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<li><a class="reference internal" href="#">Sorting HOW TO</a><ul>
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<li><a class="reference internal" href="#sorting-basics">Sorting Basics</a></li>
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<li><a class="reference internal" href="#key-functions">Key Functions</a></li>
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<li><a class="reference internal" href="#ascending-and-descending">Ascending and Descending</a></li>
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<li><a class="reference internal" href="#sort-stability-and-complex-sorts">Sort Stability and Complex Sorts</a></li>
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<li><a class="reference internal" href="#decorate-sort-undecorate">Decorate-Sort-Undecorate</a></li>
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<section id="sorting-how-to">
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<span id="sortinghowto"></span><h1>Sorting HOW TO<a class="headerlink" href="#sorting-how-to" title="Permalink to this headline">¶</a></h1>
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<dt class="field-odd">Author</dt>
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<p>Python lists have a built-in <a class="reference internal" href="../library/stdtypes.html#list.sort" title="list.sort"><code class="xref py py-meth docutils literal notranslate"><span class="pre">list.sort()</span></code></a> method that modifies the list
|
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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>
|
||
<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-pycon3 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 <a class="reference internal" href="../library/stdtypes.html#list.sort" title="list.sort"><code class="xref py py-meth docutils literal notranslate"><span class="pre">list.sort()</span></code></a> method. 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-pycon3 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>
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||
<span class="gp">>>> </span><span class="n">a</span><span class="o">.</span><span class="n">sort</span><span class="p">()</span>
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||
<span class="gp">>>> </span><span class="n">a</span>
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||
<span class="go">[1, 2, 3, 4, 5]</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Another difference is that the <a class="reference internal" href="../library/stdtypes.html#list.sort" title="list.sort"><code class="xref py py-meth docutils literal notranslate"><span class="pre">list.sort()</span></code></a> 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-pycon3 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>
|
||
</section>
|
||
<section id="key-functions">
|
||
<h2>Key Functions<a class="headerlink" href="#key-functions" title="Permalink to this headline">¶</a></h2>
|
||
<p>Both <a class="reference internal" href="../library/stdtypes.html#list.sort" title="list.sort"><code class="xref py py-meth docutils literal notranslate"><span class="pre">list.sort()</span></code></a> 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> have a <em>key</em> parameter to specify a
|
||
function (or other callable) 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-pycon3 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>
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||
<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 (or other callable) 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-pycon3 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>
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||
<span class="gp">... </span><span class="p">]</span>
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||
<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>
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||
<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-pycon3 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="fm">__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="fm">__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>
|
||
|
||
<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>
|
||
</section>
|
||
<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
|
||
<a class="reference internal" href="../library/operator.html#module-operator" title="operator: Functions corresponding to the standard operators."><code class="xref py py-mod docutils literal notranslate"><span class="pre">operator</span></code></a> 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">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">attrgetter()</span></code></a>, and a <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">methodcaller()</span></code></a> function.</p>
|
||
<p>Using those functions, the above examples become simpler and faster:</p>
|
||
<div class="highlight-pycon3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">operator</span> <span class="kn">import</span> <span class="n">itemgetter</span><span class="p">,</span> <span class="n">attrgetter</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>
|
||
|
||
<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-pycon3 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>
|
||
|
||
<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>
|
||
</section>
|
||
<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 <a class="reference internal" href="../library/stdtypes.html#list.sort" title="list.sort"><code class="xref py py-meth docutils literal notranslate"><span class="pre">list.sort()</span></code></a> 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-pycon3 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>
|
||
|
||
<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>
|
||
</section>
|
||
<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>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-pycon3 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-pycon3 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>This can be abstracted out into a wrapper function that can take a list and
|
||
tuples of field and order to sort them on multiple passes.</p>
|
||
<div class="highlight-pycon3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">multisort</span><span class="p">(</span><span class="n">xs</span><span class="p">,</span> <span class="n">specs</span><span class="p">):</span>
|
||
<span class="gp">... </span> <span class="k">for</span> <span class="n">key</span><span class="p">,</span> <span class="n">reverse</span> <span class="ow">in</span> <span class="nb">reversed</span><span class="p">(</span><span class="n">specs</span><span class="p">):</span>
|
||
<span class="gp">... </span> <span class="n">xs</span><span class="o">.</span><span class="n">sort</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="n">key</span><span class="p">),</span> <span class="n">reverse</span><span class="o">=</span><span class="n">reverse</span><span class="p">)</span>
|
||
<span class="gp">... </span> <span class="k">return</span> <span class="n">xs</span>
|
||
|
||
<span class="gp">>>> </span><span class="n">multisort</span><span class="p">(</span><span class="nb">list</span><span class="p">(</span><span class="n">student_objects</span><span class="p">),</span> <span class="p">((</span><span class="s1">'grade'</span><span class="p">,</span> <span class="kc">True</span><span class="p">),</span> <span class="p">(</span><span class="s1">'age'</span><span class="p">,</span> <span class="kc">False</span><span class="p">)))</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>
|
||
</section>
|
||
<section id="decorate-sort-undecorate">
|
||
<h2>Decorate-Sort-Undecorate<a class="headerlink" href="#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><p>First, the initial list is decorated with new values that control the sort order.</p></li>
|
||
<li><p>Second, the decorated list is sorted.</p></li>
|
||
<li><p>Finally, the decorations are removed, creating a list that contains only the
|
||
initial values in the new order.</p></li>
|
||
</ul>
|
||
<p>For example, to sort the student data by <em>grade</em> using the DSU approach:</p>
|
||
<div class="doctest 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><p>The sort is stable – if two items have the same key, their order will be
|
||
preserved in the sorted list.</p></li>
|
||
<li><p>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.</p></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>Now that Python sorting provides key-functions, this technique is not often needed.</p>
|
||
</section>
|
||
<section id="comparison-functions">
|
||
<h2>Comparison Functions<a class="headerlink" href="#comparison-functions" title="Permalink to this headline">¶</a></h2>
|
||
<p>Unlike key functions that return an absolute value for sorting, a comparison
|
||
function computes the relative ordering for two inputs.</p>
|
||
<p>For example, a <a class="reference external" href="https://upload.wikimedia.org/wikipedia/commons/1/17/Balance_à_tabac_1850.JPG">balance scale</a>
|
||
compares two samples giving a relative ordering: lighter, equal, or heavier.
|
||
Likewise, a comparison function such as <code class="docutils literal notranslate"><span class="pre">cmp(a,</span> <span class="pre">b)</span></code> will return a negative
|
||
value for less-than, zero if the inputs are equal, or a positive value for
|
||
greater-than.</p>
|
||
<p>It is common to encounter comparison functions when translating algorithms from
|
||
other languages. Also, some libraries provide comparison functions as part of
|
||
their API. For example, <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> is a comparison function.</p>
|
||
<p>To accommodate those situations, Python provides
|
||
<a class="reference internal" href="../library/functools.html#functools.cmp_to_key" title="functools.cmp_to_key"><code class="xref py py-class docutils literal notranslate"><span class="pre">functools.cmp_to_key</span></code></a> to wrap the comparison function
|
||
to make it usable as a key function:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nb">sorted</span><span class="p">(</span><span class="n">words</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">strcoll</span><span class="p">))</span> <span class="c1"># locale-aware sort order</span>
|
||
</pre></div>
|
||
</div>
|
||
</section>
|
||
<section id="odds-and-ends">
|
||
<h2>Odds and Ends<a class="headerlink" href="#odds-and-ends" title="Permalink to this headline">¶</a></h2>
|
||
<ul>
|
||
<li><p>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. This is necessary
|
||
because “alphabetical” sort orderings can vary across cultures even
|
||
if the underlying alphabet is the same.</p></li>
|
||
<li><p>The <em>reverse</em> parameter still maintains sort stability (so that records with
|
||
equal keys retain the 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-pycon3 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>The sort routines use <code class="docutils literal notranslate"><span class="pre"><</span></code> when making comparisons
|
||
between two objects. So, it is easy to add a standard sort order to a class by
|
||
defining an <a class="reference internal" href="../reference/datamodel.html#object.__lt__" title="object.__lt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__lt__()</span></code></a> method:</p>
|
||
<div class="highlight-pycon3 notranslate"><div class="highlight"><pre><span></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="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>However, note that <code class="docutils literal notranslate"><span class="pre"><</span></code> can fall back to using <a class="reference internal" href="../reference/datamodel.html#object.__gt__" title="object.__gt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__gt__()</span></code></a> if
|
||
<a class="reference internal" href="../reference/datamodel.html#object.__lt__" title="object.__lt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__lt__()</span></code></a> is not implemented (see <a class="reference internal" href="../reference/datamodel.html#object.__lt__" title="object.__lt__"><code class="xref py py-func docutils literal notranslate"><span class="pre">object.__lt__()</span></code></a>).</p>
|
||
</li>
|
||
<li><p>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-pycon3 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">newgrades</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">newgrades</span><span class="o">.</span><span class="fm">__getitem__</span><span class="p">)</span>
|
||
<span class="go">['jane', 'dave', 'john']</span>
|
||
</pre></div>
|
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