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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>4. More Control Flow Tools — 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="5. Data Structures" href="datastructures.html" /> <link rel="prev" title="3. An Informal Introduction to Python" href="introduction.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/tutorial/controlflow.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="datastructures.html" title="5. Data Structures" accesskey="N">next</a> |</li> <li class="right" > <a href="introduction.html" title="3. An Informal Introduction to Python" 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="more-control-flow-tools"> <span id="tut-morecontrol"></span><h1>4. More Control Flow Tools<a class="headerlink" href="#more-control-flow-tools" title="Permalink to this headline">¶</a></h1> <p>Besides the <a class="reference internal" href="../reference/compound_stmts.html#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a> statement just introduced, Python knows the usual control flow statements known from other languages, with some twists.</p> <div class="section" id="if-statements"> <span id="tut-if"></span><h2>4.1. <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<a class="headerlink" href="#if-statements" title="Permalink to this headline">¶</a></h2> <p>Perhaps the most well-known statement type is the <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> statement. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="nb">input</span><span class="p">(</span><span class="s2">"Please enter an integer: "</span><span class="p">))</span> <span class="go">Please enter an integer: 42</span> <span class="gp">>>> </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="gp">... </span> <span class="n">x</span> <span class="o">=</span> <span class="mi">0</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'Negative changed to zero'</span><span class="p">)</span> <span class="gp">... </span><span class="k">elif</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'Zero'</span><span class="p">)</span> <span class="gp">... </span><span class="k">elif</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'Single'</span><span class="p">)</span> <span class="gp">... </span><span class="k">else</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'More'</span><span class="p">)</span> <span class="gp">...</span> <span class="go">More</span> </pre></div> </div> <p>There can be zero or more <a class="reference internal" href="../reference/compound_stmts.html#elif"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">elif</span></code></a> parts, and the <a class="reference internal" href="../reference/compound_stmts.html#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> part is optional. The keyword ‘<a class="reference internal" href="../reference/compound_stmts.html#elif"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">elif</span></code></a>’ is short for ‘else if’, and is useful to avoid excessive indentation. An <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> … <a class="reference internal" href="../reference/compound_stmts.html#elif"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">elif</span></code></a> … <a class="reference internal" href="../reference/compound_stmts.html#elif"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">elif</span></code></a> … sequence is a substitute for the <code class="docutils literal notranslate"><span class="pre">switch</span></code> or <code class="docutils literal notranslate"><span class="pre">case</span></code> statements found in other languages.</p> </div> <div class="section" id="for-statements"> <span id="tut-for"></span><h2>4.2. <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> Statements<a class="headerlink" href="#for-statements" title="Permalink to this headline">¶</a></h2> <p id="index-0">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> statement in Python differs a bit from what you may be used to in C or Pascal. Rather than always iterating over an arithmetic progression of numbers (like in Pascal), or giving the user the ability to define both the iteration step and halting condition (as C), Python’s <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> statement iterates over the items of any sequence (a list or a string), in the order that they appear in the sequence. For example (no pun intended):</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="c1"># Measure some strings:</span> <span class="gp">... </span><span class="n">words</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'cat'</span><span class="p">,</span> <span class="s1">'window'</span><span class="p">,</span> <span class="s1">'defenestrate'</span><span class="p">]</span> <span class="gp">>>> </span><span class="k">for</span> <span class="n">w</span> <span class="ow">in</span> <span class="n">words</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">w</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="n">w</span><span class="p">))</span> <span class="gp">...</span> <span class="go">cat 3</span> <span class="go">window 6</span> <span class="go">defenestrate 12</span> </pre></div> </div> <p>If you need to modify the sequence you are iterating over while inside the loop (for example to duplicate selected items), it is recommended that you first make a copy. Iterating over a sequence does not implicitly make a copy. The slice notation makes this especially convenient:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">for</span> <span class="n">w</span> <span class="ow">in</span> <span class="n">words</span><span class="p">[:]:</span> <span class="c1"># Loop over a slice copy of the entire list.</span> <span class="gp">... </span> <span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">w</span><span class="p">)</span> <span class="o">></span> <span class="mi">6</span><span class="p">:</span> <span class="gp">... </span> <span class="n">words</span><span class="o">.</span><span class="n">insert</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">w</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">words</span> <span class="go">['defenestrate', 'cat', 'window', 'defenestrate']</span> </pre></div> </div> <p>With <code class="docutils literal notranslate"><span class="pre">for</span> <span class="pre">w</span> <span class="pre">in</span> <span class="pre">words:</span></code>, the example would attempt to create an infinite list, inserting <code class="docutils literal notranslate"><span class="pre">defenestrate</span></code> over and over again.</p> </div> <div class="section" id="the-range-function"> <span id="tut-range"></span><h2>4.3. The <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-func docutils literal notranslate"><span class="pre">range()</span></code></a> Function<a class="headerlink" href="#the-range-function" title="Permalink to this headline">¶</a></h2> <p>If you do need to iterate over a sequence of numbers, the built-in function <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-func docutils literal notranslate"><span class="pre">range()</span></code></a> comes in handy. It generates arithmetic progressions:</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">range</span><span class="p">(</span><span class="mi">5</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">0</span> <span class="go">1</span> <span class="go">2</span> <span class="go">3</span> <span class="go">4</span> </pre></div> </div> <p>The given end point is never part of the generated sequence; <code class="docutils literal notranslate"><span class="pre">range(10)</span></code> generates 10 values, the legal indices for items of a sequence of length 10. It is possible to let the range start at another number, or to specify a different increment (even negative; sometimes this is called the ‘step’):</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="mi">10</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="mi">9</span> <span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="mi">9</span> <span class="nb">range</span><span class="p">(</span><span class="o">-</span><span class="mi">10</span><span class="p">,</span> <span class="o">-</span><span class="mi">100</span><span class="p">,</span> <span class="o">-</span><span class="mi">30</span><span class="p">)</span> <span class="o">-</span><span class="mi">10</span><span class="p">,</span> <span class="o">-</span><span class="mi">40</span><span class="p">,</span> <span class="o">-</span><span class="mi">70</span> </pre></div> </div> <p>To iterate over the indices of a sequence, you can combine <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-func docutils literal notranslate"><span class="pre">range()</span></code></a> and <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a> as follows:</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="s1">'Mary'</span><span class="p">,</span> <span class="s1">'had'</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">,</span> <span class="s1">'little'</span><span class="p">,</span> <span class="s1">'lamb'</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="nb">len</span><span class="p">(</span><span class="n">a</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">a</span><span class="p">[</span><span class="n">i</span><span class="p">])</span> <span class="gp">...</span> <span class="go">0 Mary</span> <span class="go">1 had</span> <span class="go">2 a</span> <span class="go">3 little</span> <span class="go">4 lamb</span> </pre></div> </div> <p>In most such cases, however, it is convenient to use 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, see <a class="reference internal" href="datastructures.html#tut-loopidioms"><span class="std std-ref">Looping Techniques</span></a>.</p> <p>A strange thing happens if you just print a range:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span> <span class="go">range(0, 10)</span> </pre></div> </div> <p>In many ways the object returned by <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-func docutils literal notranslate"><span class="pre">range()</span></code></a> behaves as if it is a list, but in fact it isn’t. It is an object which returns the successive items of the desired sequence when you iterate over it, but it doesn’t really make the list, thus saving space.</p> <p>We say such an object is <em>iterable</em>, that is, suitable as a target for functions and constructs that expect something from which they can obtain successive items until the supply is exhausted. We have seen that 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> statement is such an <em>iterator</em>. The function <a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-func docutils literal notranslate"><span class="pre">list()</span></code></a> is another; it creates lists from iterables:</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">range</span><span class="p">(</span><span class="mi">5</span><span class="p">))</span> <span class="go">[0, 1, 2, 3, 4]</span> </pre></div> </div> <p>Later we will see more functions that return iterables and take iterables as argument.</p> </div> <div class="section" id="break-and-continue-statements-and-else-clauses-on-loops"> <span id="tut-break"></span><h2>4.4. <a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> and <a class="reference internal" href="../reference/simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> Statements, and <a class="reference internal" href="../reference/compound_stmts.html#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> Clauses on Loops<a class="headerlink" href="#break-and-continue-statements-and-else-clauses-on-loops" title="Permalink to this headline">¶</a></h2> <p>The <a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> statement, like in C, breaks out of the innermost enclosing <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#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a> loop.</p> <p>Loop statements may have an <code class="docutils literal notranslate"><span class="pre">else</span></code> clause; it is executed when the loop terminates through exhaustion of the list (with <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 when the condition becomes false (with <a class="reference internal" href="../reference/compound_stmts.html#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a>), but not when the loop is terminated by a <a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> statement. This is exemplified by the following loop, which searches for prime numbers:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">for</span> <span class="n">n</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">10</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">2</span><span class="p">,</span> <span class="n">n</span><span class="p">):</span> <span class="gp">... </span> <span class="k">if</span> <span class="n">n</span> <span class="o">%</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">n</span><span class="p">,</span> <span class="s1">'equals'</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="s1">'*'</span><span class="p">,</span> <span class="n">n</span><span class="o">//</span><span class="n">x</span><span class="p">)</span> <span class="gp">... </span> <span class="k">break</span> <span class="gp">... </span> <span class="k">else</span><span class="p">:</span> <span class="gp">... </span> <span class="c1"># loop fell through without finding a factor</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">n</span><span class="p">,</span> <span class="s1">'is a prime number'</span><span class="p">)</span> <span class="gp">...</span> <span class="go">2 is a prime number</span> <span class="go">3 is a prime number</span> <span class="go">4 equals 2 * 2</span> <span class="go">5 is a prime number</span> <span class="go">6 equals 2 * 3</span> <span class="go">7 is a prime number</span> <span class="go">8 equals 2 * 4</span> <span class="go">9 equals 3 * 3</span> </pre></div> </div> <p>(Yes, this is the correct code. Look closely: the <code class="docutils literal notranslate"><span class="pre">else</span></code> clause belongs to 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> loop, <strong>not</strong> the <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> statement.)</p> <p>When used with a loop, the <code class="docutils literal notranslate"><span class="pre">else</span></code> clause has more in common with the <code class="docutils literal notranslate"><span class="pre">else</span></code> clause of a <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement than it does that of <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: a <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement’s <code class="docutils literal notranslate"><span class="pre">else</span></code> clause runs when no exception occurs, and a loop’s <code class="docutils literal notranslate"><span class="pre">else</span></code> clause runs when no <code class="docutils literal notranslate"><span class="pre">break</span></code> occurs. For more on the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement and exceptions, see <a class="reference internal" href="errors.html#tut-handling"><span class="std std-ref">Handling Exceptions</span></a>.</p> <p>The <a class="reference internal" href="../reference/simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> statement, also borrowed from C, continues with the next iteration of the loop:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">for</span> <span class="n">num</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">10</span><span class="p">):</span> <span class="gp">... </span> <span class="k">if</span> <span class="n">num</span> <span class="o">%</span> <span class="mi">2</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Found an even number"</span><span class="p">,</span> <span class="n">num</span><span class="p">)</span> <span class="gp">... </span> <span class="k">continue</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Found a number"</span><span class="p">,</span> <span class="n">num</span><span class="p">)</span> <span class="go">Found an even number 2</span> <span class="go">Found a number 3</span> <span class="go">Found an even number 4</span> <span class="go">Found a number 5</span> <span class="go">Found an even number 6</span> <span class="go">Found a number 7</span> <span class="go">Found an even number 8</span> <span class="go">Found a number 9</span> </pre></div> </div> </div> <div class="section" id="pass-statements"> <span id="tut-pass"></span><h2>4.5. <a class="reference internal" href="../reference/simple_stmts.html#pass"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">pass</span></code></a> Statements<a class="headerlink" href="#pass-statements" title="Permalink to this headline">¶</a></h2> <p>The <a class="reference internal" href="../reference/simple_stmts.html#pass"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">pass</span></code></a> statement does nothing. It can be used when a statement is required syntactically but the program requires no action. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">while</span> <span class="kc">True</span><span class="p">:</span> <span class="gp">... </span> <span class="k">pass</span> <span class="c1"># Busy-wait for keyboard interrupt (Ctrl+C)</span> <span class="gp">...</span> </pre></div> </div> <p>This is commonly used for creating minimal classes:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">MyEmptyClass</span><span class="p">:</span> <span class="gp">... </span> <span class="k">pass</span> <span class="gp">...</span> </pre></div> </div> <p>Another place <a class="reference internal" href="../reference/simple_stmts.html#pass"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">pass</span></code></a> can be used is as a place-holder for a function or conditional body when you are working on new code, allowing you to keep thinking at a more abstract level. The <a class="reference internal" href="../reference/simple_stmts.html#pass"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">pass</span></code></a> is silently ignored:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">initlog</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">):</span> <span class="gp">... </span> <span class="k">pass</span> <span class="c1"># Remember to implement this!</span> <span class="gp">...</span> </pre></div> </div> </div> <div class="section" id="defining-functions"> <span id="tut-functions"></span><h2>4.6. Defining Functions<a class="headerlink" href="#defining-functions" title="Permalink to this headline">¶</a></h2> <p>We can create a function that writes the Fibonacci series to an arbitrary boundary:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">fib</span><span class="p">(</span><span class="n">n</span><span class="p">):</span> <span class="c1"># write Fibonacci series up to n</span> <span class="gp">... </span> <span class="sd">"""Print a Fibonacci series up to n."""</span> <span class="gp">... </span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span> <span class="gp">... </span> <span class="k">while</span> <span class="n">a</span> <span class="o"><</span> <span class="n">n</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span> <span class="gp">... </span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="n">b</span><span class="p">,</span> <span class="n">a</span><span class="o">+</span><span class="n">b</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">()</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="c1"># Now call the function we just defined:</span> <span class="gp">... </span><span class="n">fib</span><span class="p">(</span><span class="mi">2000</span><span class="p">)</span> <span class="go">0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597</span> </pre></div> </div> <p id="index-1">The keyword <a class="reference internal" href="../reference/compound_stmts.html#def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">def</span></code></a> introduces a function <em>definition</em>. It must be followed by the function name and the parenthesized list of formal parameters. The statements that form the body of the function start at the next line, and must be indented.</p> <p>The first statement of the function body can optionally be a string literal; this string literal is the function’s documentation string, or <em class="dfn">docstring</em>. (More about docstrings can be found in the section <a class="reference internal" href="#tut-docstrings"><span class="std std-ref">Documentation Strings</span></a>.) There are tools which use docstrings to automatically produce online or printed documentation, or to let the user interactively browse through code; it’s good practice to include docstrings in code that you write, so make a habit of it.</p> <p>The <em>execution</em> of a function introduces a new symbol table used for the local variables of the function. More precisely, all variable assignments in a function store the value in the local symbol table; whereas variable references first look in the local symbol table, then in the local symbol tables of enclosing functions, then in the global symbol table, and finally in the table of built-in names. Thus, global variables cannot be directly assigned a value within a function (unless named in a <a class="reference internal" href="../reference/simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement), although they may be referenced.</p> <p>The actual parameters (arguments) to a function call are introduced in the local symbol table of the called function when it is called; thus, arguments are passed using <em>call by value</em> (where the <em>value</em> is always an object <em>reference</em>, not the value of the object). <a class="footnote-reference" href="#id2" id="id1">[1]</a> When a function calls another function, a new local symbol table is created for that call.</p> <p>A function definition introduces the function name in the current symbol table. The value of the function name has a type that is recognized by the interpreter as a user-defined function. This value can be assigned to another name which can then also be used as a function. This serves as a general renaming mechanism:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">fib</span> <span class="go"><function fib at 10042ed0></span> <span class="gp">>>> </span><span class="n">f</span> <span class="o">=</span> <span class="n">fib</span> <span class="gp">>>> </span><span class="n">f</span><span class="p">(</span><span class="mi">100</span><span class="p">)</span> <span class="go">0 1 1 2 3 5 8 13 21 34 55 89</span> </pre></div> </div> <p>Coming from other languages, you might object that <code class="docutils literal notranslate"><span class="pre">fib</span></code> is not a function but a procedure since it doesn’t return a value. In fact, even functions without a <a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement do return a value, albeit a rather boring one. This value is called <code class="docutils literal notranslate"><span class="pre">None</span></code> (it’s a built-in name). Writing the value <code class="docutils literal notranslate"><span class="pre">None</span></code> is normally suppressed by the interpreter if it would be the only value written. You can see it if you really want to using <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate"><span class="pre">print()</span></code></a>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">fib</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span> <span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">fib</span><span class="p">(</span><span class="mi">0</span><span class="p">))</span> <span class="go">None</span> </pre></div> </div> <p>It is simple to write a function that returns a list of the numbers of the Fibonacci series, instead of printing it:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">fib2</span><span class="p">(</span><span class="n">n</span><span class="p">):</span> <span class="c1"># return Fibonacci series up to n</span> <span class="gp">... </span> <span class="sd">"""Return a list containing the Fibonacci series up to n."""</span> <span class="gp">... </span> <span class="n">result</span> <span class="o">=</span> <span class="p">[]</span> <span class="gp">... </span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span> <span class="gp">... </span> <span class="k">while</span> <span class="n">a</span> <span class="o"><</span> <span class="n">n</span><span class="p">:</span> <span class="gp">... </span> <span class="n">result</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">a</span><span class="p">)</span> <span class="c1"># see below</span> <span class="gp">... </span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="n">b</span><span class="p">,</span> <span class="n">a</span><span class="o">+</span><span class="n">b</span> <span class="gp">... </span> <span class="k">return</span> <span class="n">result</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">f100</span> <span class="o">=</span> <span class="n">fib2</span><span class="p">(</span><span class="mi">100</span><span class="p">)</span> <span class="c1"># call it</span> <span class="gp">>>> </span><span class="n">f100</span> <span class="c1"># write the result</span> <span class="go">[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]</span> </pre></div> </div> <p>This example, as usual, demonstrates some new Python features:</p> <ul class="simple"> <li>The <a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement returns with a value from a function. <a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> without an expression argument returns <code class="docutils literal notranslate"><span class="pre">None</span></code>. Falling off the end of a function also returns <code class="docutils literal notranslate"><span class="pre">None</span></code>.</li> <li>The statement <code class="docutils literal notranslate"><span class="pre">result.append(a)</span></code> calls a <em>method</em> of the list object <code class="docutils literal notranslate"><span class="pre">result</span></code>. A method is a function that ‘belongs’ to an object and is named <code class="docutils literal notranslate"><span class="pre">obj.methodname</span></code>, where <code class="docutils literal notranslate"><span class="pre">obj</span></code> is some object (this may be an expression), and <code class="docutils literal notranslate"><span class="pre">methodname</span></code> is the name of a method that is defined by the object’s type. Different types define different methods. Methods of different types may have the same name without causing ambiguity. (It is possible to define your own object types and methods, using <em>classes</em>, see <a class="reference internal" href="classes.html#tut-classes"><span class="std std-ref">Classes</span></a>) The method <code class="xref py py-meth docutils literal notranslate"><span class="pre">append()</span></code> shown in the example is defined for list objects; it adds a new element at the end of the list. In this example it is equivalent to <code class="docutils literal notranslate"><span class="pre">result</span> <span class="pre">=</span> <span class="pre">result</span> <span class="pre">+</span> <span class="pre">[a]</span></code>, but more efficient.</li> </ul> </div> <div class="section" id="more-on-defining-functions"> <span id="tut-defining"></span><h2>4.7. More on Defining Functions<a class="headerlink" href="#more-on-defining-functions" title="Permalink to this headline">¶</a></h2> <p>It is also possible to define functions with a variable number of arguments. There are three forms, which can be combined.</p> <div class="section" id="default-argument-values"> <span id="tut-defaultargs"></span><h3>4.7.1. Default Argument Values<a class="headerlink" href="#default-argument-values" title="Permalink to this headline">¶</a></h3> <p>The most useful form is to specify a default value for one or more arguments. This creates a function that can be called with fewer arguments than it is defined to allow. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">ask_ok</span><span class="p">(</span><span class="n">prompt</span><span class="p">,</span> <span class="n">retries</span><span class="o">=</span><span class="mi">4</span><span class="p">,</span> <span class="n">reminder</span><span class="o">=</span><span class="s1">'Please try again!'</span><span class="p">):</span> <span class="k">while</span> <span class="kc">True</span><span class="p">:</span> <span class="n">ok</span> <span class="o">=</span> <span class="nb">input</span><span class="p">(</span><span class="n">prompt</span><span class="p">)</span> <span class="k">if</span> <span class="n">ok</span> <span class="ow">in</span> <span class="p">(</span><span class="s1">'y'</span><span class="p">,</span> <span class="s1">'ye'</span><span class="p">,</span> <span class="s1">'yes'</span><span class="p">):</span> <span class="k">return</span> <span class="kc">True</span> <span class="k">if</span> <span class="n">ok</span> <span class="ow">in</span> <span class="p">(</span><span class="s1">'n'</span><span class="p">,</span> <span class="s1">'no'</span><span class="p">,</span> <span class="s1">'nop'</span><span class="p">,</span> <span class="s1">'nope'</span><span class="p">):</span> <span class="k">return</span> <span class="kc">False</span> <span class="n">retries</span> <span class="o">=</span> <span class="n">retries</span> <span class="o">-</span> <span class="mi">1</span> <span class="k">if</span> <span class="n">retries</span> <span class="o"><</span> <span class="mi">0</span><span class="p">:</span> <span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s1">'invalid user response'</span><span class="p">)</span> <span class="nb">print</span><span class="p">(</span><span class="n">reminder</span><span class="p">)</span> </pre></div> </div> <p>This function can be called in several ways:</p> <ul class="simple"> <li>giving only the mandatory argument: <code class="docutils literal notranslate"><span class="pre">ask_ok('Do</span> <span class="pre">you</span> <span class="pre">really</span> <span class="pre">want</span> <span class="pre">to</span> <span class="pre">quit?')</span></code></li> <li>giving one of the optional arguments: <code class="docutils literal notranslate"><span class="pre">ask_ok('OK</span> <span class="pre">to</span> <span class="pre">overwrite</span> <span class="pre">the</span> <span class="pre">file?',</span> <span class="pre">2)</span></code></li> <li>or even giving all arguments: <code class="docutils literal notranslate"><span class="pre">ask_ok('OK</span> <span class="pre">to</span> <span class="pre">overwrite</span> <span class="pre">the</span> <span class="pre">file?',</span> <span class="pre">2,</span> <span class="pre">'Come</span> <span class="pre">on,</span> <span class="pre">only</span> <span class="pre">yes</span> <span class="pre">or</span> <span class="pre">no!')</span></code></li> </ul> <p>This example also introduces 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. This tests whether or not a sequence contains a certain value.</p> <p>The default values are evaluated at the point of function definition in the <em>defining</em> scope, so that</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">i</span> <span class="o">=</span> <span class="mi">5</span> <span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">arg</span><span class="o">=</span><span class="n">i</span><span class="p">):</span> <span class="nb">print</span><span class="p">(</span><span class="n">arg</span><span class="p">)</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">6</span> <span class="n">f</span><span class="p">()</span> </pre></div> </div> <p>will print <code class="docutils literal notranslate"><span class="pre">5</span></code>.</p> <p><strong>Important warning:</strong> The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes. For example, the following function accumulates the arguments passed to it on subsequent calls:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">L</span><span class="o">=</span><span class="p">[]):</span> <span class="n">L</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">a</span><span class="p">)</span> <span class="k">return</span> <span class="n">L</span> <span class="nb">print</span><span class="p">(</span><span class="n">f</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span> <span class="nb">print</span><span class="p">(</span><span class="n">f</span><span class="p">(</span><span class="mi">2</span><span class="p">))</span> <span class="nb">print</span><span class="p">(</span><span class="n">f</span><span class="p">(</span><span class="mi">3</span><span class="p">))</span> </pre></div> </div> <p>This will print</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="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="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> </pre></div> </div> <p>If you don’t want the default to be shared between subsequent calls, you can write the function like this instead:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">L</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span> <span class="k">if</span> <span class="n">L</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span> <span class="n">L</span> <span class="o">=</span> <span class="p">[]</span> <span class="n">L</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">a</span><span class="p">)</span> <span class="k">return</span> <span class="n">L</span> </pre></div> </div> </div> <div class="section" id="keyword-arguments"> <span id="tut-keywordargs"></span><h3>4.7.2. Keyword Arguments<a class="headerlink" href="#keyword-arguments" title="Permalink to this headline">¶</a></h3> <p>Functions can also be called using <a class="reference internal" href="../glossary.html#term-keyword-argument"><span class="xref std std-term">keyword arguments</span></a> of the form <code class="docutils literal notranslate"><span class="pre">kwarg=value</span></code>. For instance, the following function:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">parrot</span><span class="p">(</span><span class="n">voltage</span><span class="p">,</span> <span class="n">state</span><span class="o">=</span><span class="s1">'a stiff'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'voom'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="s1">'Norwegian Blue'</span><span class="p">):</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"-- This parrot wouldn't"</span><span class="p">,</span> <span class="n">action</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"if you put"</span><span class="p">,</span> <span class="n">voltage</span><span class="p">,</span> <span class="s2">"volts through it."</span><span class="p">)</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"-- Lovely plumage, the"</span><span class="p">,</span> <span class="nb">type</span><span class="p">)</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"-- It's"</span><span class="p">,</span> <span class="n">state</span><span class="p">,</span> <span class="s2">"!"</span><span class="p">)</span> </pre></div> </div> <p>accepts one required argument (<code class="docutils literal notranslate"><span class="pre">voltage</span></code>) and three optional arguments (<code class="docutils literal notranslate"><span class="pre">state</span></code>, <code class="docutils literal notranslate"><span class="pre">action</span></code>, and <code class="docutils literal notranslate"><span class="pre">type</span></code>). This function can be called in any of the following ways:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">parrot</span><span class="p">(</span><span class="mi">1000</span><span class="p">)</span> <span class="c1"># 1 positional argument</span> <span class="n">parrot</span><span class="p">(</span><span class="n">voltage</span><span class="o">=</span><span class="mi">1000</span><span class="p">)</span> <span class="c1"># 1 keyword argument</span> <span class="n">parrot</span><span class="p">(</span><span class="n">voltage</span><span class="o">=</span><span class="mi">1000000</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'VOOOOOM'</span><span class="p">)</span> <span class="c1"># 2 keyword arguments</span> <span class="n">parrot</span><span class="p">(</span><span class="n">action</span><span class="o">=</span><span class="s1">'VOOOOOM'</span><span class="p">,</span> <span class="n">voltage</span><span class="o">=</span><span class="mi">1000000</span><span class="p">)</span> <span class="c1"># 2 keyword arguments</span> <span class="n">parrot</span><span class="p">(</span><span class="s1">'a million'</span><span class="p">,</span> <span class="s1">'bereft of life'</span><span class="p">,</span> <span class="s1">'jump'</span><span class="p">)</span> <span class="c1"># 3 positional arguments</span> <span class="n">parrot</span><span class="p">(</span><span class="s1">'a thousand'</span><span class="p">,</span> <span class="n">state</span><span class="o">=</span><span class="s1">'pushing up the daisies'</span><span class="p">)</span> <span class="c1"># 1 positional, 1 keyword</span> </pre></div> </div> <p>but all the following calls would be invalid:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">parrot</span><span class="p">()</span> <span class="c1"># required argument missing</span> <span class="n">parrot</span><span class="p">(</span><span class="n">voltage</span><span class="o">=</span><span class="mf">5.0</span><span class="p">,</span> <span class="s1">'dead'</span><span class="p">)</span> <span class="c1"># non-keyword argument after a keyword argument</span> <span class="n">parrot</span><span class="p">(</span><span class="mi">110</span><span class="p">,</span> <span class="n">voltage</span><span class="o">=</span><span class="mi">220</span><span class="p">)</span> <span class="c1"># duplicate value for the same argument</span> <span class="n">parrot</span><span class="p">(</span><span class="n">actor</span><span class="o">=</span><span class="s1">'John Cleese'</span><span class="p">)</span> <span class="c1"># unknown keyword argument</span> </pre></div> </div> <p>In a function call, keyword arguments must follow positional arguments. All the keyword arguments passed must match one of the arguments accepted by the function (e.g. <code class="docutils literal notranslate"><span class="pre">actor</span></code> is not a valid argument for the <code class="docutils literal notranslate"><span class="pre">parrot</span></code> function), and their order is not important. This also includes non-optional arguments (e.g. <code class="docutils literal notranslate"><span class="pre">parrot(voltage=1000)</span></code> is valid too). No argument may receive a value more than once. Here’s an example that fails due to this restriction:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">function</span><span class="p">(</span><span class="n">a</span><span class="p">):</span> <span class="gp">... </span> <span class="k">pass</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">function</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">a</span><span class="o">=</span><span class="mi">0</span><span class="p">)</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">function() got multiple values for keyword argument 'a'</span> </pre></div> </div> <p>When a final formal parameter of the form <code class="docutils literal notranslate"><span class="pre">**name</span></code> is present, it receives a dictionary (see <a class="reference internal" href="../library/stdtypes.html#typesmapping"><span class="std std-ref">Mapping Types — dict</span></a>) containing all keyword arguments except for those corresponding to a formal parameter. This may be combined with a formal parameter of the form <code class="docutils literal notranslate"><span class="pre">*name</span></code> (described in the next subsection) which receives a tuple containing the positional arguments beyond the formal parameter list. (<code class="docutils literal notranslate"><span class="pre">*name</span></code> must occur before <code class="docutils literal notranslate"><span class="pre">**name</span></code>.) For example, if we define a function like this:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">cheeseshop</span><span class="p">(</span><span class="n">kind</span><span class="p">,</span> <span class="o">*</span><span class="n">arguments</span><span class="p">,</span> <span class="o">**</span><span class="n">keywords</span><span class="p">):</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"-- Do you have any"</span><span class="p">,</span> <span class="n">kind</span><span class="p">,</span> <span class="s2">"?"</span><span class="p">)</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"-- I'm sorry, we're all out of"</span><span class="p">,</span> <span class="n">kind</span><span class="p">)</span> <span class="k">for</span> <span class="n">arg</span> <span class="ow">in</span> <span class="n">arguments</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="n">arg</span><span class="p">)</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"-"</span> <span class="o">*</span> <span class="mi">40</span><span class="p">)</span> <span class="k">for</span> <span class="n">kw</span> <span class="ow">in</span> <span class="n">keywords</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="n">kw</span><span class="p">,</span> <span class="s2">":"</span><span class="p">,</span> <span class="n">keywords</span><span class="p">[</span><span class="n">kw</span><span class="p">])</span> </pre></div> </div> <p>It could be called like this:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">cheeseshop</span><span class="p">(</span><span class="s2">"Limburger"</span><span class="p">,</span> <span class="s2">"It's very runny, sir."</span><span class="p">,</span> <span class="s2">"It's really very, VERY runny, sir."</span><span class="p">,</span> <span class="n">shopkeeper</span><span class="o">=</span><span class="s2">"Michael Palin"</span><span class="p">,</span> <span class="n">client</span><span class="o">=</span><span class="s2">"John Cleese"</span><span class="p">,</span> <span class="n">sketch</span><span class="o">=</span><span class="s2">"Cheese Shop Sketch"</span><span class="p">)</span> </pre></div> </div> <p>and of course it would print:</p> <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>-- Do you have any Limburger ? -- I'm sorry, we're all out of Limburger It's very runny, sir. It's really very, VERY runny, sir. ---------------------------------------- shopkeeper : Michael Palin client : John Cleese sketch : Cheese Shop Sketch </pre></div> </div> <p>Note that the order in which the keyword arguments are printed is guaranteed to match the order in which they were provided in the function call.</p> </div> <div class="section" id="arbitrary-argument-lists"> <span id="tut-arbitraryargs"></span><h3>4.7.3. Arbitrary Argument Lists<a class="headerlink" href="#arbitrary-argument-lists" title="Permalink to this headline">¶</a></h3> <p id="index-2">Finally, the least frequently used option is to specify that a function can be called with an arbitrary number of arguments. These arguments will be wrapped up in a tuple (see <a class="reference internal" href="datastructures.html#tut-tuples"><span class="std std-ref">Tuples and Sequences</span></a>). Before the variable number of arguments, zero or more normal arguments may occur.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">write_multiple_items</span><span class="p">(</span><span class="n">file</span><span class="p">,</span> <span class="n">separator</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">):</span> <span class="n">file</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">separator</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">args</span><span class="p">))</span> </pre></div> </div> <p>Normally, these <code class="docutils literal notranslate"><span class="pre">variadic</span></code> arguments will be last in the list of formal parameters, because they scoop up all remaining input arguments that are passed to the function. Any formal parameters which occur after the <code class="docutils literal notranslate"><span class="pre">*args</span></code> parameter are ‘keyword-only’ arguments, meaning that they can only be used as keywords rather than positional arguments.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">concat</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="n">sep</span><span class="o">=</span><span class="s2">"/"</span><span class="p">):</span> <span class="gp">... </span> <span class="k">return</span> <span class="n">sep</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">args</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">concat</span><span class="p">(</span><span class="s2">"earth"</span><span class="p">,</span> <span class="s2">"mars"</span><span class="p">,</span> <span class="s2">"venus"</span><span class="p">)</span> <span class="go">'earth/mars/venus'</span> <span class="gp">>>> </span><span class="n">concat</span><span class="p">(</span><span class="s2">"earth"</span><span class="p">,</span> <span class="s2">"mars"</span><span class="p">,</span> <span class="s2">"venus"</span><span class="p">,</span> <span class="n">sep</span><span class="o">=</span><span class="s2">"."</span><span class="p">)</span> <span class="go">'earth.mars.venus'</span> </pre></div> </div> </div> <div class="section" id="unpacking-argument-lists"> <span id="tut-unpacking-arguments"></span><h3>4.7.4. Unpacking Argument Lists<a class="headerlink" href="#unpacking-argument-lists" title="Permalink to this headline">¶</a></h3> <p>The reverse situation occurs when the arguments are already in a list or tuple but need to be unpacked for a function call requiring separate positional arguments. For instance, the built-in <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-func docutils literal notranslate"><span class="pre">range()</span></code></a> function expects separate <em>start</em> and <em>stop</em> arguments. If they are not available separately, write the function call with the <code class="docutils literal notranslate"><span class="pre">*</span></code>-operator to unpack the arguments out of a list or tuple:</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">range</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">6</span><span class="p">))</span> <span class="c1"># normal call with separate arguments</span> <span class="go">[3, 4, 5]</span> <span class="gp">>>> </span><span class="n">args</span> <span class="o">=</span> <span class="p">[</span><span class="mi">3</span><span class="p">,</span> <span class="mi">6</span><span class="p">]</span> <span class="gp">>>> </span><span class="nb">list</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">))</span> <span class="c1"># call with arguments unpacked from a list</span> <span class="go">[3, 4, 5]</span> </pre></div> </div> <p id="index-3">In the same fashion, dictionaries can deliver keyword arguments with the <code class="docutils literal notranslate"><span class="pre">**</span></code>-operator:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">parrot</span><span class="p">(</span><span class="n">voltage</span><span class="p">,</span> <span class="n">state</span><span class="o">=</span><span class="s1">'a stiff'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'voom'</span><span class="p">):</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"-- This parrot wouldn't"</span><span class="p">,</span> <span class="n">action</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"if you put"</span><span class="p">,</span> <span class="n">voltage</span><span class="p">,</span> <span class="s2">"volts through it."</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s1">' '</span><span class="p">)</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"E's"</span><span class="p">,</span> <span class="n">state</span><span class="p">,</span> <span class="s2">"!"</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s2">"voltage"</span><span class="p">:</span> <span class="s2">"four million"</span><span class="p">,</span> <span class="s2">"state"</span><span class="p">:</span> <span class="s2">"bleedin' demised"</span><span class="p">,</span> <span class="s2">"action"</span><span class="p">:</span> <span class="s2">"VOOM"</span><span class="p">}</span> <span class="gp">>>> </span><span class="n">parrot</span><span class="p">(</span><span class="o">**</span><span class="n">d</span><span class="p">)</span> <span class="go">-- This parrot wouldn't VOOM if you put four million volts through it. E's bleedin' demised !</span> </pre></div> </div> </div> <div class="section" id="lambda-expressions"> <span id="tut-lambda"></span><h3>4.7.5. Lambda Expressions<a class="headerlink" href="#lambda-expressions" title="Permalink to this headline">¶</a></h3> <p>Small anonymous functions can be created with the <a class="reference internal" href="../reference/expressions.html#lambda"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">lambda</span></code></a> keyword. This function returns the sum of its two arguments: <code class="docutils literal notranslate"><span class="pre">lambda</span> <span class="pre">a,</span> <span class="pre">b:</span> <span class="pre">a+b</span></code>. Lambda functions can be used wherever function objects are required. They are syntactically restricted to a single expression. Semantically, they are just syntactic sugar for a normal function definition. Like nested function definitions, lambda functions can reference variables from the containing scope:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">make_incrementor</span><span class="p">(</span><span class="n">n</span><span class="p">):</span> <span class="gp">... </span> <span class="k">return</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="n">n</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">f</span> <span class="o">=</span> <span class="n">make_incrementor</span><span class="p">(</span><span class="mi">42</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">f</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span> <span class="go">42</span> <span class="gp">>>> </span><span class="n">f</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="go">43</span> </pre></div> </div> <p>The above example uses a lambda expression to return a function. Another use is to pass a small function as an argument:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">pairs</span> <span class="o">=</span> <span class="p">[(</span><span class="mi">1</span><span class="p">,</span> <span class="s1">'one'</span><span class="p">),</span> <span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="s1">'two'</span><span class="p">),</span> <span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="s1">'three'</span><span class="p">),</span> <span class="p">(</span><span class="mi">4</span><span class="p">,</span> <span class="s1">'four'</span><span class="p">)]</span> <span class="gp">>>> </span><span class="n">pairs</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="k">lambda</span> <span class="n">pair</span><span class="p">:</span> <span class="n">pair</span><span class="p">[</span><span class="mi">1</span><span class="p">])</span> <span class="gp">>>> </span><span class="n">pairs</span> <span class="go">[(4, 'four'), (1, 'one'), (3, 'three'), (2, 'two')]</span> </pre></div> </div> </div> <div class="section" id="documentation-strings"> <span id="tut-docstrings"></span><h3>4.7.6. Documentation Strings<a class="headerlink" href="#documentation-strings" title="Permalink to this headline">¶</a></h3> <p id="index-4">Here are some conventions about the content and formatting of documentation strings.</p> <p>The first line should always be a short, concise summary of the object’s purpose. For brevity, it should not explicitly state the object’s name or type, since these are available by other means (except if the name happens to be a verb describing a function’s operation). This line should begin with a capital letter and end with a period.</p> <p>If there are more lines in the documentation string, the second line should be blank, visually separating the summary from the rest of the description. The following lines should be one or more paragraphs describing the object’s calling conventions, its side effects, etc.</p> <p>The Python parser does not strip indentation from multi-line string literals in Python, so tools that process documentation have to strip indentation if desired. This is done using the following convention. The first non-blank line <em>after</em> the first line of the string determines the amount of indentation for the entire documentation string. (We can’t use the first line since it is generally adjacent to the string’s opening quotes so its indentation is not apparent in the string literal.) Whitespace “equivalent” to this indentation is then stripped from the start of all lines of the string. Lines that are indented less should not occur, but if they occur all their leading whitespace should be stripped. Equivalence of whitespace should be tested after expansion of tabs (to 8 spaces, normally).</p> <p>Here is an example of a multi-line docstring:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">my_function</span><span class="p">():</span> <span class="gp">... </span> <span class="sd">"""Do nothing, but document it.</span> <span class="gp">...</span><span class="sd"></span> <span class="gp">... </span><span class="sd"> No, really, it doesn't do anything.</span> <span class="gp">... </span><span class="sd"> """</span> <span class="gp">... </span> <span class="k">pass</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">my_function</span><span class="o">.</span><span class="vm">__doc__</span><span class="p">)</span> <span class="go">Do nothing, but document it.</span> <span class="go"> No, really, it doesn't do anything.</span> </pre></div> </div> </div> <div class="section" id="function-annotations"> <span id="tut-annotations"></span><h3>4.7.7. Function Annotations<a class="headerlink" href="#function-annotations" title="Permalink to this headline">¶</a></h3> <p id="index-5"><a class="reference internal" href="../reference/compound_stmts.html#function"><span class="std std-ref">Function annotations</span></a> are completely optional metadata information about the types used by user-defined functions (see <span class="target" id="index-6"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3107"><strong>PEP 3107</strong></a> and <span class="target" id="index-7"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0484"><strong>PEP 484</strong></a> for more information).</p> <p>Annotations are stored in the <code class="xref py py-attr docutils literal notranslate"><span class="pre">__annotations__</span></code> attribute of the function as a dictionary and have no effect on any other part of the function. Parameter annotations are defined by a colon after the parameter name, followed by an expression evaluating to the value of the annotation. Return annotations are defined by a literal <code class="docutils literal notranslate"><span class="pre">-></span></code>, followed by an expression, between the parameter list and the colon denoting the end of the <a class="reference internal" href="../reference/compound_stmts.html#def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">def</span></code></a> statement. The following example has a positional argument, a keyword argument, and the return value annotated:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">ham</span><span class="p">:</span> <span class="nb">str</span><span class="p">,</span> <span class="n">eggs</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s1">'eggs'</span><span class="p">)</span> <span class="o">-></span> <span class="nb">str</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Annotations:"</span><span class="p">,</span> <span class="n">f</span><span class="o">.</span><span class="vm">__annotations__</span><span class="p">)</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Arguments:"</span><span class="p">,</span> <span class="n">ham</span><span class="p">,</span> <span class="n">eggs</span><span class="p">)</span> <span class="gp">... </span> <span class="k">return</span> <span class="n">ham</span> <span class="o">+</span> <span class="s1">' and '</span> <span class="o">+</span> <span class="n">eggs</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">f</span><span class="p">(</span><span class="s1">'spam'</span><span class="p">)</span> <span class="go">Annotations: {'ham': <class 'str'>, 'return': <class 'str'>, 'eggs': <class 'str'>}</span> <span class="go">Arguments: spam eggs</span> <span class="go">'spam and eggs'</span> </pre></div> </div> </div> </div> <div class="section" id="intermezzo-coding-style"> <span id="tut-codingstyle"></span><h2>4.8. Intermezzo: Coding Style<a class="headerlink" href="#intermezzo-coding-style" title="Permalink to this headline">¶</a></h2> <p id="index-8">Now that you are about to write longer, more complex pieces of Python, it is a good time to talk about <em>coding style</em>. Most languages can be written (or more concise, <em>formatted</em>) in different styles; some are more readable than others. Making it easy for others to read your code is always a good idea, and adopting a nice coding style helps tremendously for that.</p> <p>For Python, <span class="target" id="index-9"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0008"><strong>PEP 8</strong></a> has emerged as the style guide that most projects adhere to; it promotes a very readable and eye-pleasing coding style. Every Python developer should read it at some point; here are the most important points extracted for you:</p> <ul> <li><p class="first">Use 4-space indentation, and no tabs.</p> <p>4 spaces are a good compromise between small indentation (allows greater nesting depth) and large indentation (easier to read). Tabs introduce confusion, and are best left out.</p> </li> <li><p class="first">Wrap lines so that they don’t exceed 79 characters.</p> <p>This helps users with small displays and makes it possible to have several code files side-by-side on larger displays.</p> </li> <li><p class="first">Use blank lines to separate functions and classes, and larger blocks of code inside functions.</p> </li> <li><p class="first">When possible, put comments on a line of their own.</p> </li> <li><p class="first">Use docstrings.</p> </li> <li><p class="first">Use spaces around operators and after commas, but not directly inside bracketing constructs: <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">=</span> <span class="pre">f(1,</span> <span class="pre">2)</span> <span class="pre">+</span> <span class="pre">g(3,</span> <span class="pre">4)</span></code>.</p> </li> <li><p class="first">Name your classes and functions consistently; the convention is to use <code class="docutils literal notranslate"><span class="pre">CamelCase</span></code> for classes and <code class="docutils literal notranslate"><span class="pre">lower_case_with_underscores</span></code> for functions and methods. Always use <code class="docutils literal notranslate"><span class="pre">self</span></code> as the name for the first method argument (see <a class="reference internal" href="classes.html#tut-firstclasses"><span class="std std-ref">A First Look at Classes</span></a> for more on classes and methods).</p> </li> <li><p class="first">Don’t use fancy encodings if your code is meant to be used in international environments. Python’s default, UTF-8, or even plain ASCII work best in any case.</p> </li> <li><p class="first">Likewise, don’t use non-ASCII characters in identifiers if there is only the slightest chance people speaking a different language will read or maintain the code.</p> </li> </ul> <p class="rubric">Footnotes</p> <table class="docutils footnote" frame="void" id="id2" 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>Actually, <em>call by object reference</em> would be a better description, since if a mutable object is passed, the caller will see any changes the callee makes to it (items inserted into a list).</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="#">4. More Control Flow Tools</a><ul> <li><a class="reference internal" href="#if-statements">4.1. <code class="docutils literal notranslate"><span class="pre">if</span></code> Statements</a></li> <li><a class="reference internal" href="#for-statements">4.2. <code class="docutils literal notranslate"><span class="pre">for</span></code> Statements</a></li> <li><a class="reference internal" href="#the-range-function">4.3. The <code class="docutils literal notranslate"><span class="pre">range()</span></code> Function</a></li> <li><a class="reference internal" href="#break-and-continue-statements-and-else-clauses-on-loops">4.4. <code class="docutils literal notranslate"><span class="pre">break</span></code> and <code class="docutils literal notranslate"><span class="pre">continue</span></code> Statements, and <code class="docutils literal notranslate"><span class="pre">else</span></code> Clauses on Loops</a></li> <li><a class="reference internal" href="#pass-statements">4.5. <code class="docutils literal notranslate"><span class="pre">pass</span></code> Statements</a></li> <li><a class="reference internal" href="#defining-functions">4.6. Defining Functions</a></li> <li><a class="reference internal" href="#more-on-defining-functions">4.7. More on Defining Functions</a><ul> <li><a class="reference internal" href="#default-argument-values">4.7.1. Default Argument Values</a></li> <li><a class="reference internal" href="#keyword-arguments">4.7.2. Keyword Arguments</a></li> <li><a class="reference internal" href="#arbitrary-argument-lists">4.7.3. Arbitrary Argument Lists</a></li> <li><a class="reference internal" href="#unpacking-argument-lists">4.7.4. Unpacking Argument Lists</a></li> <li><a class="reference internal" href="#lambda-expressions">4.7.5. Lambda Expressions</a></li> <li><a class="reference internal" href="#documentation-strings">4.7.6. Documentation Strings</a></li> <li><a class="reference internal" href="#function-annotations">4.7.7. Function Annotations</a></li> </ul> </li> <li><a class="reference internal" href="#intermezzo-coding-style">4.8. Intermezzo: Coding Style</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="introduction.html" title="previous chapter">3. An Informal Introduction to Python</a></p> <h4>Next topic</h4> <p class="topless"><a href="datastructures.html" title="next chapter">5. Data Structures</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li> <a href="https://github.com/python/cpython/blob/3.6/Doc/tutorial/controlflow.rst" rel="nofollow">Show Source </a> </li> </ul> </div> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="datastructures.html" title="5. Data Structures" >next</a> |</li> <li class="right" > <a href="introduction.html" title="3. 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