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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="Content-Type" content="text/html; charset=utf-8" /> <title>Argparse Tutorial &mdash; Python 2.7.5 documentation</title> <link rel="stylesheet" href="../_static/default.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '../', VERSION: '2.7.5', COLLAPSE_INDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </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 2.7.5 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="top" title="Python 2.7.5 documentation" href="../index.html" /> <link rel="up" title="Python HOWTOs" href="index.html" /> <link rel="next" title="Python Frequently Asked Questions" href="../faq/index.html" /> <link rel="prev" title="HOWTO Use Python in the web" href="webservers.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head> <body> <div class="related"> <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="../faq/index.html" title="Python Frequently Asked Questions" accesskey="N">next</a> |</li> <li class="right" > <a href="webservers.html" title="HOWTO Use Python in the web" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="http://www.python.org/">Python</a> &raquo;</li> <li> <a href="../index.html">Python 2.7.5 documentation</a> &raquo; </li> <li><a href="index.html" accesskey="U">Python HOWTOs</a> &raquo;</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="argparse-tutorial"> <h1>Argparse Tutorial<a class="headerlink" href="#argparse-tutorial" title="Permalink to this headline">¶</a></h1> <table class="docutils field-list" frame="void" rules="none"> <col class="field-name" /> <col class="field-body" /> <tbody valign="top"> <tr class="field-odd field"><th class="field-name">author:</th><td class="field-body">Tshepang Lekhonkhobe</td> </tr> </tbody> </table> <p id="id1">This tutorial is intended to be a gentle introduction to <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal"><span class="pre">argparse</span></tt></a>, the recommended command-line parsing module in the Python standard library.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">There are two other modules that fulfill the same task, namely <a class="reference internal" href="../library/getopt.html#module-getopt" title="getopt: Portable parser for command line options; support both short and long option names."><tt class="xref py py-mod docutils literal"><span class="pre">getopt</span></tt></a> (an equivalent for <tt class="xref c c-func docutils literal"><span class="pre">getopt()</span></tt> from the C language) and the deprecated <a class="reference internal" href="../library/optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><tt class="xref py py-mod docutils literal"><span class="pre">optparse</span></tt></a>. Note also that <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal"><span class="pre">argparse</span></tt></a> is based on <a class="reference internal" href="../library/optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><tt class="xref py py-mod docutils literal"><span class="pre">optparse</span></tt></a>, and therefore very similar in terms of usage.</p> </div> <div class="section" id="concepts"> <h2>Concepts<a class="headerlink" href="#concepts" title="Permalink to this headline">¶</a></h2> <p>Let&#8217;s show the sort of functionality that we are going to explore in this introductory tutorial by making use of the <strong class="command">ls</strong> command:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>ls cpython devguide prog.py pypy rm-unused-function.patch <span class="nv">$ </span>ls pypy ctypes_configure demo dotviewer include lib_pypy lib-python ... <span class="nv">$ </span>ls -l total 20 drwxr-xr-x 19 wena wena 4096 Feb 18 18:51 cpython drwxr-xr-x 4 wena wena 4096 Feb 8 12:04 devguide -rwxr-xr-x 1 wena wena 535 Feb 19 00:05 prog.py drwxr-xr-x 14 wena wena 4096 Feb 7 00:59 pypy -rw-r--r-- 1 wena wena 741 Feb 18 01:01 rm-unused-function.patch <span class="nv">$ </span>ls --help Usage: ls <span class="o">[</span>OPTION<span class="o">]</span>... <span class="o">[</span>FILE<span class="o">]</span>... List information about the FILEs <span class="o">(</span>the current directory by default<span class="o">)</span>. Sort entries alphabetically <span class="k">if </span>none of -cftuvSUX nor --sort is specified. ... </pre></div> </div> <p>A few concepts we can learn from the four commands:</p> <ul class="simple"> <li>The <strong class="command">ls</strong> command is useful when run without any options at all. It defaults to displaying the contents of the current directory.</li> <li>If we want beyond what it provides by default, we tell it a bit more. In this case, we want it to display a different directory, <tt class="docutils literal"><span class="pre">pypy</span></tt>. What we did is specify what is known as a positional argument. It&#8217;s named so because the program should know what to do with the value, solely based on where it appears on the command line. This concept is more relevant to a command like <strong class="command">cp</strong>, whose most basic usage is <tt class="docutils literal"><span class="pre">cp</span> <span class="pre">SRC</span> <span class="pre">DEST</span></tt>. The first position is <em>what you want copied,</em> and the second position is <em>where you want it copied to</em>.</li> <li>Now, say we want to change behaviour of the program. In our example, we display more info for each file instead of just showing the file names. The <tt class="docutils literal"><span class="pre">-l</span></tt> in that case is known as an optional argument.</li> <li>That&#8217;s a snippet of the help text. It&#8217;s very useful in that you can come across a program you have never used before, and can figure out how it works simply by reading it&#8217;s help text.</li> </ul> </div> <div class="section" id="the-basics"> <h2>The basics<a class="headerlink" href="#the-basics" title="Permalink to this headline">¶</a></h2> <p>Let us start with a very simple example which does (almost) nothing:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> </pre></div> </div> <p>Following is a result of running the code:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py <span class="nv">$ </span>python prog.py --help usage: prog.py <span class="o">[</span>-h<span class="o">]</span> optional arguments: -h, --help show this <span class="nb">help </span>message and <span class="nb">exit</span> <span class="nv">$ </span>python prog.py --verbose usage: prog.py <span class="o">[</span>-h<span class="o">]</span> prog.py: error: unrecognized arguments: --verbose <span class="nv">$ </span>python prog.py foo usage: prog.py <span class="o">[</span>-h<span class="o">]</span> prog.py: error: unrecognized arguments: foo </pre></div> </div> <p>Here is what is happening:</p> <ul class="simple"> <li>Running the script without any options results in nothing displayed to stdout. Not so useful.</li> <li>The second one starts to display the usefulness of the <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal"><span class="pre">argparse</span></tt></a> module. We have done almost nothing, but already we get a nice help message.</li> <li>The <tt class="docutils literal"><span class="pre">--help</span></tt> option, which can also be shortened to <tt class="docutils literal"><span class="pre">-h</span></tt>, is the only option we get for free (i.e. no need to specify it). Specifying anything else results in an error. But even then, we do get a useful usage message, also for free.</li> </ul> </div> <div class="section" id="introducing-positional-arguments"> <h2>Introducing Positional arguments<a class="headerlink" href="#introducing-positional-arguments" title="Permalink to this headline">¶</a></h2> <p>An example:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;echo&quot;</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="k">print</span> <span class="n">args</span><span class="o">.</span><span class="n">echo</span> </pre></div> </div> <p>And running the code:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="nb">echo</span> prog.py: error: the following arguments are required: <span class="nb">echo</span> <span class="nv">$ </span>python prog.py --help usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="nb">echo</span> positional arguments: <span class="nb">echo</span> optional arguments: -h, --help show this <span class="nb">help </span>message and <span class="nb">exit</span> <span class="nv">$ </span>python prog.py foo foo </pre></div> </div> <p>Here is what&#8217;s happening:</p> <ul class="simple"> <li>We&#8217;ve added the <tt class="xref py py-meth docutils literal"><span class="pre">add_argument()</span></tt> method, which is what we use to specify which command-line options the program is willing to accept. In this case, I&#8217;ve named it <tt class="docutils literal"><span class="pre">echo</span></tt> so that it&#8217;s in line with its function.</li> <li>Calling our program now requires us to specify an option.</li> <li>The <tt class="xref py py-meth docutils literal"><span class="pre">parse_args()</span></tt> method actually returns some data from the options specified, in this case, <tt class="docutils literal"><span class="pre">echo</span></tt>.</li> <li>The variable is some form of &#8216;magic&#8217; that <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal"><span class="pre">argparse</span></tt></a> performs for free (i.e. no need to specify which variable that value is stored in). You will also notice that its name matches the string argument given to the method, <tt class="docutils literal"><span class="pre">echo</span></tt>.</li> </ul> <p>Note however that, although the help display looks nice and all, it currently is not as helpful as it can be. For example we see that we got <tt class="docutils literal"><span class="pre">echo</span></tt> as a positional argument, but we don&#8217;t know what it does, other than by guessing or by reading the source code. So, let&#8217;s make it a bit more useful:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;echo&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;echo the string you use here&quot;</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="k">print</span> <span class="n">args</span><span class="o">.</span><span class="n">echo</span> </pre></div> </div> <p>And we get:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py -h usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="nb">echo</span> positional arguments: <span class="nb">echo echo </span>the string you use here optional arguments: -h, --help show this <span class="nb">help </span>message and <span class="nb">exit</span> </pre></div> </div> <p>Now, how about doing something even more useful:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;square&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;display a square of a given number&quot;</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="k">print</span> <span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span> </pre></div> </div> <p>Following is a result of running the code:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py 4 Traceback <span class="o">(</span>most recent call last<span class="o">)</span>: File <span class="s2">&quot;prog.py&quot;</span>, line 5, in &lt;module&gt; print args.square**2 TypeError: unsupported operand <span class="nb">type</span><span class="o">(</span>s<span class="o">)</span> <span class="k">for</span> ** or pow<span class="o">()</span>: <span class="s1">&#39;str&#39;</span> and <span class="s1">&#39;int&#39;</span> </pre></div> </div> <p>That didn&#8217;t go so well. That&#8217;s because <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal"><span class="pre">argparse</span></tt></a> treats the options we give it as strings, unless we tell it otherwise. So, let&#8217;s tell <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal"><span class="pre">argparse</span></tt></a> to treat that input as an integer:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;square&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;display a square of a given number&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="k">print</span> <span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span> </pre></div> </div> <p>Following is a result of running the code:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py 4 16 <span class="nv">$ </span>python prog.py four usage: prog.py <span class="o">[</span>-h<span class="o">]</span> square prog.py: error: argument square: invalid int value: <span class="s1">&#39;four&#39;</span> </pre></div> </div> <p>That went well. The program now even helpfully quits on bad illegal input before proceeding.</p> </div> <div class="section" id="introducing-optional-arguments"> <h2>Introducing Optional arguments<a class="headerlink" href="#introducing-optional-arguments" title="Permalink to this headline">¶</a></h2> <p>So far we, have been playing with positional arguments. Let us have a look on how to add optional ones:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;increase output verbosity&quot;</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;verbosity turned on&quot;</span> </pre></div> </div> <p>And the output:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py --verbosity 1 verbosity turned on <span class="nv">$ </span>python prog.py <span class="nv">$ </span>python prog.py --help usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>--verbosity VERBOSITY<span class="o">]</span> optional arguments: -h, --help show this <span class="nb">help </span>message and <span class="nb">exit</span> --verbosity VERBOSITY increase output verbosity <span class="nv">$ </span>python prog.py --verbosity usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>--verbosity VERBOSITY<span class="o">]</span> prog.py: error: argument --verbosity: expected one argument </pre></div> </div> <p>Here is what is happening:</p> <ul class="simple"> <li>The program is written so as to display something when <tt class="docutils literal"><span class="pre">--verbosity</span></tt> is specified and display nothing when not.</li> <li>To show that the option is actually optional, there is no error when running the program without it. Note that by default, if an optional argument isn&#8217;t used, the relevant variable, in this case <tt class="xref py py-attr docutils literal"><span class="pre">args.verbosity</span></tt>, is given <tt class="docutils literal"><span class="pre">None</span></tt> as a value, which is the reason it fails the truth test of the <a class="reference internal" href="../reference/compound_stmts.html#if"><tt class="xref std std-keyword docutils literal"><span class="pre">if</span></tt></a> statement.</li> <li>The help message is a bit different.</li> <li>When using the <tt class="docutils literal"><span class="pre">--verbosity</span></tt> option, one must also specify some value, any value.</li> </ul> <p>The above example accepts arbitrary integer values for <tt class="docutils literal"><span class="pre">--verbosity</span></tt>, but for our simple program, only two values are actually useful, <tt class="docutils literal"><span class="pre">True</span></tt> or <tt class="docutils literal"><span class="pre">False</span></tt>. Let&#8217;s modify the code accordingly:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;--verbose&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;increase output verbosity&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s">&quot;store_true&quot;</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbose</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;verbosity turned on&quot;</span> </pre></div> </div> <p>And the output:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py --verbose verbosity turned on <span class="nv">$ </span>python prog.py --verbose 1 usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>--verbose<span class="o">]</span> prog.py: error: unrecognized arguments: 1 <span class="nv">$ </span>python prog.py --help usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>--verbose<span class="o">]</span> optional arguments: -h, --help show this <span class="nb">help </span>message and <span class="nb">exit</span> --verbose increase output verbosity </pre></div> </div> <p>Here is what is happening:</p> <ul class="simple"> <li>The option is now more of a flag than something that requires a value. We even changed the name of the option to match that idea. Note that we now specify a new keyword, <tt class="docutils literal"><span class="pre">action</span></tt>, and give it the value <tt class="docutils literal"><span class="pre">&quot;store_true&quot;</span></tt>. This means that, if the option is specified, assign the value <tt class="docutils literal"><span class="pre">True</span></tt> to <tt class="xref py py-data docutils literal"><span class="pre">args.verbose</span></tt>. Not specifying it implies <tt class="docutils literal"><span class="pre">False</span></tt>.</li> <li>It complains when you specify a value, in true spirit of what flags actually are.</li> <li>Notice the different help text.</li> </ul> <div class="section" id="short-options"> <h3>Short options<a class="headerlink" href="#short-options" title="Permalink to this headline">¶</a></h3> <p>If you are familiar with command line usage, you will notice that I haven&#8217;t yet touched on the topic of short versions of the options. It&#8217;s quite simple:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;-v&quot;</span><span class="p">,</span> <span class="s">&quot;--verbose&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;increase output verbosity&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s">&quot;store_true&quot;</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbose</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;verbosity turned on&quot;</span> </pre></div> </div> <p>And here goes:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py -v verbosity turned on <span class="nv">$ </span>python prog.py --help usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>-v<span class="o">]</span> optional arguments: -h, --help show this <span class="nb">help </span>message and <span class="nb">exit</span> -v, --verbose increase output verbosity </pre></div> </div> <p>Note that the new ability is also reflected in the help text.</p> </div> </div> <div class="section" id="combining-positional-and-optional-arguments"> <h2>Combining Positional and Optional arguments<a class="headerlink" href="#combining-positional-and-optional-arguments" title="Permalink to this headline">¶</a></h2> <p>Our program keeps growing in complexity:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;square&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;display a square of a given number&quot;</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;-v&quot;</span><span class="p">,</span> <span class="s">&quot;--verbose&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s">&quot;store_true&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;increase output verbosity&quot;</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span> <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbose</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;the square of {} equals {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">else</span><span class="p">:</span> <span class="k">print</span> <span class="n">answer</span> </pre></div> </div> <p>And now the output:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>-v<span class="o">]</span> square prog.py: error: the following arguments are required: square <span class="nv">$ </span>python prog.py 4 16 <span class="nv">$ </span>python prog.py 4 --verbose the square of 4 equals 16 <span class="nv">$ </span>python prog.py --verbose 4 the square of 4 equals 16 </pre></div> </div> <ul class="simple"> <li>We&#8217;ve brought back a positional argument, hence the complaint.</li> <li>Note that the order does not matter.</li> </ul> <p>How about we give this program of ours back the ability to have multiple verbosity values, and actually get to use them:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;square&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;display a square of a given number&quot;</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;-v&quot;</span><span class="p">,</span> <span class="s">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;increase output verbosity&quot;</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span> <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">==</span> <span class="mi">2</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;the square of {} equals {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;{}^2 == {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">else</span><span class="p">:</span> <span class="k">print</span> <span class="n">answer</span> </pre></div> </div> <p>And the output:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py 4 16 <span class="nv">$ </span>python prog.py 4 -v usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>-v VERBOSITY<span class="o">]</span> square prog.py: error: argument -v/--verbosity: expected one argument <span class="nv">$ </span>python prog.py 4 -v 1 4^2 <span class="o">==</span> 16 <span class="nv">$ </span>python prog.py 4 -v 2 the square of 4 equals 16 <span class="nv">$ </span>python prog.py 4 -v 3 16 </pre></div> </div> <p>These all look good except the last one, which exposes a bug in our program. Let&#8217;s fix it by restricting the values the <tt class="docutils literal"><span class="pre">--verbosity</span></tt> option can accept:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;square&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;display a square of a given number&quot;</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;-v&quot;</span><span class="p">,</span> <span class="s">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">choices</span><span class="o">=</span><span class="p">[</span><span class="mi">0</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="n">help</span><span class="o">=</span><span class="s">&quot;increase output verbosity&quot;</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span> <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">==</span> <span class="mi">2</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;the square of {} equals {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;{}^2 == {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">else</span><span class="p">:</span> <span class="k">print</span> <span class="n">answer</span> </pre></div> </div> <p>And the output:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py 4 -v 3 usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>-v <span class="o">{</span>0,1,2<span class="o">}]</span> square prog.py: error: argument -v/--verbosity: invalid choice: 3 <span class="o">(</span>choose from 0, 1, 2<span class="o">)</span> <span class="nv">$ </span>python prog.py 4 -h usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>-v <span class="o">{</span>0,1,2<span class="o">}]</span> square positional arguments: square display a square of a given number optional arguments: -h, --help show this <span class="nb">help </span>message and <span class="nb">exit</span> -v <span class="o">{</span>0,1,2<span class="o">}</span>, --verbosity <span class="o">{</span>0,1,2<span class="o">}</span> increase output verbosity </pre></div> </div> <p>Note that the change also reflects both in the error message as well as the help string.</p> <p>Now, let&#8217;s use a different approach of playing with verbosity, which is pretty common. It also matches the way the CPython executable handles its own verbosity argument (check the output of <tt class="docutils literal"><span class="pre">python</span> <span class="pre">--help</span></tt>):</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;square&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;display the square of a given number&quot;</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;-v&quot;</span><span class="p">,</span> <span class="s">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s">&quot;count&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;increase output verbosity&quot;</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span> <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">==</span> <span class="mi">2</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;the square of {} equals {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;{}^2 == {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">else</span><span class="p">:</span> <span class="k">print</span> <span class="n">answer</span> </pre></div> </div> <p>We have introduced another action, &#8220;count&#8221;, to count the number of occurences of a specific optional arguments:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py 4 16 <span class="nv">$ </span>python prog.py 4 -v 4^2 <span class="o">==</span> 16 <span class="nv">$ </span>python prog.py 4 -vv the square of 4 equals 16 <span class="nv">$ </span>python prog.py 4 --verbosity --verbosity the square of 4 equals 16 <span class="nv">$ </span>python prog.py 4 -v 1 usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>-v<span class="o">]</span> square prog.py: error: unrecognized arguments: 1 <span class="nv">$ </span>python prog.py 4 -h usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>-v<span class="o">]</span> square positional arguments: square display a square of a given number optional arguments: -h, --help show this <span class="nb">help </span>message and <span class="nb">exit</span> -v, --verbosity increase output verbosity <span class="nv">$ </span>python prog.py 4 -vvv 16 </pre></div> </div> <ul class="simple"> <li>Yes, it&#8217;s now more of a flag (similar to <tt class="docutils literal"><span class="pre">action=&quot;store_true&quot;</span></tt>) in the previous version of our script. That should explain the complaint.</li> <li>It also behaves similar to &#8220;store_true&#8221; action.</li> <li>Now here&#8217;s a demonstration of what the &#8220;count&#8221; action gives. You&#8217;ve probably seen this sort of usage before.</li> <li>And, just like the &#8220;store_true&#8221; action, if you don&#8217;t specify the <tt class="docutils literal"><span class="pre">-v</span></tt> flag, that flag is considered to have <tt class="docutils literal"><span class="pre">None</span></tt> value.</li> <li>As should be expected, specifying the long form of the flag, we should get the same output.</li> <li>Sadly, our help output isn&#8217;t very informative on the new ability our script has acquired, but that can always be fixed by improving the documentation for out script (e.g. via the <tt class="docutils literal"><span class="pre">help</span></tt> keyword argument).</li> <li>That last output exposes a bug in our program.</li> </ul> <p>Let&#8217;s fix:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;square&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;display a square of a given number&quot;</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;-v&quot;</span><span class="p">,</span> <span class="s">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s">&quot;count&quot;</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;increase output verbosity&quot;</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span> <span class="c"># bugfix: replace == with &gt;=</span> <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">2</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;the square of {} equals {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">1</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;{}^2 == {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">else</span><span class="p">:</span> <span class="k">print</span> <span class="n">answer</span> </pre></div> </div> <p>And this is what it gives:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py 4 -vvv the square of 4 equals 16 <span class="nv">$ </span>python prog.py 4 -vvvv the square of 4 equals 16 <span class="nv">$ </span>python prog.py 4 Traceback <span class="o">(</span>most recent call last<span class="o">)</span>: File <span class="s2">&quot;prog.py&quot;</span>, line 11, in &lt;module&gt; <span class="k">if </span>args.verbosity &gt;<span class="o">=</span> 2: TypeError: unorderable types: NoneType<span class="o">()</span> &gt;<span class="o">=</span> int<span class="o">()</span> </pre></div> </div> <ul class="simple"> <li>First output went well, and fixes the bug we had before. That is, we want any value &gt;= 2 to be as verbose as possible.</li> <li>Third output not so good.</li> </ul> <p>Let&#8217;s fix that bug:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;square&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;display a square of a given number&quot;</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;-v&quot;</span><span class="p">,</span> <span class="s">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s">&quot;count&quot;</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;increase output verbosity&quot;</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="o">**</span><span class="mi">2</span> <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">2</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;the square of {} equals {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">1</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;{}^2 == {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">square</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">else</span><span class="p">:</span> <span class="k">print</span> <span class="n">answer</span> </pre></div> </div> <p>We&#8217;ve just introduced yet another keyword, <tt class="docutils literal"><span class="pre">default</span></tt>. We&#8217;ve set it to <tt class="docutils literal"><span class="pre">0</span></tt> in order to make it comparable to the other int values. Remember that by default, if an optional argument isn&#8217;t specified, it gets the <tt class="docutils literal"><span class="pre">None</span></tt> value, and that cannot be compared to an int value (hence the <a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><tt class="xref py py-exc docutils literal"><span class="pre">TypeError</span></tt></a> exception).</p> <p>And:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py 4 16 </pre></div> </div> <p>You can go quite far just with what we&#8217;ve learned so far, and we have only scratched the surface. The <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal"><span class="pre">argparse</span></tt></a> module is very powerful, and we&#8217;ll explore a bit more of it before we end this tutorial.</p> </div> <div class="section" id="getting-a-little-more-advanced"> <h2>Getting a little more advanced<a class="headerlink" href="#getting-a-little-more-advanced" title="Permalink to this headline">¶</a></h2> <p>What if we wanted to expand our tiny program to perform other powers, not just squares:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;x&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;the base&quot;</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;y&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;the exponent&quot;</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;-v&quot;</span><span class="p">,</span> <span class="s">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s">&quot;count&quot;</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="o">**</span><span class="n">args</span><span class="o">.</span><span class="n">y</span> <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">2</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;{} to the power {} equals {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="p">,</span> <span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">1</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;{}^{} == {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="p">,</span> <span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">else</span><span class="p">:</span> <span class="k">print</span> <span class="n">answer</span> </pre></div> </div> <p>Output:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>-v<span class="o">]</span> x y prog.py: error: the following arguments are required: x, y <span class="nv">$ </span>python prog.py -h usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>-v<span class="o">]</span> x y positional arguments: x the base y the exponent optional arguments: -h, --help show this <span class="nb">help </span>message and <span class="nb">exit</span> -v, --verbosity <span class="nv">$ </span>python prog.py 4 2 -v 4^2 <span class="o">==</span> 16 </pre></div> </div> <p>Notice that so far we&#8217;ve been using verbosity level to <em>change</em> the text that gets displayed. The following example instead uses verbosity level to display <em>more</em> text instead:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;x&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;the base&quot;</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;y&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;the exponent&quot;</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;-v&quot;</span><span class="p">,</span> <span class="s">&quot;--verbosity&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s">&quot;count&quot;</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="o">**</span><span class="n">args</span><span class="o">.</span><span class="n">y</span> <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">2</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;Running &#39;{}&#39;&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">__file__</span><span class="p">)</span> <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span> <span class="o">&gt;=</span> <span class="mi">1</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;{}^{} ==&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="p">,</span> <span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="p">),</span> <span class="k">print</span> <span class="n">answer</span> </pre></div> </div> <p>Output:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py 4 2 16 <span class="nv">$ </span>python prog.py 4 2 -v 4^2 <span class="o">==</span> 16 <span class="nv">$ </span>python prog.py 4 2 -vv Running <span class="s1">&#39;prog.py&#39;</span> 4^2 <span class="o">==</span> 16 </pre></div> </div> <div class="section" id="conflicting-options"> <h3>Conflicting options<a class="headerlink" href="#conflicting-options" title="Permalink to this headline">¶</a></h3> <p>So far, we have been working with two methods of an <a class="reference internal" href="../library/argparse.html#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal"><span class="pre">argparse.ArgumentParser</span></tt></a> instance. Let&#8217;s introduce a third one, <tt class="xref py py-meth docutils literal"><span class="pre">add_mutually_exclusive_group()</span></tt>. It allows for us to specify options that conflict with each other. Let&#8217;s also change the rest of the program so that the new functionality makes more sense: we&#8217;ll introduce the <tt class="docutils literal"><span class="pre">--quiet</span></tt> option, which will be the opposite of the <tt class="docutils literal"><span class="pre">--verbose</span></tt> one:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span> <span class="n">group</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_mutually_exclusive_group</span><span class="p">()</span> <span class="n">group</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;-v&quot;</span><span class="p">,</span> <span class="s">&quot;--verbose&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s">&quot;store_true&quot;</span><span class="p">)</span> <span class="n">group</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;-q&quot;</span><span class="p">,</span> <span class="s">&quot;--quiet&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s">&quot;store_true&quot;</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;x&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;the base&quot;</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;y&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;the exponent&quot;</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="o">**</span><span class="n">args</span><span class="o">.</span><span class="n">y</span> <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">quiet</span><span class="p">:</span> <span class="k">print</span> <span class="n">answer</span> <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbose</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;{} to the power {} equals {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="p">,</span> <span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">else</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;{}^{} == {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="p">,</span> <span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> </pre></div> </div> <p>Our program is now simpler, and we&#8217;ve lost some functionality for the sake of demonstration. Anyways, here&#8217;s the output:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py 4 2 4^2 <span class="o">==</span> 16 <span class="nv">$ </span>python prog.py 4 2 -q 16 <span class="nv">$ </span>python prog.py 4 2 -v 4 to the power 2 equals 16 <span class="nv">$ </span>python prog.py 4 2 -vq usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>-v | -q<span class="o">]</span> x y prog.py: error: argument -q/--quiet: not allowed with argument -v/--verbose <span class="nv">$ </span>python prog.py 4 2 -v --quiet usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>-v | -q<span class="o">]</span> x y prog.py: error: argument -q/--quiet: not allowed with argument -v/--verbose </pre></div> </div> <p>That should be easy to follow. I&#8217;ve added that last output so you can see the sort of flexibility you get, i.e. mixing long form options with short form ones.</p> <p>Before we conclude, you probably want to tell your users the main purpose of your program, just in case they don&#8217;t know:</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">argparse</span> <span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">description</span><span class="o">=</span><span class="s">&quot;calculate X to the power of Y&quot;</span><span class="p">)</span> <span class="n">group</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_mutually_exclusive_group</span><span class="p">()</span> <span class="n">group</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;-v&quot;</span><span class="p">,</span> <span class="s">&quot;--verbose&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s">&quot;store_true&quot;</span><span class="p">)</span> <span class="n">group</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;-q&quot;</span><span class="p">,</span> <span class="s">&quot;--quiet&quot;</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s">&quot;store_true&quot;</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;x&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;the base&quot;</span><span class="p">)</span> <span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">&quot;y&quot;</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">&quot;the exponent&quot;</span><span class="p">)</span> <span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span> <span class="n">answer</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="o">**</span><span class="n">args</span><span class="o">.</span><span class="n">y</span> <span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">quiet</span><span class="p">:</span> <span class="k">print</span> <span class="n">answer</span> <span class="k">elif</span> <span class="n">args</span><span class="o">.</span><span class="n">verbose</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;{} to the power {} equals {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="p">,</span> <span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> <span class="k">else</span><span class="p">:</span> <span class="k">print</span> <span class="s">&quot;{}^{} == {}&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">x</span><span class="p">,</span> <span class="n">args</span><span class="o">.</span><span class="n">y</span><span class="p">,</span> <span class="n">answer</span><span class="p">)</span> </pre></div> </div> <p>Note that slight difference in the usage text. Note the <tt class="docutils literal"><span class="pre">[-v</span> <span class="pre">|</span> <span class="pre">-q]</span></tt>, which tells us that we can either use <tt class="docutils literal"><span class="pre">-v</span></tt> or <tt class="docutils literal"><span class="pre">-q</span></tt>, but not both at the same time:</p> <div class="highlight-sh"><div class="highlight"><pre><span class="nv">$ </span>python prog.py --help usage: prog.py <span class="o">[</span>-h<span class="o">]</span> <span class="o">[</span>-v | -q<span class="o">]</span> x y calculate X to the power of Y positional arguments: x the base y the exponent optional arguments: -h, --help show this <span class="nb">help </span>message and <span class="nb">exit</span> -v, --verbose -q, --quiet </pre></div> </div> </div> </div> <div class="section" id="conclusion"> <h2>Conclusion<a class="headerlink" href="#conclusion" title="Permalink to this headline">¶</a></h2> <p>The <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal"><span class="pre">argparse</span></tt></a> module offers a lot more than shown here. Its docs are quite detailed and thorough, and full of examples. Having gone through this tutorial, you should easily digest them without feeling overwhelmed.</p> </div> </div> </div> </div> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">Argparse Tutorial</a><ul> <li><a class="reference internal" href="#concepts">Concepts</a></li> <li><a class="reference internal" href="#the-basics">The basics</a></li> <li><a class="reference internal" href="#introducing-positional-arguments">Introducing Positional arguments</a></li> <li><a class="reference internal" href="#introducing-optional-arguments">Introducing Optional arguments</a><ul> <li><a class="reference internal" href="#short-options">Short options</a></li> </ul> </li> <li><a class="reference internal" href="#combining-positional-and-optional-arguments">Combining Positional and Optional arguments</a></li> <li><a class="reference internal" href="#getting-a-little-more-advanced">Getting a little more advanced</a><ul> <li><a class="reference internal" href="#conflicting-options">Conflicting options</a></li> </ul> </li> <li><a class="reference internal" href="#conclusion">Conclusion</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="webservers.html" title="previous chapter">HOWTO Use Python in the web</a></p> <h4>Next topic</h4> <p class="topless"><a href="../faq/index.html" title="next chapter">Python Frequently Asked Questions</a></p> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li><a href="../_sources/howto/argparse.txt" rel="nofollow">Show Source</a></li> </ul> <div id="searchbox" style="display: none"> <h3>Quick search</h3> <form class="search" action="../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related"> <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="../faq/index.html" title="Python Frequently Asked Questions" >next</a> |</li> <li class="right" > <a href="webservers.html" title="HOWTO Use Python in the web" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="http://www.python.org/">Python</a> &raquo;</li> <li> <a href="../index.html">Python 2.7.5 documentation</a> &raquo; </li> <li><a href="index.html" >Python HOWTOs</a> &raquo;</li> </ul> </div> <div class="footer"> &copy; <a href="../copyright.html">Copyright</a> 1990-2020, Python Software Foundation. <br /> The Python Software Foundation is a non-profit corporation. <a href="http://www.python.org/psf/donations/">Please donate.</a> <br /> Last updated on Oct 13, 2020. <a href="../bugs.html">Found a bug</a>? <br /> Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.1.3. </div> </body> </html>