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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>5. Data Structures — Python 3.6.7 documentation</title> <link rel="stylesheet" href="../_static/pydoctheme.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/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 3.6.7 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="6. Modules" href="modules.html" /> <link rel="prev" title="4. More Control Flow Tools" href="controlflow.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/tutorial/datastructures.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <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="modules.html" title="6. Modules" accesskey="N">next</a> |</li> <li class="right" > <a href="controlflow.html" title="4. More Control Flow Tools" 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">3.6.7 Documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">The Python Tutorial</a> »</li> <li class="right"> <div class="inline-search" style="display: none" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" 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> <script type="text/javascript">$('.inline-search').show(0);</script> | </li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="data-structures"> <span id="tut-structures"></span><h1>5. Data Structures<a class="headerlink" href="#data-structures" title="Permalink to this headline">¶</a></h1> <p>This chapter describes some things you’ve learned about already in more detail, and adds some new things as well.</p> <div class="section" id="more-on-lists"> <span id="tut-morelists"></span><h2>5.1. More on Lists<a class="headerlink" href="#more-on-lists" title="Permalink to this headline">¶</a></h2> <p>The list data type has some more methods. Here are all of the methods of list objects:</p> <dl class="method"> <dt> <code class="descclassname">list.</code><code class="descname">append</code><span class="sig-paren">(</span><em>x</em><span class="sig-paren">)</span></dt> <dd><p>Add an item to the end of the list. Equivalent to <code class="docutils literal notranslate"><span class="pre">a[len(a):]</span> <span class="pre">=</span> <span class="pre">[x]</span></code>.</p> </dd></dl> <dl class="method"> <dt> <code class="descclassname">list.</code><code class="descname">extend</code><span class="sig-paren">(</span><em>iterable</em><span class="sig-paren">)</span></dt> <dd><p>Extend the list by appending all the items from the iterable. Equivalent to <code class="docutils literal notranslate"><span class="pre">a[len(a):]</span> <span class="pre">=</span> <span class="pre">iterable</span></code>.</p> </dd></dl> <dl class="method"> <dt> <code class="descclassname">list.</code><code class="descname">insert</code><span class="sig-paren">(</span><em>i</em>, <em>x</em><span class="sig-paren">)</span></dt> <dd><p>Insert an item at a given position. The first argument is the index of the element before which to insert, so <code class="docutils literal notranslate"><span class="pre">a.insert(0,</span> <span class="pre">x)</span></code> inserts at the front of the list, and <code class="docutils literal notranslate"><span class="pre">a.insert(len(a),</span> <span class="pre">x)</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">a.append(x)</span></code>.</p> </dd></dl> <dl class="method"> <dt> <code class="descclassname">list.</code><code class="descname">remove</code><span class="sig-paren">(</span><em>x</em><span class="sig-paren">)</span></dt> <dd><p>Remove the first item from the list whose value is <em>x</em>. It is an error if there is no such item.</p> </dd></dl> <dl class="method"> <dt> <code class="descclassname">list.</code><code class="descname">pop</code><span class="sig-paren">(</span><span class="optional">[</span><em>i</em><span class="optional">]</span><span class="sig-paren">)</span></dt> <dd><p>Remove the item at the given position in the list, and return it. If no index is specified, <code class="docutils literal notranslate"><span class="pre">a.pop()</span></code> removes and returns the last item in the list. (The square brackets around the <em>i</em> in the method signature denote that the parameter is optional, not that you should type square brackets at that position. You will see this notation frequently in the Python Library Reference.)</p> </dd></dl> <dl class="method"> <dt> <code class="descclassname">list.</code><code class="descname">clear</code><span class="sig-paren">(</span><span class="sig-paren">)</span></dt> <dd><p>Remove all items from the list. Equivalent to <code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">a[:]</span></code>.</p> </dd></dl> <dl class="method"> <dt> <code class="descclassname">list.</code><code class="descname">index</code><span class="sig-paren">(</span><em>x</em><span class="optional">[</span>, <em>start</em><span class="optional">[</span>, <em>end</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span></dt> <dd><p>Return zero-based index in the list of the first item whose value is <em>x</em>. Raises a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> if there is no such item.</p> <p>The optional arguments <em>start</em> and <em>end</em> are interpreted as in the slice notation and are used to limit the search to a particular subsequence of the list. The returned index is computed relative to the beginning of the full sequence rather than the <em>start</em> argument.</p> </dd></dl> <dl class="method"> <dt> <code class="descclassname">list.</code><code class="descname">count</code><span class="sig-paren">(</span><em>x</em><span class="sig-paren">)</span></dt> <dd><p>Return the number of times <em>x</em> appears in the list.</p> </dd></dl> <dl class="method"> <dt> <code class="descclassname">list.</code><code class="descname">sort</code><span class="sig-paren">(</span><em>key=None</em>, <em>reverse=False</em><span class="sig-paren">)</span></dt> <dd><p>Sort the items of the list in place (the arguments can be used for sort customization, see <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> for their explanation).</p> </dd></dl> <dl class="method"> <dt> <code class="descclassname">list.</code><code class="descname">reverse</code><span class="sig-paren">(</span><span class="sig-paren">)</span></dt> <dd><p>Reverse the elements of the list in place.</p> </dd></dl> <dl class="method"> <dt> <code class="descclassname">list.</code><code class="descname">copy</code><span class="sig-paren">(</span><span class="sig-paren">)</span></dt> <dd><p>Return a shallow copy of the list. Equivalent to <code class="docutils literal notranslate"><span class="pre">a[:]</span></code>.</p> </dd></dl> <p>An example that uses most of the list methods:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">fruits</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'orange'</span><span class="p">,</span> <span class="s1">'apple'</span><span class="p">,</span> <span class="s1">'pear'</span><span class="p">,</span> <span class="s1">'banana'</span><span class="p">,</span> <span class="s1">'kiwi'</span><span class="p">,</span> <span class="s1">'apple'</span><span class="p">,</span> <span class="s1">'banana'</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">count</span><span class="p">(</span><span class="s1">'apple'</span><span class="p">)</span> <span class="go">2</span> <span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">count</span><span class="p">(</span><span class="s1">'tangerine'</span><span class="p">)</span> <span class="go">0</span> <span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">index</span><span class="p">(</span><span class="s1">'banana'</span><span class="p">)</span> <span class="go">3</span> <span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">index</span><span class="p">(</span><span class="s1">'banana'</span><span class="p">,</span> <span class="mi">4</span><span class="p">)</span> <span class="c1"># Find next banana starting a position 4</span> <span class="go">6</span> <span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">reverse</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">fruits</span> <span class="go">['banana', 'apple', 'kiwi', 'banana', 'pear', 'apple', 'orange']</span> <span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="s1">'grape'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">fruits</span> <span class="go">['banana', 'apple', 'kiwi', 'banana', 'pear', 'apple', 'orange', 'grape']</span> <span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">sort</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">fruits</span> <span class="go">['apple', 'apple', 'banana', 'banana', 'grape', 'kiwi', 'orange', 'pear']</span> <span class="gp">>>> </span><span class="n">fruits</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span> <span class="go">'pear'</span> </pre></div> </div> <p>You might have noticed that methods like <code class="docutils literal notranslate"><span class="pre">insert</span></code>, <code class="docutils literal notranslate"><span class="pre">remove</span></code> or <code class="docutils literal notranslate"><span class="pre">sort</span></code> that only modify the list have no return value printed – they return the default <code class="docutils literal notranslate"><span class="pre">None</span></code>. <a class="footnote-reference" href="#id3" id="id1">[1]</a> This is a design principle for all mutable data structures in Python.</p> <div class="section" id="using-lists-as-stacks"> <span id="tut-lists-as-stacks"></span><h3>5.1.1. Using Lists as Stacks<a class="headerlink" href="#using-lists-as-stacks" title="Permalink to this headline">¶</a></h3> <p>The list methods make it very easy to use a list as a stack, where the last element added is the first element retrieved (“last-in, first-out”). To add an item to the top of the stack, use <code class="xref py py-meth docutils literal notranslate"><span class="pre">append()</span></code>. To retrieve an item from the top of the stack, use <code class="xref py py-meth docutils literal notranslate"><span class="pre">pop()</span></code> without an explicit index. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">stack</span> <span class="o">=</span> <span class="p">[</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">stack</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">6</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">stack</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">7</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">stack</span> <span class="go">[3, 4, 5, 6, 7]</span> <span class="gp">>>> </span><span class="n">stack</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span> <span class="go">7</span> <span class="gp">>>> </span><span class="n">stack</span> <span class="go">[3, 4, 5, 6]</span> <span class="gp">>>> </span><span class="n">stack</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span> <span class="go">6</span> <span class="gp">>>> </span><span class="n">stack</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span> <span class="go">5</span> <span class="gp">>>> </span><span class="n">stack</span> <span class="go">[3, 4]</span> </pre></div> </div> </div> <div class="section" id="using-lists-as-queues"> <span id="tut-lists-as-queues"></span><h3>5.1.2. Using Lists as Queues<a class="headerlink" href="#using-lists-as-queues" title="Permalink to this headline">¶</a></h3> <p>It is also possible to use a list as a queue, where the first element added is the first element retrieved (“first-in, first-out”); however, lists are not efficient for this purpose. While appends and pops from the end of list are fast, doing inserts or pops from the beginning of a list is slow (because all of the other elements have to be shifted by one).</p> <p>To implement a queue, use <a class="reference internal" href="../library/collections.html#collections.deque" title="collections.deque"><code class="xref py py-class docutils literal notranslate"><span class="pre">collections.deque</span></code></a> which was designed to have fast appends and pops from both ends. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">collections</span> <span class="k">import</span> <span class="n">deque</span> <span class="gp">>>> </span><span class="n">queue</span> <span class="o">=</span> <span class="n">deque</span><span class="p">([</span><span class="s2">"Eric"</span><span class="p">,</span> <span class="s2">"John"</span><span class="p">,</span> <span class="s2">"Michael"</span><span class="p">])</span> <span class="gp">>>> </span><span class="n">queue</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="s2">"Terry"</span><span class="p">)</span> <span class="c1"># Terry arrives</span> <span class="gp">>>> </span><span class="n">queue</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="s2">"Graham"</span><span class="p">)</span> <span class="c1"># Graham arrives</span> <span class="gp">>>> </span><span class="n">queue</span><span class="o">.</span><span class="n">popleft</span><span class="p">()</span> <span class="c1"># The first to arrive now leaves</span> <span class="go">'Eric'</span> <span class="gp">>>> </span><span class="n">queue</span><span class="o">.</span><span class="n">popleft</span><span class="p">()</span> <span class="c1"># The second to arrive now leaves</span> <span class="go">'John'</span> <span class="gp">>>> </span><span class="n">queue</span> <span class="c1"># Remaining queue in order of arrival</span> <span class="go">deque(['Michael', 'Terry', 'Graham'])</span> </pre></div> </div> </div> <div class="section" id="list-comprehensions"> <span id="tut-listcomps"></span><h3>5.1.3. List Comprehensions<a class="headerlink" href="#list-comprehensions" title="Permalink to this headline">¶</a></h3> <p>List comprehensions provide a concise way to create lists. Common applications are to make new lists where each element is the result of some operations applied to each member of another sequence or iterable, or to create a subsequence of those elements that satisfy a certain condition.</p> <p>For example, assume we want to create a list of squares, like:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">squares</span> <span class="o">=</span> <span class="p">[]</span> <span class="gp">>>> </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span> <span class="gp">... </span> <span class="n">squares</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">x</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">squares</span> <span class="go">[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]</span> </pre></div> </div> <p>Note that this creates (or overwrites) a variable named <code class="docutils literal notranslate"><span class="pre">x</span></code> that still exists after the loop completes. We can calculate the list of squares without any side effects using:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">squares</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="nb">map</span><span class="p">(</span><span class="k">lambda</span> <span class="n">x</span><span class="p">:</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span><span class="p">,</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)))</span> </pre></div> </div> <p>or, equivalently:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">squares</span> <span class="o">=</span> <span class="p">[</span><span class="n">x</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)]</span> </pre></div> </div> <p>which is more concise and readable.</p> <p>A list comprehension consists of brackets containing an expression followed by a <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> clause, then zero or more <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> or <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> clauses. The result will be a new list resulting from evaluating the expression in the context of the <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> and <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> clauses which follow it. For example, this listcomp combines the elements of two lists if they are not equal:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="k">for</span> <span class="n">x</span> <span class="ow">in</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="mi">3</span><span class="p">]</span> <span class="k">for</span> <span class="n">y</span> <span class="ow">in</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="k">if</span> <span class="n">x</span> <span class="o">!=</span> <span class="n">y</span><span class="p">]</span> <span class="go">[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]</span> </pre></div> </div> <p>and it’s equivalent to:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">combs</span> <span class="o">=</span> <span class="p">[]</span> <span class="gp">>>> </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</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="mi">3</span><span class="p">]:</span> <span class="gp">... </span> <span class="k">for</span> <span class="n">y</span> <span class="ow">in</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="k">if</span> <span class="n">x</span> <span class="o">!=</span> <span class="n">y</span><span class="p">:</span> <span class="gp">... </span> <span class="n">combs</span><span class="o">.</span><span class="n">append</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="gp">>>> </span><span class="n">combs</span> <span class="go">[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]</span> </pre></div> </div> <p>Note how the order of the <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> and <a class="reference internal" href="../reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> statements is the same in both these snippets.</p> <p>If the expression is a tuple (e.g. the <code class="docutils literal notranslate"><span class="pre">(x,</span> <span class="pre">y)</span></code> in the previous example), it must be parenthesized.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">vec</span> <span class="o">=</span> <span class="p">[</span><span class="o">-</span><span class="mi">4</span><span class="p">,</span> <span class="o">-</span><span class="mi">2</span><span class="p">,</span> <span class="mi">0</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="gp">>>> </span><span class="c1"># create a new list with the values doubled</span> <span class="gp">>>> </span><span class="p">[</span><span class="n">x</span><span class="o">*</span><span class="mi">2</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">vec</span><span class="p">]</span> <span class="go">[-8, -4, 0, 4, 8]</span> <span class="gp">>>> </span><span class="c1"># filter the list to exclude negative numbers</span> <span class="gp">>>> </span><span class="p">[</span><span class="n">x</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">vec</span> <span class="k">if</span> <span class="n">x</span> <span class="o">>=</span> <span class="mi">0</span><span class="p">]</span> <span class="go">[0, 2, 4]</span> <span class="gp">>>> </span><span class="c1"># apply a function to all the elements</span> <span class="gp">>>> </span><span class="p">[</span><span class="nb">abs</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">vec</span><span class="p">]</span> <span class="go">[4, 2, 0, 2, 4]</span> <span class="gp">>>> </span><span class="c1"># call a method on each element</span> <span class="gp">>>> </span><span class="n">freshfruit</span> <span class="o">=</span> <span class="p">[</span><span class="s1">' banana'</span><span class="p">,</span> <span class="s1">' loganberry '</span><span class="p">,</span> <span class="s1">'passion fruit '</span><span class="p">]</span> <span class="gp">>>> </span><span class="p">[</span><span class="n">weapon</span><span class="o">.</span><span class="n">strip</span><span class="p">()</span> <span class="k">for</span> <span class="n">weapon</span> <span class="ow">in</span> <span class="n">freshfruit</span><span class="p">]</span> <span class="go">['banana', 'loganberry', 'passion fruit']</span> <span class="gp">>>> </span><span class="c1"># create a list of 2-tuples like (number, square)</span> <span class="gp">>>> </span><span class="p">[(</span><span class="n">x</span><span class="p">,</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">6</span><span class="p">)]</span> <span class="go">[(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25)]</span> <span class="gp">>>> </span><span class="c1"># the tuple must be parenthesized, otherwise an error is raised</span> <span class="gp">>>> </span><span class="p">[</span><span class="n">x</span><span class="p">,</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">6</span><span class="p">)]</span> <span class="go"> File "<stdin>", line 1, in <module></span> <span class="go"> [x, x**2 for x in range(6)]</span> <span class="go"> ^</span> <span class="go">SyntaxError: invalid syntax</span> <span class="gp">>>> </span><span class="c1"># flatten a list using a listcomp with two 'for'</span> <span class="gp">>>> </span><span class="n">vec</span> <span class="o">=</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="mi">3</span><span class="p">],</span> <span class="p">[</span><span class="mi">4</span><span class="p">,</span><span class="mi">5</span><span class="p">,</span><span class="mi">6</span><span class="p">],</span> <span class="p">[</span><span class="mi">7</span><span class="p">,</span><span class="mi">8</span><span class="p">,</span><span class="mi">9</span><span class="p">]]</span> <span class="gp">>>> </span><span class="p">[</span><span class="n">num</span> <span class="k">for</span> <span class="n">elem</span> <span class="ow">in</span> <span class="n">vec</span> <span class="k">for</span> <span class="n">num</span> <span class="ow">in</span> <span class="n">elem</span><span class="p">]</span> <span class="go">[1, 2, 3, 4, 5, 6, 7, 8, 9]</span> </pre></div> </div> <p>List comprehensions can contain complex expressions and nested functions:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">math</span> <span class="k">import</span> <span class="n">pi</span> <span class="gp">>>> </span><span class="p">[</span><span class="nb">str</span><span class="p">(</span><span class="nb">round</span><span class="p">(</span><span class="n">pi</span><span class="p">,</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">1</span><span class="p">,</span> <span class="mi">6</span><span class="p">)]</span> <span class="go">['3.1', '3.14', '3.142', '3.1416', '3.14159']</span> </pre></div> </div> </div> <div class="section" id="nested-list-comprehensions"> <h3>5.1.4. Nested List Comprehensions<a class="headerlink" href="#nested-list-comprehensions" title="Permalink to this headline">¶</a></h3> <p>The initial expression in a list comprehension can be any arbitrary expression, including another list comprehension.</p> <p>Consider the following example of a 3x4 matrix implemented as a list of 3 lists of length 4:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">matrix</span> <span class="o">=</span> <span class="p">[</span> <span class="gp">... </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="mi">3</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">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">8</span><span class="p">],</span> <span class="gp">... </span> <span class="p">[</span><span class="mi">9</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">11</span><span class="p">,</span> <span class="mi">12</span><span class="p">],</span> <span class="gp">... </span><span class="p">]</span> </pre></div> </div> <p>The following list comprehension will transpose rows and columns:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="p">[[</span><span class="n">row</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="k">for</span> <span class="n">row</span> <span class="ow">in</span> <span class="n">matrix</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">[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]</span> </pre></div> </div> <p>As we saw in the previous section, the nested listcomp is evaluated in the context of the <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> that follows it, so this example is equivalent to:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">transposed</span> <span class="o">=</span> <span class="p">[]</span> <span class="gp">>>> </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="gp">... </span> <span class="n">transposed</span><span class="o">.</span><span class="n">append</span><span class="p">([</span><span class="n">row</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="k">for</span> <span class="n">row</span> <span class="ow">in</span> <span class="n">matrix</span><span class="p">])</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">transposed</span> <span class="go">[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]</span> </pre></div> </div> <p>which, in turn, is the same as:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">transposed</span> <span class="o">=</span> <span class="p">[]</span> <span class="gp">>>> </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="gp">... </span> <span class="c1"># the following 3 lines implement the nested listcomp</span> <span class="gp">... </span> <span class="n">transposed_row</span> <span class="o">=</span> <span class="p">[]</span> <span class="gp">... </span> <span class="k">for</span> <span class="n">row</span> <span class="ow">in</span> <span class="n">matrix</span><span class="p">:</span> <span class="gp">... </span> <span class="n">transposed_row</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">row</span><span class="p">[</span><span class="n">i</span><span class="p">])</span> <span class="gp">... </span> <span class="n">transposed</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">transposed_row</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">transposed</span> <span class="go">[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]</span> </pre></div> </div> <p>In the real world, you should prefer built-in functions to complex flow statements. The <a class="reference internal" href="../library/functions.html#zip" title="zip"><code class="xref py py-func docutils literal notranslate"><span class="pre">zip()</span></code></a> function would do a great job for this use case:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">list</span><span class="p">(</span><span class="nb">zip</span><span class="p">(</span><span class="o">*</span><span class="n">matrix</span><span class="p">))</span> <span class="go">[(1, 5, 9), (2, 6, 10), (3, 7, 11), (4, 8, 12)]</span> </pre></div> </div> <p>See <a class="reference internal" href="controlflow.html#tut-unpacking-arguments"><span class="std std-ref">Unpacking Argument Lists</span></a> for details on the asterisk in this line.</p> </div> </div> <div class="section" id="the-del-statement"> <span id="tut-del"></span><h2>5.2. The <a class="reference internal" href="../reference/simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> statement<a class="headerlink" href="#the-del-statement" title="Permalink to this headline">¶</a></h2> <p>There is a way to remove an item from a list given its index instead of its value: the <a class="reference internal" href="../reference/simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> statement. This differs from the <code class="xref py py-meth docutils literal notranslate"><span class="pre">pop()</span></code> method which returns a value. The <a class="reference internal" href="../reference/simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> statement can also be used to remove slices from a list or clear the entire list (which we did earlier by assignment of an empty list to the slice). For example:</p> <div class="highlight-python3 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="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mf">66.25</span><span class="p">,</span> <span class="mi">333</span><span class="p">,</span> <span class="mi">333</span><span class="p">,</span> <span class="mf">1234.5</span><span class="p">]</span> <span class="gp">>>> </span><span class="k">del</span> <span class="n">a</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">a</span> <span class="go">[1, 66.25, 333, 333, 1234.5]</span> <span class="gp">>>> </span><span class="k">del</span> <span class="n">a</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="gp">>>> </span><span class="n">a</span> <span class="go">[1, 66.25, 1234.5]</span> <span class="gp">>>> </span><span class="k">del</span> <span class="n">a</span><span class="p">[:]</span> <span class="gp">>>> </span><span class="n">a</span> <span class="go">[]</span> </pre></div> </div> <p><a class="reference internal" href="../reference/simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> can also be used to delete entire variables:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">del</span> <span class="n">a</span> </pre></div> </div> <p>Referencing the name <code class="docutils literal notranslate"><span class="pre">a</span></code> hereafter is an error (at least until another value is assigned to it). We’ll find other uses for <a class="reference internal" href="../reference/simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> later.</p> </div> <div class="section" id="tuples-and-sequences"> <span id="tut-tuples"></span><h2>5.3. Tuples and Sequences<a class="headerlink" href="#tuples-and-sequences" title="Permalink to this headline">¶</a></h2> <p>We saw that lists and strings have many common properties, such as indexing and slicing operations. They are two examples of <em>sequence</em> data types (see <a class="reference internal" href="../library/stdtypes.html#typesseq"><span class="std std-ref">Sequence Types — list, tuple, range</span></a>). Since Python is an evolving language, other sequence data types may be added. There is also another standard sequence data type: the <em>tuple</em>.</p> <p>A tuple consists of a number of values separated by commas, for instance:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">t</span> <span class="o">=</span> <span class="mi">12345</span><span class="p">,</span> <span class="mi">54321</span><span class="p">,</span> <span class="s1">'hello!'</span> <span class="gp">>>> </span><span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="go">12345</span> <span class="gp">>>> </span><span class="n">t</span> <span class="go">(12345, 54321, 'hello!')</span> <span class="gp">>>> </span><span class="c1"># Tuples may be nested:</span> <span class="gp">... </span><span class="n">u</span> <span class="o">=</span> <span class="n">t</span><span class="p">,</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="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">u</span> <span class="go">((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))</span> <span class="gp">>>> </span><span class="c1"># Tuples are immutable:</span> <span class="gp">... </span><span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">88888</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> <span class="gr">TypeError</span>: <span class="n">'tuple' object does not support item assignment</span> <span class="gp">>>> </span><span class="c1"># but they can contain mutable objects:</span> <span class="gp">... </span><span class="n">v</span> <span class="o">=</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="mi">3</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">1</span><span class="p">])</span> <span class="gp">>>> </span><span class="n">v</span> <span class="go">([1, 2, 3], [3, 2, 1])</span> </pre></div> </div> <p>As you see, on output tuples are always enclosed in parentheses, so that nested tuples are interpreted correctly; they may be input with or without surrounding parentheses, although often parentheses are necessary anyway (if the tuple is part of a larger expression). It is not possible to assign to the individual items of a tuple, however it is possible to create tuples which contain mutable objects, such as lists.</p> <p>Though tuples may seem similar to lists, they are often used in different situations and for different purposes. Tuples are <a class="reference internal" href="../glossary.html#term-immutable"><span class="xref std std-term">immutable</span></a>, and usually contain a heterogeneous sequence of elements that are accessed via unpacking (see later in this section) or indexing (or even by attribute in the case of <a class="reference internal" href="../library/collections.html#collections.namedtuple" title="collections.namedtuple"><code class="xref py py-func docutils literal notranslate"><span class="pre">namedtuples</span></code></a>). Lists are <a class="reference internal" href="../glossary.html#term-mutable"><span class="xref std std-term">mutable</span></a>, and their elements are usually homogeneous and are accessed by iterating over the list.</p> <p>A special problem is the construction of tuples containing 0 or 1 items: the syntax has some extra quirks to accommodate these. Empty tuples are constructed by an empty pair of parentheses; a tuple with one item is constructed by following a value with a comma (it is not sufficient to enclose a single value in parentheses). Ugly, but effective. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">empty</span> <span class="o">=</span> <span class="p">()</span> <span class="gp">>>> </span><span class="n">singleton</span> <span class="o">=</span> <span class="s1">'hello'</span><span class="p">,</span> <span class="c1"># <-- note trailing comma</span> <span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">empty</span><span class="p">)</span> <span class="go">0</span> <span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">singleton</span><span class="p">)</span> <span class="go">1</span> <span class="gp">>>> </span><span class="n">singleton</span> <span class="go">('hello',)</span> </pre></div> </div> <p>The statement <code class="docutils literal notranslate"><span class="pre">t</span> <span class="pre">=</span> <span class="pre">12345,</span> <span class="pre">54321,</span> <span class="pre">'hello!'</span></code> is an example of <em>tuple packing</em>: the values <code class="docutils literal notranslate"><span class="pre">12345</span></code>, <code class="docutils literal notranslate"><span class="pre">54321</span></code> and <code class="docutils literal notranslate"><span class="pre">'hello!'</span></code> are packed together in a tuple. The reverse operation is also possible:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">z</span> <span class="o">=</span> <span class="n">t</span> </pre></div> </div> <p>This is called, appropriately enough, <em>sequence unpacking</em> and works for any sequence on the right-hand side. Sequence unpacking requires that there are as many variables on the left side of the equals sign as there are elements in the sequence. Note that multiple assignment is really just a combination of tuple packing and sequence unpacking.</p> </div> <div class="section" id="sets"> <span id="tut-sets"></span><h2>5.4. Sets<a class="headerlink" href="#sets" title="Permalink to this headline">¶</a></h2> <p>Python also includes a data type for <em>sets</em>. A set is an unordered collection with no duplicate elements. Basic uses include membership testing and eliminating duplicate entries. Set objects also support mathematical operations like union, intersection, difference, and symmetric difference.</p> <p>Curly braces or the <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-func docutils literal notranslate"><span class="pre">set()</span></code></a> function can be used to create sets. Note: to create an empty set you have to use <code class="docutils literal notranslate"><span class="pre">set()</span></code>, not <code class="docutils literal notranslate"><span class="pre">{}</span></code>; the latter creates an empty dictionary, a data structure that we discuss in the next section.</p> <p>Here is a brief demonstration:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">basket</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'apple'</span><span class="p">,</span> <span class="s1">'orange'</span><span class="p">,</span> <span class="s1">'apple'</span><span class="p">,</span> <span class="s1">'pear'</span><span class="p">,</span> <span class="s1">'orange'</span><span class="p">,</span> <span class="s1">'banana'</span><span class="p">}</span> <span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">basket</span><span class="p">)</span> <span class="c1"># show that duplicates have been removed</span> <span class="go">{'orange', 'banana', 'pear', 'apple'}</span> <span class="gp">>>> </span><span class="s1">'orange'</span> <span class="ow">in</span> <span class="n">basket</span> <span class="c1"># fast membership testing</span> <span class="go">True</span> <span class="gp">>>> </span><span class="s1">'crabgrass'</span> <span class="ow">in</span> <span class="n">basket</span> <span class="go">False</span> <span class="gp">>>> </span><span class="c1"># Demonstrate set operations on unique letters from two words</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="nb">set</span><span class="p">(</span><span class="s1">'abracadabra'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">b</span> <span class="o">=</span> <span class="nb">set</span><span class="p">(</span><span class="s1">'alacazam'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">a</span> <span class="c1"># unique letters in a</span> <span class="go">{'a', 'r', 'b', 'c', 'd'}</span> <span class="gp">>>> </span><span class="n">a</span> <span class="o">-</span> <span class="n">b</span> <span class="c1"># letters in a but not in b</span> <span class="go">{'r', 'd', 'b'}</span> <span class="gp">>>> </span><span class="n">a</span> <span class="o">|</span> <span class="n">b</span> <span class="c1"># letters in a or b or both</span> <span class="go">{'a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'}</span> <span class="gp">>>> </span><span class="n">a</span> <span class="o">&</span> <span class="n">b</span> <span class="c1"># letters in both a and b</span> <span class="go">{'a', 'c'}</span> <span class="gp">>>> </span><span class="n">a</span> <span class="o">^</span> <span class="n">b</span> <span class="c1"># letters in a or b but not both</span> <span class="go">{'r', 'd', 'b', 'm', 'z', 'l'}</span> </pre></div> </div> <p>Similarly to <a class="reference internal" href="#tut-listcomps"><span class="std std-ref">list comprehensions</span></a>, set comprehensions are also supported:</p> <div class="highlight-python3 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="n">x</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="s1">'abracadabra'</span> <span class="k">if</span> <span class="n">x</span> <span class="ow">not</span> <span class="ow">in</span> <span class="s1">'abc'</span><span class="p">}</span> <span class="gp">>>> </span><span class="n">a</span> <span class="go">{'r', 'd'}</span> </pre></div> </div> </div> <div class="section" id="dictionaries"> <span id="tut-dictionaries"></span><h2>5.5. Dictionaries<a class="headerlink" href="#dictionaries" title="Permalink to this headline">¶</a></h2> <p>Another useful data type built into Python is the <em>dictionary</em> (see <a class="reference internal" href="../library/stdtypes.html#typesmapping"><span class="std std-ref">Mapping Types — dict</span></a>). Dictionaries are sometimes found in other languages as “associative memories” or “associative arrays”. Unlike sequences, which are indexed by a range of numbers, dictionaries are indexed by <em>keys</em>, which can be any immutable type; strings and numbers can always be keys. Tuples can be used as keys if they contain only strings, numbers, or tuples; if a tuple contains any mutable object either directly or indirectly, it cannot be used as a key. You can’t use lists as keys, since lists can be modified in place using index assignments, slice assignments, or methods like <code class="xref py py-meth docutils literal notranslate"><span class="pre">append()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">extend()</span></code>.</p> <p>It is best to think of a dictionary as an unordered set of <em>key: value</em> pairs, with the requirement that the keys are unique (within one dictionary). A pair of braces creates an empty dictionary: <code class="docutils literal notranslate"><span class="pre">{}</span></code>. Placing a comma-separated list of key:value pairs within the braces adds initial key:value pairs to the dictionary; this is also the way dictionaries are written on output.</p> <p>The main operations on a dictionary are storing a value with some key and extracting the value given the key. It is also possible to delete a key:value pair with <code class="docutils literal notranslate"><span class="pre">del</span></code>. If you store using a key that is already in use, the old value associated with that key is forgotten. It is an error to extract a value using a non-existent key.</p> <p>Performing <code class="docutils literal notranslate"><span class="pre">list(d.keys())</span></code> on a dictionary returns a list of all the keys used in the dictionary, in arbitrary order (if you want it sorted, just use <code class="docutils literal notranslate"><span class="pre">sorted(d.keys())</span></code> instead). <a class="footnote-reference" href="#id4" id="id2">[2]</a> To check whether a single key is in the dictionary, use the <a class="reference internal" href="../reference/expressions.html#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a> keyword.</p> <p>Here is a small example using a dictionary:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">tel</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'jack'</span><span class="p">:</span> <span class="mi">4098</span><span class="p">,</span> <span class="s1">'sape'</span><span class="p">:</span> <span class="mi">4139</span><span class="p">}</span> <span class="gp">>>> </span><span class="n">tel</span><span class="p">[</span><span class="s1">'guido'</span><span class="p">]</span> <span class="o">=</span> <span class="mi">4127</span> <span class="gp">>>> </span><span class="n">tel</span> <span class="go">{'sape': 4139, 'guido': 4127, 'jack': 4098}</span> <span class="gp">>>> </span><span class="n">tel</span><span class="p">[</span><span class="s1">'jack'</span><span class="p">]</span> <span class="go">4098</span> <span class="gp">>>> </span><span class="k">del</span> <span class="n">tel</span><span class="p">[</span><span class="s1">'sape'</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">tel</span><span class="p">[</span><span class="s1">'irv'</span><span class="p">]</span> <span class="o">=</span> <span class="mi">4127</span> <span class="gp">>>> </span><span class="n">tel</span> <span class="go">{'guido': 4127, 'irv': 4127, 'jack': 4098}</span> <span class="gp">>>> </span><span class="nb">list</span><span class="p">(</span><span class="n">tel</span><span class="o">.</span><span class="n">keys</span><span class="p">())</span> <span class="go">['irv', 'guido', 'jack']</span> <span class="gp">>>> </span><span class="nb">sorted</span><span class="p">(</span><span class="n">tel</span><span class="o">.</span><span class="n">keys</span><span class="p">())</span> <span class="go">['guido', 'irv', 'jack']</span> <span class="gp">>>> </span><span class="s1">'guido'</span> <span class="ow">in</span> <span class="n">tel</span> <span class="go">True</span> <span class="gp">>>> </span><span class="s1">'jack'</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">tel</span> <span class="go">False</span> </pre></div> </div> <p>The <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-func docutils literal notranslate"><span class="pre">dict()</span></code></a> constructor builds dictionaries directly from sequences of key-value pairs:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">dict</span><span class="p">([(</span><span class="s1">'sape'</span><span class="p">,</span> <span class="mi">4139</span><span class="p">),</span> <span class="p">(</span><span class="s1">'guido'</span><span class="p">,</span> <span class="mi">4127</span><span class="p">),</span> <span class="p">(</span><span class="s1">'jack'</span><span class="p">,</span> <span class="mi">4098</span><span class="p">)])</span> <span class="go">{'sape': 4139, 'jack': 4098, 'guido': 4127}</span> </pre></div> </div> <p>In addition, dict comprehensions can be used to create dictionaries from arbitrary key and value expressions:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="p">{</span><span class="n">x</span><span class="p">:</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</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">6</span><span class="p">)}</span> <span class="go">{2: 4, 4: 16, 6: 36}</span> </pre></div> </div> <p>When the keys are simple strings, it is sometimes easier to specify pairs using keyword arguments:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">dict</span><span class="p">(</span><span class="n">sape</span><span class="o">=</span><span class="mi">4139</span><span class="p">,</span> <span class="n">guido</span><span class="o">=</span><span class="mi">4127</span><span class="p">,</span> <span class="n">jack</span><span class="o">=</span><span class="mi">4098</span><span class="p">)</span> <span class="go">{'sape': 4139, 'jack': 4098, 'guido': 4127}</span> </pre></div> </div> </div> <div class="section" id="looping-techniques"> <span id="tut-loopidioms"></span><h2>5.6. Looping Techniques<a class="headerlink" href="#looping-techniques" title="Permalink to this headline">¶</a></h2> <p>When looping through dictionaries, the key and corresponding value can be retrieved at the same time using the <code class="xref py py-meth docutils literal notranslate"><span class="pre">items()</span></code> method.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">knights</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'gallahad'</span><span class="p">:</span> <span class="s1">'the pure'</span><span class="p">,</span> <span class="s1">'robin'</span><span class="p">:</span> <span class="s1">'the brave'</span><span class="p">}</span> <span class="gp">>>> </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">knights</span><span class="o">.</span><span class="n">items</span><span class="p">():</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="p">)</span> <span class="gp">...</span> <span class="go">gallahad the pure</span> <span class="go">robin the brave</span> </pre></div> </div> <p>When looping through a sequence, the position index and corresponding value can be retrieved at the same time using the <a class="reference internal" href="../library/functions.html#enumerate" title="enumerate"><code class="xref py py-func docutils literal notranslate"><span class="pre">enumerate()</span></code></a> function.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">v</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">([</span><span class="s1">'tic'</span><span class="p">,</span> <span class="s1">'tac'</span><span class="p">,</span> <span class="s1">'toe'</span><span class="p">]):</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="n">v</span><span class="p">)</span> <span class="gp">...</span> <span class="go">0 tic</span> <span class="go">1 tac</span> <span class="go">2 toe</span> </pre></div> </div> <p>To loop over two or more sequences at the same time, the entries can be paired with the <a class="reference internal" href="../library/functions.html#zip" title="zip"><code class="xref py py-func docutils literal notranslate"><span class="pre">zip()</span></code></a> function.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">questions</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'name'</span><span class="p">,</span> <span class="s1">'quest'</span><span class="p">,</span> <span class="s1">'favorite color'</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">answers</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'lancelot'</span><span class="p">,</span> <span class="s1">'the holy grail'</span><span class="p">,</span> <span class="s1">'blue'</span><span class="p">]</span> <span class="gp">>>> </span><span class="k">for</span> <span class="n">q</span><span class="p">,</span> <span class="n">a</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="n">questions</span><span class="p">,</span> <span class="n">answers</span><span class="p">):</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'What is your </span><span class="si">{0}</span><span class="s1">? It is </span><span class="si">{1}</span><span class="s1">.'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">q</span><span class="p">,</span> <span class="n">a</span><span class="p">))</span> <span class="gp">...</span> <span class="go">What is your name? It is lancelot.</span> <span class="go">What is your quest? It is the holy grail.</span> <span class="go">What is your favorite color? It is blue.</span> </pre></div> </div> <p>To loop over a sequence in reverse, first specify the sequence in a forward direction and then call the <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.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">reversed</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">2</span><span class="p">)):</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="gp">...</span> <span class="go">9</span> <span class="go">7</span> <span class="go">5</span> <span class="go">3</span> <span class="go">1</span> </pre></div> </div> <p>To loop over a sequence in sorted order, use 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 which returns a new sorted list while leaving the source unaltered.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">basket</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'apple'</span><span class="p">,</span> <span class="s1">'orange'</span><span class="p">,</span> <span class="s1">'apple'</span><span class="p">,</span> <span class="s1">'pear'</span><span class="p">,</span> <span class="s1">'orange'</span><span class="p">,</span> <span class="s1">'banana'</span><span class="p">]</span> <span class="gp">>>> </span><span class="k">for</span> <span class="n">f</span> <span class="ow">in</span> <span class="nb">sorted</span><span class="p">(</span><span class="nb">set</span><span class="p">(</span><span class="n">basket</span><span class="p">)):</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">f</span><span class="p">)</span> <span class="gp">...</span> <span class="go">apple</span> <span class="go">banana</span> <span class="go">orange</span> <span class="go">pear</span> </pre></div> </div> <p>It is sometimes tempting to change a list while you are looping over it; however, it is often simpler and safer to create a new list instead.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">math</span> <span class="gp">>>> </span><span class="n">raw_data</span> <span class="o">=</span> <span class="p">[</span><span class="mf">56.2</span><span class="p">,</span> <span class="nb">float</span><span class="p">(</span><span class="s1">'NaN'</span><span class="p">),</span> <span class="mf">51.7</span><span class="p">,</span> <span class="mf">55.3</span><span class="p">,</span> <span class="mf">52.5</span><span class="p">,</span> <span class="nb">float</span><span class="p">(</span><span class="s1">'NaN'</span><span class="p">),</span> <span class="mf">47.8</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">filtered_data</span> <span class="o">=</span> <span class="p">[]</span> <span class="gp">>>> </span><span class="k">for</span> <span class="n">value</span> <span class="ow">in</span> <span class="n">raw_data</span><span class="p">:</span> <span class="gp">... </span> <span class="k">if</span> <span class="ow">not</span> <span class="n">math</span><span class="o">.</span><span class="n">isnan</span><span class="p">(</span><span class="n">value</span><span class="p">):</span> <span class="gp">... </span> <span class="n">filtered_data</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">value</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">filtered_data</span> <span class="go">[56.2, 51.7, 55.3, 52.5, 47.8]</span> </pre></div> </div> </div> <div class="section" id="more-on-conditions"> <span id="tut-conditions"></span><h2>5.7. More on Conditions<a class="headerlink" href="#more-on-conditions" title="Permalink to this headline">¶</a></h2> <p>The conditions used in <code class="docutils literal notranslate"><span class="pre">while</span></code> and <code class="docutils literal notranslate"><span class="pre">if</span></code> statements can contain any operators, not just comparisons.</p> <p>The comparison operators <code class="docutils literal notranslate"><span class="pre">in</span></code> and <code class="docutils literal notranslate"><span class="pre">not</span> <span class="pre">in</span></code> check whether a value occurs (does not occur) in a sequence. The operators <code class="docutils literal notranslate"><span class="pre">is</span></code> and <code class="docutils literal notranslate"><span class="pre">is</span> <span class="pre">not</span></code> compare whether two objects are really the same object; this only matters for mutable objects like lists. All comparison operators have the same priority, which is lower than that of all numerical operators.</p> <p>Comparisons can be chained. For example, <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre"><</span> <span class="pre">b</span> <span class="pre">==</span> <span class="pre">c</span></code> tests whether <code class="docutils literal notranslate"><span class="pre">a</span></code> is less than <code class="docutils literal notranslate"><span class="pre">b</span></code> and moreover <code class="docutils literal notranslate"><span class="pre">b</span></code> equals <code class="docutils literal notranslate"><span class="pre">c</span></code>.</p> <p>Comparisons may be combined using the Boolean operators <code class="docutils literal notranslate"><span class="pre">and</span></code> and <code class="docutils literal notranslate"><span class="pre">or</span></code>, and the outcome of a comparison (or of any other Boolean expression) may be negated with <code class="docutils literal notranslate"><span class="pre">not</span></code>. These have lower priorities than comparison operators; between them, <code class="docutils literal notranslate"><span class="pre">not</span></code> has the highest priority and <code class="docutils literal notranslate"><span class="pre">or</span></code> the lowest, so that <code class="docutils literal notranslate"><span class="pre">A</span> <span class="pre">and</span> <span class="pre">not</span> <span class="pre">B</span> <span class="pre">or</span> <span class="pre">C</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">(A</span> <span class="pre">and</span> <span class="pre">(not</span> <span class="pre">B))</span> <span class="pre">or</span> <span class="pre">C</span></code>. As always, parentheses can be used to express the desired composition.</p> <p>The Boolean operators <code class="docutils literal notranslate"><span class="pre">and</span></code> and <code class="docutils literal notranslate"><span class="pre">or</span></code> are so-called <em>short-circuit</em> operators: their arguments are evaluated from left to right, and evaluation stops as soon as the outcome is determined. For example, if <code class="docutils literal notranslate"><span class="pre">A</span></code> and <code class="docutils literal notranslate"><span class="pre">C</span></code> are true but <code class="docutils literal notranslate"><span class="pre">B</span></code> is false, <code class="docutils literal notranslate"><span class="pre">A</span> <span class="pre">and</span> <span class="pre">B</span> <span class="pre">and</span> <span class="pre">C</span></code> does not evaluate the expression <code class="docutils literal notranslate"><span class="pre">C</span></code>. When used as a general value and not as a Boolean, the return value of a short-circuit operator is the last evaluated argument.</p> <p>It is possible to assign the result of a comparison or other Boolean expression to a variable. For example,</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">string1</span><span class="p">,</span> <span class="n">string2</span><span class="p">,</span> <span class="n">string3</span> <span class="o">=</span> <span class="s1">''</span><span class="p">,</span> <span class="s1">'Trondheim'</span><span class="p">,</span> <span class="s1">'Hammer Dance'</span> <span class="gp">>>> </span><span class="n">non_null</span> <span class="o">=</span> <span class="n">string1</span> <span class="ow">or</span> <span class="n">string2</span> <span class="ow">or</span> <span class="n">string3</span> <span class="gp">>>> </span><span class="n">non_null</span> <span class="go">'Trondheim'</span> </pre></div> </div> <p>Note that in Python, unlike C, assignment cannot occur inside expressions. C programmers may grumble about this, but it avoids a common class of problems encountered in C programs: typing <code class="docutils literal notranslate"><span class="pre">=</span></code> in an expression when <code class="docutils literal notranslate"><span class="pre">==</span></code> was intended.</p> </div> <div class="section" id="comparing-sequences-and-other-types"> <span id="tut-comparing"></span><h2>5.8. Comparing Sequences and Other Types<a class="headerlink" href="#comparing-sequences-and-other-types" title="Permalink to this headline">¶</a></h2> <p>Sequence objects may be compared to other objects with the same sequence type. The comparison uses <em>lexicographical</em> ordering: first the first two items are compared, and if they differ this determines the outcome of the comparison; if they are equal, the next two items are compared, and so on, until either sequence is exhausted. If two items to be compared are themselves sequences of the same type, the lexicographical comparison is carried out recursively. If all items of two sequences compare equal, the sequences are considered equal. If one sequence is an initial sub-sequence of the other, the shorter sequence is the smaller (lesser) one. Lexicographical ordering for strings uses the Unicode code point number to order individual characters. Some examples of comparisons between sequences of the same type:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></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="mi">3</span><span class="p">)</span> <span class="o"><</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="mi">4</span><span class="p">)</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="mi">3</span><span class="p">]</span> <span class="o"><</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="mi">4</span><span class="p">]</span> <span class="s1">'ABC'</span> <span class="o"><</span> <span class="s1">'C'</span> <span class="o"><</span> <span class="s1">'Pascal'</span> <span class="o"><</span> <span class="s1">'Python'</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="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">)</span> <span class="o"><</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="mi">4</span><span class="p">)</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="o"><</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="o">-</span><span class="mi">1</span><span class="p">)</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="mi">3</span><span class="p">)</span> <span class="o">==</span> <span class="p">(</span><span class="mf">1.0</span><span class="p">,</span> <span class="mf">2.0</span><span class="p">,</span> <span class="mf">3.0</span><span class="p">)</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="p">(</span><span class="s1">'aa'</span><span class="p">,</span> <span class="s1">'ab'</span><span class="p">))</span> <span class="o"><</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="p">(</span><span class="s1">'abc'</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">),</span> <span class="mi">4</span><span class="p">)</span> </pre></div> </div> <p>Note that comparing objects of different types with <code class="docutils literal notranslate"><span class="pre"><</span></code> or <code class="docutils literal notranslate"><span class="pre">></span></code> is legal provided that the objects have appropriate comparison methods. For example, mixed numeric types are compared according to their numeric value, so 0 equals 0.0, etc. Otherwise, rather than providing an arbitrary ordering, the interpreter will raise a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> exception.</p> <p class="rubric">Footnotes</p> <table class="docutils footnote" frame="void" id="id3" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>Other languages may return the mutated object, which allows method chaining, such as <code class="docutils literal notranslate"><span class="pre">d->insert("a")->remove("b")->sort();</span></code>.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id4" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id2">[2]</a></td><td>Calling <code class="docutils literal notranslate"><span class="pre">d.keys()</span></code> will return a <em class="dfn">dictionary view</em> object. It supports operations like membership test and iteration, but its contents are not independent of the original dictionary – it is only a <em>view</em>.</td></tr> </tbody> </table> </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="#">5. Data Structures</a><ul> <li><a class="reference internal" href="#more-on-lists">5.1. More on Lists</a><ul> <li><a class="reference internal" href="#using-lists-as-stacks">5.1.1. Using Lists as Stacks</a></li> <li><a class="reference internal" href="#using-lists-as-queues">5.1.2. Using Lists as Queues</a></li> <li><a class="reference internal" href="#list-comprehensions">5.1.3. List Comprehensions</a></li> <li><a class="reference internal" href="#nested-list-comprehensions">5.1.4. Nested List Comprehensions</a></li> </ul> </li> <li><a class="reference internal" href="#the-del-statement">5.2. The <code class="docutils literal notranslate"><span class="pre">del</span></code> statement</a></li> <li><a class="reference internal" href="#tuples-and-sequences">5.3. Tuples and Sequences</a></li> <li><a class="reference internal" href="#sets">5.4. Sets</a></li> <li><a class="reference internal" href="#dictionaries">5.5. Dictionaries</a></li> <li><a class="reference internal" href="#looping-techniques">5.6. Looping Techniques</a></li> <li><a class="reference internal" href="#more-on-conditions">5.7. More on Conditions</a></li> <li><a class="reference internal" href="#comparing-sequences-and-other-types">5.8. Comparing Sequences and Other Types</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="controlflow.html" title="previous chapter">4. 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