate lists based on the expected size, as estimated by <code class="docutils literal notranslate"><span class="pre">__length_hint__</span></code>, can be a significant optimization. CPython has been observed to run some code faster than PyPy, purely because of this optimization being present.</p> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> <p>This document has been placed into 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-0424.rst">https://github.com/python/peps/blob/main/peps/pep-0424.rst</a></p> <p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0424.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="#specification">Specification</a></li> <li><a class="reference internal" href="#rationale">Rationale</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-0424.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>

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PEP 424 – A method for exposing a length hint | peps.python.org