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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>Sorting HOW TO — Python 2.7.18 documentation</title> <link rel="stylesheet" href="../_static/classic.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/language_data.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.18 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="Unicode HOWTO" href="unicode.html" /> <link rel="prev" title="Socket Programming HOWTO" href="sockets.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="file:///usr/share/doc/python2.7/html/howto/sorting.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div id="outdated-warning" style="padding: .5em; text-align: center; background-color: #FFBABA; color: #6A0E0E;"> This document is for an old version of Python that is <a href="https://devguide.python.org/devcycle/#end-of-life-branches">no longer supported</a>. You should install the python3 and python3-doc packages and read the <a href="file:///usr/share/doc/python3-doc/html/howto/sorting.html"> Python documentation for the Python3 version packaged in this release</a>. </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="unicode.html" title="Unicode HOWTO" accesskey="N">next</a> |</li> <li class="right" > <a href="sockets.html" title="Socket Programming HOWTO" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.18 documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">Python HOWTOs</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="sorting-how-to"> <span id="sortinghowto"></span><h1>Sorting HOW TO<a class="headerlink" href="#sorting-how-to" title="Permalink to this headline">¶</a></h1> <table class="docutils field-list" frame="void" rules="none"> <col class="field-name" /> <col class="field-body" /> <tbody valign="top"> <tr class="field-odd field"><th class="field-name">Author:</th><td class="field-body">Andrew Dalke and Raymond Hettinger</td> </tr> <tr class="field-even field"><th class="field-name">Release:</th><td class="field-body">0.1</td> </tr> </tbody> </table> <p>Python lists have a built-in <code class="xref py py-meth docutils literal notranslate"><span class="pre">list.sort()</span></code> method that modifies the list in-place. There is also a <a class="reference internal" href="../library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate"><span class="pre">sorted()</span></code></a> built-in function that builds a new sorted list from an iterable.</p> <p>In this document, we explore the various techniques for sorting data using Python.</p> <div class="section" id="sorting-basics"> <h2>Sorting Basics<a class="headerlink" href="#sorting-basics" title="Permalink to this headline">¶</a></h2> <p>A simple ascending sort is very easy: just call the <a class="reference internal" href="../library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate"><span class="pre">sorted()</span></code></a> function. It returns a new sorted list:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">sorted</span><span class="p">([</span><span class="mi">5</span><span class="p">,</span> <span class="mi">2</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="go">[1, 2, 3, 4, 5]</span> </pre></div> </div> <p>You can also use the <code class="xref py py-meth docutils literal notranslate"><span class="pre">list.sort()</span></code> method of a list. It modifies the list in-place (and returns <code class="docutils literal notranslate"><span class="pre">None</span></code> to avoid confusion). Usually it’s less convenient than <a class="reference internal" href="../library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate"><span class="pre">sorted()</span></code></a> - but if you don’t need the original list, it’s slightly more efficient.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="p">[</span><span class="mi">5</span><span class="p">,</span> <span class="mi">2</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="gp">>>> </span><span class="n">a</span><span class="o">.</span><span class="n">sort</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">a</span> <span class="go">[1, 2, 3, 4, 5]</span> </pre></div> </div> <p>Another difference is that the <code class="xref py py-meth docutils literal notranslate"><span class="pre">list.sort()</span></code> method is only defined for lists. In contrast, the <a class="reference internal" href="../library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate"><span class="pre">sorted()</span></code></a> function accepts any iterable.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">sorted</span><span class="p">({</span><span class="mi">1</span><span class="p">:</span> <span class="s1">'D'</span><span class="p">,</span> <span class="mi">2</span><span class="p">:</span> <span class="s1">'B'</span><span class="p">,</span> <span class="mi">3</span><span class="p">:</span> <span class="s1">'B'</span><span class="p">,</span> <span class="mi">4</span><span class="p">:</span> <span class="s1">'E'</span><span class="p">,</span> <span class="mi">5</span><span class="p">:</span> <span class="s1">'A'</span><span class="p">})</span> <span class="go">[1, 2, 3, 4, 5]</span> </pre></div> </div> </div> <div class="section" id="key-functions"> <h2>Key Functions<a class="headerlink" href="#key-functions" title="Permalink to this headline">¶</a></h2> <p>Starting with Python 2.4, both <code class="xref py py-meth docutils literal notranslate"><span class="pre">list.sort()</span></code> and <a class="reference internal" href="../library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate"><span class="pre">sorted()</span></code></a> added a <em>key</em> parameter to specify a function to be called on each list element prior to making comparisons.</p> <p>For example, here’s a case-insensitive string comparison:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="s2">"This is a test string from Andrew"</span><span class="o">.</span><span class="n">split</span><span class="p">(),</span> <span class="n">key</span><span class="o">=</span><span class="nb">str</span><span class="o">.</span><span class="n">lower</span><span class="p">)</span> <span class="go">['a', 'Andrew', 'from', 'is', 'string', 'test', 'This']</span> </pre></div> </div> <p>The value of the <em>key</em> parameter should be a function that takes a single argument and returns a key to use for sorting purposes. This technique is fast because the key function is called exactly once for each input record.</p> <p>A common pattern is to sort complex objects using some of the object’s indices as keys. For example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">student_tuples</span> <span class="o">=</span> <span class="p">[</span> <span class="gp">... </span> <span class="p">(</span><span class="s1">'john'</span><span class="p">,</span> <span class="s1">'A'</span><span class="p">,</span> <span class="mi">15</span><span class="p">),</span> <span class="gp">... </span> <span class="p">(</span><span class="s1">'jane'</span><span class="p">,</span> <span class="s1">'B'</span><span class="p">,</span> <span class="mi">12</span><span class="p">),</span> <span class="gp">... </span> <span class="p">(</span><span class="s1">'dave'</span><span class="p">,</span> <span class="s1">'B'</span><span class="p">,</span> <span class="mi">10</span><span class="p">),</span> <span class="gp">... </span><span class="p">]</span> <span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">student_tuples</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="k">lambda</span> <span class="n">student</span><span class="p">:</span> <span class="n">student</span><span class="p">[</span><span class="mi">2</span><span class="p">])</span> <span class="c1"># sort by age</span> <span class="go">[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]</span> </pre></div> </div> <p>The same technique works for objects with named attributes. For example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">Student</span><span class="p">:</span> <span class="gp">... </span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">grade</span><span class="p">,</span> <span class="n">age</span><span class="p">):</span> <span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span> <span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">grade</span> <span class="o">=</span> <span class="n">grade</span> <span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">age</span> <span class="o">=</span> <span class="n">age</span> <span class="gp">... </span> <span class="k">def</span> <span class="nf">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="gp">... </span> <span class="k">return</span> <span class="nb">repr</span><span class="p">((</span><span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">grade</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">age</span><span class="p">))</span> </pre></div> </div> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">student_objects</span> <span class="o">=</span> <span class="p">[</span> <span class="gp">... </span> <span class="n">Student</span><span class="p">(</span><span class="s1">'john'</span><span class="p">,</span> <span class="s1">'A'</span><span class="p">,</span> <span class="mi">15</span><span class="p">),</span> <span class="gp">... </span> <span class="n">Student</span><span class="p">(</span><span class="s1">'jane'</span><span class="p">,</span> <span class="s1">'B'</span><span class="p">,</span> <span class="mi">12</span><span class="p">),</span> <span class="gp">... </span> <span class="n">Student</span><span class="p">(</span><span class="s1">'dave'</span><span class="p">,</span> <span class="s1">'B'</span><span class="p">,</span> <span class="mi">10</span><span class="p">),</span> <span class="gp">... </span><span class="p">]</span> <span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">student_objects</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="k">lambda</span> <span class="n">student</span><span class="p">:</span> <span class="n">student</span><span class="o">.</span><span class="n">age</span><span class="p">)</span> <span class="c1"># sort by age</span> <span class="go">[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]</span> </pre></div> </div> </div> <div class="section" id="operator-module-functions"> <h2>Operator Module Functions<a class="headerlink" href="#operator-module-functions" title="Permalink to this headline">¶</a></h2> <p>The key-function patterns shown above are very common, so Python provides convenience functions to make accessor functions easier and faster. The operator module has <a class="reference internal" href="../library/operator.html#operator.itemgetter" title="operator.itemgetter"><code class="xref py py-func docutils literal notranslate"><span class="pre">operator.itemgetter()</span></code></a>, <a class="reference internal" href="../library/operator.html#operator.attrgetter" title="operator.attrgetter"><code class="xref py py-func docutils literal notranslate"><span class="pre">operator.attrgetter()</span></code></a>, and starting in Python 2.5 an <a class="reference internal" href="../library/operator.html#operator.methodcaller" title="operator.methodcaller"><code class="xref py py-func docutils literal notranslate"><span class="pre">operator.methodcaller()</span></code></a> function.</p> <p>Using those functions, the above examples become simpler and faster:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">operator</span> <span class="k">import</span> <span class="n">itemgetter</span><span class="p">,</span> <span class="n">attrgetter</span> </pre></div> </div> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">student_tuples</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="n">itemgetter</span><span class="p">(</span><span class="mi">2</span><span class="p">))</span> <span class="go">[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]</span> </pre></div> </div> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">student_objects</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="n">attrgetter</span><span class="p">(</span><span class="s1">'age'</span><span class="p">))</span> <span class="go">[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]</span> </pre></div> </div> <p>The operator module functions allow multiple levels of sorting. For example, to sort by <em>grade</em> then by <em>age</em>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">student_tuples</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="n">itemgetter</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">))</span> <span class="go">[('john', 'A', 15), ('dave', 'B', 10), ('jane', 'B', 12)]</span> </pre></div> </div> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">student_objects</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="n">attrgetter</span><span class="p">(</span><span class="s1">'grade'</span><span class="p">,</span> <span class="s1">'age'</span><span class="p">))</span> <span class="go">[('john', 'A', 15), ('dave', 'B', 10), ('jane', 'B', 12)]</span> </pre></div> </div> <p>The <a class="reference internal" href="../library/operator.html#operator.methodcaller" title="operator.methodcaller"><code class="xref py py-func docutils literal notranslate"><span class="pre">operator.methodcaller()</span></code></a> function makes method calls with fixed parameters for each object being sorted. For example, the <a class="reference internal" href="../library/stdtypes.html#str.count" title="str.count"><code class="xref py py-meth docutils literal notranslate"><span class="pre">str.count()</span></code></a> method could be used to compute message priority by counting the number of exclamation marks in a message:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">operator</span> <span class="k">import</span> <span class="n">methodcaller</span> <span class="gp">>>> </span><span class="n">messages</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'critical!!!'</span><span class="p">,</span> <span class="s1">'hurry!'</span><span class="p">,</span> <span class="s1">'standby'</span><span class="p">,</span> <span class="s1">'immediate!!'</span><span class="p">]</span> <span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">messages</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="n">methodcaller</span><span class="p">(</span><span class="s1">'count'</span><span class="p">,</span> <span class="s1">'!'</span><span class="p">))</span> <span class="go">['standby', 'hurry!', 'immediate!!', 'critical!!!']</span> </pre></div> </div> </div> <div class="section" id="ascending-and-descending"> <h2>Ascending and Descending<a class="headerlink" href="#ascending-and-descending" title="Permalink to this headline">¶</a></h2> <p>Both <code class="xref py py-meth docutils literal notranslate"><span class="pre">list.sort()</span></code> and <a class="reference internal" href="../library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate"><span class="pre">sorted()</span></code></a> accept a <em>reverse</em> parameter with a boolean value. This is used to flag descending sorts. For example, to get the student data in reverse <em>age</em> order:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">student_tuples</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="n">itemgetter</span><span class="p">(</span><span class="mi">2</span><span class="p">),</span> <span class="n">reverse</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span> <span class="go">[('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]</span> </pre></div> </div> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">student_objects</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="n">attrgetter</span><span class="p">(</span><span class="s1">'age'</span><span class="p">),</span> <span class="n">reverse</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span> <span class="go">[('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]</span> </pre></div> </div> </div> <div class="section" id="sort-stability-and-complex-sorts"> <h2>Sort Stability and Complex Sorts<a class="headerlink" href="#sort-stability-and-complex-sorts" title="Permalink to this headline">¶</a></h2> <p>Starting with Python 2.2, sorts are guaranteed to be <a class="reference external" href="https://en.wikipedia.org/wiki/Sorting_algorithm#Stability">stable</a>. That means that when multiple records have the same key, their original order is preserved.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">data</span> <span class="o">=</span> <span class="p">[(</span><span class="s1">'red'</span><span class="p">,</span> <span class="mi">1</span><span class="p">),</span> <span class="p">(</span><span class="s1">'blue'</span><span class="p">,</span> <span class="mi">1</span><span class="p">),</span> <span class="p">(</span><span class="s1">'red'</span><span class="p">,</span> <span class="mi">2</span><span class="p">),</span> <span class="p">(</span><span class="s1">'blue'</span><span class="p">,</span> <span class="mi">2</span><span class="p">)]</span> <span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="n">itemgetter</span><span class="p">(</span><span class="mi">0</span><span class="p">))</span> <span class="go">[('blue', 1), ('blue', 2), ('red', 1), ('red', 2)]</span> </pre></div> </div> <p>Notice how the two records for <em>blue</em> retain their original order so that <code class="docutils literal notranslate"><span class="pre">('blue',</span> <span class="pre">1)</span></code> is guaranteed to precede <code class="docutils literal notranslate"><span class="pre">('blue',</span> <span class="pre">2)</span></code>.</p> <p>This wonderful property lets you build complex sorts in a series of sorting steps. For example, to sort the student data by descending <em>grade</em> and then ascending <em>age</em>, do the <em>age</em> sort first and then sort again using <em>grade</em>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="nb">sorted</span><span class="p">(</span><span class="n">student_objects</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="n">attrgetter</span><span class="p">(</span><span class="s1">'age'</span><span class="p">))</span> <span class="c1"># sort on secondary key</span> <span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="n">attrgetter</span><span class="p">(</span><span class="s1">'grade'</span><span class="p">),</span> <span class="n">reverse</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span> <span class="c1"># now sort on primary key, descending</span> <span class="go">[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]</span> </pre></div> </div> <p>The <a class="reference external" href="https://en.wikipedia.org/wiki/Timsort">Timsort</a> algorithm used in Python does multiple sorts efficiently because it can take advantage of any ordering already present in a dataset.</p> </div> <div class="section" id="the-old-way-using-decorate-sort-undecorate"> <h2>The Old Way Using Decorate-Sort-Undecorate<a class="headerlink" href="#the-old-way-using-decorate-sort-undecorate" title="Permalink to this headline">¶</a></h2> <p>This idiom is called Decorate-Sort-Undecorate after its three steps:</p> <ul class="simple"> <li>First, the initial list is decorated with new values that control the sort order.</li> <li>Second, the decorated list is sorted.</li> <li>Finally, the decorations are removed, creating a list that contains only the initial values in the new order.</li> </ul> <p>For example, to sort the student data by <em>grade</em> using the DSU approach:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">decorated</span> <span class="o">=</span> <span class="p">[(</span><span class="n">student</span><span class="o">.</span><span class="n">grade</span><span class="p">,</span> <span class="n">i</span><span class="p">,</span> <span class="n">student</span><span class="p">)</span> <span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">student</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="n">student_objects</span><span class="p">)]</span> <span class="gp">>>> </span><span class="n">decorated</span><span class="o">.</span><span class="n">sort</span><span class="p">()</span> <span class="gp">>>> </span><span class="p">[</span><span class="n">student</span> <span class="k">for</span> <span class="n">grade</span><span class="p">,</span> <span class="n">i</span><span class="p">,</span> <span class="n">student</span> <span class="ow">in</span> <span class="n">decorated</span><span class="p">]</span> <span class="c1"># undecorate</span> <span class="go">[('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]</span> </pre></div> </div> <p>This idiom works because tuples are compared lexicographically; the first items are compared; if they are the same then the second items are compared, and so on.</p> <p>It is not strictly necessary in all cases to include the index <em>i</em> in the decorated list, but including it gives two benefits:</p> <ul class="simple"> <li>The sort is stable – if two items have the same key, their order will be preserved in the sorted list.</li> <li>The original items do not have to be comparable because the ordering of the decorated tuples will be determined by at most the first two items. So for example the original list could contain complex numbers which cannot be sorted directly.</li> </ul> <p>Another name for this idiom is <a class="reference external" href="https://en.wikipedia.org/wiki/Schwartzian_transform">Schwartzian transform</a>, after Randal L. Schwartz, who popularized it among Perl programmers.</p> <p>For large lists and lists where the comparison information is expensive to calculate, and Python versions before 2.4, DSU is likely to be the fastest way to sort the list. For 2.4 and later, key functions provide the same functionality.</p> </div> <div class="section" id="the-old-way-using-the-cmp-parameter"> <h2>The Old Way Using the <em>cmp</em> Parameter<a class="headerlink" href="#the-old-way-using-the-cmp-parameter" title="Permalink to this headline">¶</a></h2> <p>Many constructs given in this HOWTO assume Python 2.4 or later. Before that, there was no <a class="reference internal" href="../library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate"><span class="pre">sorted()</span></code></a> builtin and <code class="xref py py-meth docutils literal notranslate"><span class="pre">list.sort()</span></code> took no keyword arguments. Instead, all of the Py2.x versions supported a <em>cmp</em> parameter to handle user specified comparison functions.</p> <p>In Python 3, the <em>cmp</em> parameter was removed entirely (as part of a larger effort to simplify and unify the language, eliminating the conflict between rich comparisons and the <a class="reference internal" href="../reference/datamodel.html#object.__cmp__" title="object.__cmp__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__cmp__()</span></code></a> magic method).</p> <p>In Python 2, <code class="xref py py-meth docutils literal notranslate"><span class="pre">sort()</span></code> allowed an optional function which can be called for doing the comparisons. That function should take two arguments to be compared and then return a negative value for less-than, return zero if they are equal, or return a positive value for greater-than. For example, we can do:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">numeric_compare</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span> <span class="gp">... </span> <span class="k">return</span> <span class="n">x</span> <span class="o">-</span> <span class="n">y</span> <span class="gp">>>> </span><span class="nb">sorted</span><span class="p">([</span><span class="mi">5</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">4</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="nb">cmp</span><span class="o">=</span><span class="n">numeric_compare</span><span class="p">)</span> <span class="c1"># doctest: +SKIP</span> <span class="go">[1, 2, 3, 4, 5]</span> </pre></div> </div> <p>Or you can reverse the order of comparison with:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">reverse_numeric</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span> <span class="gp">... </span> <span class="k">return</span> <span class="n">y</span> <span class="o">-</span> <span class="n">x</span> <span class="gp">>>> </span><span class="nb">sorted</span><span class="p">([</span><span class="mi">5</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">4</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="nb">cmp</span><span class="o">=</span><span class="n">reverse_numeric</span><span class="p">)</span> <span class="c1"># doctest: +SKIP</span> <span class="go">[5, 4, 3, 2, 1]</span> </pre></div> </div> <p>When porting code from Python 2.x to 3.x, the situation can arise when you have the user supplying a comparison function and you need to convert that to a key function. The following wrapper makes that easy to do:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">cmp_to_key</span><span class="p">(</span><span class="n">mycmp</span><span class="p">):</span> <span class="s1">'Convert a cmp= function into a key= function'</span> <span class="k">class</span> <span class="nc">K</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">obj</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">obj</span> <span class="o">=</span> <span class="n">obj</span> <span class="k">def</span> <span class="nf">__lt__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span> <span class="k">return</span> <span class="n">mycmp</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">obj</span><span class="p">,</span> <span class="n">other</span><span class="o">.</span><span class="n">obj</span><span class="p">)</span> <span class="o"><</span> <span class="mi">0</span> <span class="k">def</span> <span class="nf">__gt__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span> <span class="k">return</span> <span class="n">mycmp</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">obj</span><span class="p">,</span> <span class="n">other</span><span class="o">.</span><span class="n">obj</span><span class="p">)</span> <span class="o">></span> <span class="mi">0</span> <span class="k">def</span> <span class="nf">__eq__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span> <span class="k">return</span> <span class="n">mycmp</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">obj</span><span class="p">,</span> <span class="n">other</span><span class="o">.</span><span class="n">obj</span><span class="p">)</span> <span class="o">==</span> <span class="mi">0</span> <span class="k">def</span> <span class="nf">__le__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span> <span class="k">return</span> <span class="n">mycmp</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">obj</span><span class="p">,</span> <span class="n">other</span><span class="o">.</span><span class="n">obj</span><span class="p">)</span> <span class="o"><=</span> <span class="mi">0</span> <span class="k">def</span> <span class="nf">__ge__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span> <span class="k">return</span> <span class="n">mycmp</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">obj</span><span class="p">,</span> <span class="n">other</span><span class="o">.</span><span class="n">obj</span><span class="p">)</span> <span class="o">>=</span> <span class="mi">0</span> <span class="k">def</span> <span class="nf">__ne__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span> <span class="k">return</span> <span class="n">mycmp</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">obj</span><span class="p">,</span> <span class="n">other</span><span class="o">.</span><span class="n">obj</span><span class="p">)</span> <span class="o">!=</span> <span class="mi">0</span> <span class="k">return</span> <span class="n">K</span> </pre></div> </div> <p>To convert to a key function, just wrap the old comparison function:</p> <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">sorted</span><span class="p">([</span><span class="mi">5</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">4</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="n">key</span><span class="o">=</span><span class="n">cmp_to_key</span><span class="p">(</span><span class="n">reverse_numeric</span><span class="p">))</span> <span class="go">[5, 4, 3, 2, 1]</span> </pre></div> </div> <p>In Python 2.7, the <a class="reference internal" href="../library/functools.html#functools.cmp_to_key" title="functools.cmp_to_key"><code class="xref py py-func docutils literal notranslate"><span class="pre">functools.cmp_to_key()</span></code></a> function was added to the functools module.</p> </div> <div class="section" id="odd-and-ends"> <h2>Odd and Ends<a class="headerlink" href="#odd-and-ends" title="Permalink to this headline">¶</a></h2> <ul> <li><p class="first">For locale aware sorting, use <a class="reference internal" href="../library/locale.html#locale.strxfrm" title="locale.strxfrm"><code class="xref py py-func docutils literal notranslate"><span class="pre">locale.strxfrm()</span></code></a> for a key function or <a class="reference internal" href="../library/locale.html#locale.strcoll" title="locale.strcoll"><code class="xref py py-func docutils literal notranslate"><span class="pre">locale.strcoll()</span></code></a> for a comparison function.</p> </li> <li><p class="first">The <em>reverse</em> parameter still maintains sort stability (so that records with equal keys retain their original order). Interestingly, that effect can be simulated without the parameter by using the builtin <a class="reference internal" href="../library/functions.html#reversed" title="reversed"><code class="xref py py-func docutils literal notranslate"><span class="pre">reversed()</span></code></a> function twice:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">data</span> <span class="o">=</span> <span class="p">[(</span><span class="s1">'red'</span><span class="p">,</span> <span class="mi">1</span><span class="p">),</span> <span class="p">(</span><span class="s1">'blue'</span><span class="p">,</span> <span class="mi">1</span><span class="p">),</span> <span class="p">(</span><span class="s1">'red'</span><span class="p">,</span> <span class="mi">2</span><span class="p">),</span> <span class="p">(</span><span class="s1">'blue'</span><span class="p">,</span> <span class="mi">2</span><span class="p">)]</span> <span class="gp">>>> </span><span class="n">standard_way</span> <span class="o">=</span> <span class="nb">sorted</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="n">itemgetter</span><span class="p">(</span><span class="mi">0</span><span class="p">),</span> <span class="n">reverse</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">double_reversed</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="nb">reversed</span><span class="p">(</span><span class="nb">sorted</span><span class="p">(</span><span class="nb">reversed</span><span class="p">(</span><span class="n">data</span><span class="p">),</span> <span class="n">key</span><span class="o">=</span><span class="n">itemgetter</span><span class="p">(</span><span class="mi">0</span><span class="p">))))</span> <span class="gp">>>> </span><span class="k">assert</span> <span class="n">standard_way</span> <span class="o">==</span> <span class="n">double_reversed</span> <span class="gp">>>> </span><span class="n">standard_way</span> <span class="go">[('red', 1), ('red', 2), ('blue', 1), ('blue', 2)]</span> </pre></div> </div> </li> <li><p class="first">To create a standard sort order for a class, just add the appropriate rich comparison methods:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">Student</span><span class="o">.</span><span class="fm">__eq__</span> <span class="o">=</span> <span class="k">lambda</span> <span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">age</span> <span class="o">==</span> <span class="n">other</span><span class="o">.</span><span class="n">age</span> <span class="gp">>>> </span><span class="n">Student</span><span class="o">.</span><span class="fm">__ne__</span> <span class="o">=</span> <span class="k">lambda</span> <span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">age</span> <span class="o">!=</span> <span class="n">other</span><span class="o">.</span><span class="n">age</span> <span class="gp">>>> </span><span class="n">Student</span><span class="o">.</span><span class="fm">__lt__</span> <span class="o">=</span> <span class="k">lambda</span> <span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">age</span> <span class="o"><</span> <span class="n">other</span><span class="o">.</span><span class="n">age</span> <span class="gp">>>> </span><span class="n">Student</span><span class="o">.</span><span class="fm">__le__</span> <span class="o">=</span> <span class="k">lambda</span> <span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">age</span> <span class="o"><=</span> <span class="n">other</span><span class="o">.</span><span class="n">age</span> <span class="gp">>>> </span><span class="n">Student</span><span class="o">.</span><span class="fm">__gt__</span> <span class="o">=</span> <span class="k">lambda</span> <span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">age</span> <span class="o">></span> <span class="n">other</span><span class="o">.</span><span class="n">age</span> <span class="gp">>>> </span><span class="n">Student</span><span class="o">.</span><span class="fm">__ge__</span> <span class="o">=</span> <span class="k">lambda</span> <span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">age</span> <span class="o">>=</span> <span class="n">other</span><span class="o">.</span><span class="n">age</span> <span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">student_objects</span><span class="p">)</span> <span class="go">[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]</span> </pre></div> </div> <p>For general purpose comparisons, the recommended approach is to define all six rich comparison operators. The <a class="reference internal" href="../library/functools.html#functools.total_ordering" title="functools.total_ordering"><code class="xref py py-func docutils literal notranslate"><span class="pre">functools.total_ordering()</span></code></a> class decorator makes this easy to implement.</p> </li> <li><p class="first">Key functions need not depend directly on the objects being sorted. A key function can also access external resources. For instance, if the student grades are stored in a dictionary, they can be used to sort a separate list of student names:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">students</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'dave'</span><span class="p">,</span> <span class="s1">'john'</span><span class="p">,</span> <span class="s1">'jane'</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">grades</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'john'</span><span class="p">:</span> <span class="s1">'F'</span><span class="p">,</span> <span class="s1">'jane'</span><span class="p">:</span><span class="s1">'A'</span><span class="p">,</span> <span class="s1">'dave'</span><span class="p">:</span> <span class="s1">'C'</span><span class="p">}</span> <span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">students</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="n">grades</span><span class="o">.</span><span class="fm">__getitem__</span><span class="p">)</span> <span class="go">['jane', 'dave', 'john']</span> </pre></div> </div> </li> </ul> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table of Contents</a></h3> <ul> <li><a class="reference internal" href="#">Sorting HOW TO</a><ul> <li><a class="reference internal" href="#sorting-basics">Sorting Basics</a></li> <li><a class="reference internal" href="#key-functions">Key Functions</a></li> <li><a class="reference internal" href="#operator-module-functions">Operator Module Functions</a></li> <li><a class="reference internal" href="#ascending-and-descending">Ascending and Descending</a></li> <li><a class="reference internal" href="#sort-stability-and-complex-sorts">Sort Stability and Complex Sorts</a></li> <li><a class="reference internal" href="#the-old-way-using-decorate-sort-undecorate">The Old Way Using Decorate-Sort-Undecorate</a></li> <li><a class="reference internal" href="#the-old-way-using-the-cmp-parameter">The Old Way Using the <em>cmp</em> Parameter</a></li> <li><a class="reference internal" href="#odd-and-ends">Odd and Ends</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="sockets.html" title="previous chapter">Socket Programming HOWTO</a></p> <h4>Next topic</h4> <p class="topless"><a href="unicode.html" title="next chapter">Unicode HOWTO</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../_sources/howto/sorting.rst.txt" rel="nofollow">Show Source</a></li> </ul> </div> <div id="searchbox" style="display: none" role="search"> <h3>Quick search</h3> <div class="searchformwrapper"> <form class="search" action="../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="unicode.html" title="Unicode HOWTO" >next</a> |</li> <li class="right" > <a href="sockets.html" title="Socket Programming HOWTO" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.18 documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >Python HOWTOs</a> »</li> </ul> </div> <div class="footer"> © <a href="../copyright.html">Copyright</a> 1990-2024, Python Software Foundation. <br /> The Python Software Foundation is a non-profit corporation. <a href="https://www.python.org/psf/donations/">Please donate.</a> <br /> Last updated on December 09, 2024. <a href="../bugs.html">Found a bug</a>? <br /> Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.8.5. </div> </body> </html>