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<div class="section" id="argparse-tutorial">
<h1>Argparse Tutorial<a class="headerlink" href="#argparse-tutorial" title="Permalink to this headline">¶</a></h1>
<dl class="field-list simple">
<dt class="field-odd">author</dt>
<dd class="field-odd"><p>Tshepang Lekhonkhobe</p>
</dd>
</dl>
<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."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a>, the
recommended command-line parsing module in the Python standard library.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>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."><code class="xref py py-mod docutils literal notranslate"><span class="pre">getopt</span></code></a> (an equivalent for <code class="xref c c-func docutils literal notranslate"><span class="pre">getopt()</span></code> 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)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">optparse</span></code></a>.
Note also that <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> is based on <a class="reference internal" href="../library/optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">optparse</span></code></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’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-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> ls
<span class="go">cpython devguide prog.py pypy rm-unused-function.patch</span>
<span class="gp">$</span> ls pypy
<span class="go">ctypes_configure demo dotviewer include lib_pypy lib-python ...</span>
<span class="gp">$</span> ls -l
<span class="go">total 20</span>
<span class="go">drwxr-xr-x 19 wena wena 4096 Feb 18 18:51 cpython</span>
<span class="go">drwxr-xr-x 4 wena wena 4096 Feb 8 12:04 devguide</span>
<span class="go">-rwxr-xr-x 1 wena wena 535 Feb 19 00:05 prog.py</span>
<span class="go">drwxr-xr-x 14 wena wena 4096 Feb 7 00:59 pypy</span>
<span class="go">-rw-r--r-- 1 wena wena 741 Feb 18 01:01 rm-unused-function.patch</span>
<span class="gp">$</span> ls --help
<span class="go">Usage: ls [OPTION]... [FILE]...</span>
<span class="go">List information about the FILEs (the current directory by default).</span>
<span class="go">Sort entries alphabetically if none of -cftuvSUX nor --sort is specified.</span>
<span class="go">...</span>
</pre></div>
</div>
<p>A few concepts we can learn from the four commands:</p>
<ul class="simple">
<li><p>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.</p></li>
<li><p>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, <code class="docutils literal notranslate"><span class="pre">pypy</span></code>.
What we did is specify what is known as a positional argument. It’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 <code class="docutils literal notranslate"><span class="pre">cp</span> <span class="pre">SRC</span> <span class="pre">DEST</span></code>.
The first position is <em>what you want copied,</em> and the second
position is <em>where you want it copied to</em>.</p></li>
<li><p>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 <code class="docutils literal notranslate"><span class="pre">-l</span></code> in that case is known as an optional argument.</p></li>
<li><p>That’s a snippet of the help text. It’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 its help text.</p></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-python3 notranslate"><div class="highlight"><pre><span></span><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-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py
<span class="gp">$</span> python3 prog.py --help
<span class="go">usage: prog.py [-h]</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="gp">$</span> python3 prog.py --verbose
<span class="go">usage: prog.py [-h]</span>
<span class="go">prog.py: error: unrecognized arguments: --verbose</span>
<span class="gp">$</span> python3 prog.py foo
<span class="go">usage: prog.py [-h]</span>
<span class="go">prog.py: error: unrecognized arguments: foo</span>
</pre></div>
</div>
<p>Here is what is happening:</p>
<ul class="simple">
<li><p>Running the script without any options results in nothing displayed to
stdout. Not so useful.</p></li>
<li><p>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."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a>
module. We have done almost nothing, but already we get a nice help message.</p></li>
<li><p>The <code class="docutils literal notranslate"><span class="pre">--help</span></code> option, which can also be shortened to <code class="docutils literal notranslate"><span class="pre">-h</span></code>, 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.</p></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-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"echo"</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="nb">print</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">echo</span><span class="p">)</span>
</pre></div>
</div>
<p>And running the code:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py
<span class="go">usage: prog.py [-h] echo</span>
<span class="go">prog.py: error: the following arguments are required: echo</span>
<span class="gp">$</span> python3 prog.py --help
<span class="go">usage: prog.py [-h] echo</span>
<span class="go">positional arguments:</span>
<span class="go"> echo</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="gp">$</span> python3 prog.py foo
<span class="go">foo</span>
</pre></div>
</div>
<p>Here is what’s happening:</p>
<ul class="simple">
<li><p>We’ve added the <code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code> method, which is what we use to specify
which command-line options the program is willing to accept. In this case,
I’ve named it <code class="docutils literal notranslate"><span class="pre">echo</span></code> so that it’s in line with its function.</p></li>
<li><p>Calling our program now requires us to specify an option.</p></li>
<li><p>The <code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code> method actually returns some data from the
options specified, in this case, <code class="docutils literal notranslate"><span class="pre">echo</span></code>.</p></li>
<li><p>The variable is some form of ‘magic’ that <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></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, <code class="docutils literal notranslate"><span class="pre">echo</span></code>.</p></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 <code class="docutils literal notranslate"><span class="pre">echo</span></code> as a
positional argument, but we don’t know what it does, other than by guessing or
by reading the source code. So, let’s make it a bit more useful:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"echo"</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">"echo the string you use here"</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="nb">print</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">echo</span><span class="p">)</span>
</pre></div>
</div>
<p>And we get:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py -h
<span class="go">usage: prog.py [-h] echo</span>
<span class="go">positional arguments:</span>
<span class="go"> echo echo the string you use here</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
</pre></div>
</div>
<p>Now, how about doing something even more useful:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"square"</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">"display a square of a given number"</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="nb">print</span><span class="p">(</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="p">)</span>
</pre></div>
</div>
<p>Following is a result of running the code:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py <span class="m">4</span>
<span class="go">Traceback (most recent call last):</span>
<span class="go"> File "prog.py", line 5, in <module></span>
<span class="go"> print(args.square**2)</span>
<span class="go">TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'int'</span>
</pre></div>
</div>
<p>That didn’t go so well. That’s because <a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> treats the options we
give it as strings, unless we tell it otherwise. So, let’s tell
<a class="reference internal" href="../library/argparse.html#module-argparse" title="argparse: Command-line option and argument parsing library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> to treat that input as an integer:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"square"</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">"display a square of a given number"</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="nb">print</span><span class="p">(</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="p">)</span>
</pre></div>
</div>
<p>Following is a result of running the code:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py <span class="m">4</span>
<span class="go">16</span>
<span class="gp">$</span> python3 prog.py four
<span class="go">usage: prog.py [-h] square</span>
<span class="go">prog.py: error: argument square: invalid int value: 'four'</span>
</pre></div>
</div>
<p>That went well. The program now even helpfully quits on bad illegal input
before proceeding.</p>
</div>
<div class="section" id="introducing-optional-arguments">
<h2>Introducing Optional arguments<a class="headerlink" href="#introducing-optional-arguments" title="Permalink to this headline">¶</a></h2>
<p>So far we have been playing with positional arguments. Let us
have a look on how to add optional ones:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">argparse</span>
<span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>
<span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s2">"--verbosity"</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">"increase output verbosity"</span><span class="p">)</span>
<span class="n">args</span> <span class="o">=</span> <span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>
<span class="k">if</span> <span class="n">args</span><span class="o">.</span><span class="n">verbosity</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"verbosity turned on"</span><span class="p">)</span>
</pre></div>
</div>
<p>And the output:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py --verbosity <span class="m">1</span>
<span class="go">verbosity turned on</span>
<span class="gp">$</span> python3 prog.py
<span class="gp">$</span> python3 prog.py --help
<span class="go">usage: prog.py [-h] [--verbosity VERBOSITY]</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> --verbosity VERBOSITY</span>
<span class="go"> increase output verbosity</span>
<span class="gp">$</span> python3 prog.py --verbosity
<span class="go">usage: prog.py [-h] [--verbosity VERBOSITY]</span>
<span class="go">prog.py: error: argument --verbosity: expected one argument</span>
</pre></div>
</div>
<p>Here is what is happening:</p>
<ul class="simple">
<li><p>The program is written so as to display something when <code class="docutils literal notranslate"><span class="pre">--verbosity</span></code> is
specified and display nothing when not.</p></li>
<li><p>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’t
used, the relevant variable, in this case <code class="xref py py-attr docutils literal notranslate"><span class="pre">args.verbosity</span></code>, is
given <code class="docutils literal notranslate"><span class="pre">None</span></code> as a value, which is the reason it fails the truth
test of 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></li>
<li><p>The help message is a bit different.</p></li>
<li><p>When using the <code class="docutils literal notranslate"><span class="pre">--verbosity</span></code> option, one must also specify some value,
any value.</p></li>
</ul>
<p>The above example accepts arbitrary integer values for <code class="docutils literal notranslate"><span class="pre">--verbosity</span></code>, but for
our simple program, only two values are actually useful, <code class="docutils literal notranslate"><span class="pre">True</span></code> or <code class="docutils literal notranslate"><span class="pre">False</span></code>.
Let’s modify the code accordingly:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"--verbose"</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">"increase output verbosity"</span><span class="p">,</span>
<span class="n">action</span><span class="o">=</span><span class="s2">"store_true"</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="nb">print</span><span class="p">(</span><span class="s2">"verbosity turned on"</span><span class="p">)</span>
</pre></div>
</div>
<p>And the output:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py --verbose
<span class="go">verbosity turned on</span>
<span class="gp">$</span> python3 prog.py --verbose <span class="m">1</span>
<span class="go">usage: prog.py [-h] [--verbose]</span>
<span class="go">prog.py: error: unrecognized arguments: 1</span>
<span class="gp">$</span> python3 prog.py --help
<span class="go">usage: prog.py [-h] [--verbose]</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> --verbose increase output verbosity</span>
</pre></div>
</div>
<p>Here is what is happening:</p>
<ul class="simple">
<li><p>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, <code class="docutils literal notranslate"><span class="pre">action</span></code>, and give it the value
<code class="docutils literal notranslate"><span class="pre">"store_true"</span></code>. This means that, if the option is specified,
assign the value <code class="docutils literal notranslate"><span class="pre">True</span></code> to <code class="xref py py-data docutils literal notranslate"><span class="pre">args.verbose</span></code>.
Not specifying it implies <code class="docutils literal notranslate"><span class="pre">False</span></code>.</p></li>
<li><p>It complains when you specify a value, in true spirit of what flags
actually are.</p></li>
<li><p>Notice the different help text.</p></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’t yet touched on the topic of short
versions of the options. It’s quite simple:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"-v"</span><span class="p">,</span> <span class="s2">"--verbose"</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s2">"increase output verbosity"</span><span class="p">,</span>
<span class="n">action</span><span class="o">=</span><span class="s2">"store_true"</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="nb">print</span><span class="p">(</span><span class="s2">"verbosity turned on"</span><span class="p">)</span>
</pre></div>
</div>
<p>And here goes:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py -v
<span class="go">verbosity turned on</span>
<span class="gp">$</span> python3 prog.py --help
<span class="go">usage: prog.py [-h] [-v]</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> -v, --verbose increase output verbosity</span>
</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-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"square"</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="s2">"display a square of a given number"</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="s2">"-v"</span><span class="p">,</span> <span class="s2">"--verbose"</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">"store_true"</span><span class="p">,</span>
<span class="n">help</span><span class="o">=</span><span class="s2">"increase output verbosity"</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="nb">print</span><span class="p">(</span><span class="s2">"the square of </span><span class="si">{}</span><span class="s2"> equals </span><span class="si">{}</span><span class="s2">"</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="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>
</pre></div>
</div>
<p>And now the output:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py
<span class="go">usage: prog.py [-h] [-v] square</span>
<span class="go">prog.py: error: the following arguments are required: square</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span>
<span class="go">16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> --verbose
<span class="go">the square of 4 equals 16</span>
<span class="gp">$</span> python3 prog.py --verbose <span class="m">4</span>
<span class="go">the square of 4 equals 16</span>
</pre></div>
</div>
<ul class="simple">
<li><p>We’ve brought back a positional argument, hence the complaint.</p></li>
<li><p>Note that the order does not matter.</p></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-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"square"</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="s2">"display a square of a given number"</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="s2">"-v"</span><span class="p">,</span> <span class="s2">"--verbosity"</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="s2">"increase output verbosity"</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="nb">print</span><span class="p">(</span><span class="s2">"the square of </span><span class="si">{}</span><span class="s2"> equals </span><span class="si">{}</span><span class="s2">"</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="nb">print</span><span class="p">(</span><span class="s2">"</span><span class="si">{}</span><span class="s2">^2 == </span><span class="si">{}</span><span class="s2">"</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="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>
</pre></div>
</div>
<p>And the output:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py <span class="m">4</span>
<span class="go">16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> -v
<span class="go">usage: prog.py [-h] [-v VERBOSITY] square</span>
<span class="go">prog.py: error: argument -v/--verbosity: expected one argument</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> -v <span class="m">1</span>
<span class="go">4^2 == 16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> -v <span class="m">2</span>
<span class="go">the square of 4 equals 16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> -v <span class="m">3</span>
<span class="go">16</span>
</pre></div>
</div>
<p>These all look good except the last one, which exposes a bug in our program.
Let’s fix it by restricting the values the <code class="docutils literal notranslate"><span class="pre">--verbosity</span></code> option can accept:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"square"</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="s2">"display a square of a given number"</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="s2">"-v"</span><span class="p">,</span> <span class="s2">"--verbosity"</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="s2">"increase output verbosity"</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="nb">print</span><span class="p">(</span><span class="s2">"the square of </span><span class="si">{}</span><span class="s2"> equals </span><span class="si">{}</span><span class="s2">"</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="nb">print</span><span class="p">(</span><span class="s2">"</span><span class="si">{}</span><span class="s2">^2 == </span><span class="si">{}</span><span class="s2">"</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="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>
</pre></div>
</div>
<p>And the output:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py <span class="m">4</span> -v <span class="m">3</span>
<span class="go">usage: prog.py [-h] [-v {0,1,2}] square</span>
<span class="go">prog.py: error: argument -v/--verbosity: invalid choice: 3 (choose from 0, 1, 2)</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> -h
<span class="go">usage: prog.py [-h] [-v {0,1,2}] square</span>
<span class="go">positional arguments:</span>
<span class="go"> square display a square of a given number</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> -v {0,1,2}, --verbosity {0,1,2}</span>
<span class="go"> increase output verbosity</span>
</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’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 <code class="docutils literal notranslate"><span class="pre">python</span> <span class="pre">--help</span></code>):</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"square"</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="s2">"display the square of a given number"</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="s2">"-v"</span><span class="p">,</span> <span class="s2">"--verbosity"</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">"count"</span><span class="p">,</span>
<span class="n">help</span><span class="o">=</span><span class="s2">"increase output verbosity"</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="nb">print</span><span class="p">(</span><span class="s2">"the square of </span><span class="si">{}</span><span class="s2"> equals </span><span class="si">{}</span><span class="s2">"</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="nb">print</span><span class="p">(</span><span class="s2">"</span><span class="si">{}</span><span class="s2">^2 == </span><span class="si">{}</span><span class="s2">"</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="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>
</pre></div>
</div>
<p>We have introduced another action, “count”,
to count the number of occurrences of a specific optional arguments:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py <span class="m">4</span>
<span class="go">16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> -v
<span class="go">4^2 == 16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> -vv
<span class="go">the square of 4 equals 16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> --verbosity --verbosity
<span class="go">the square of 4 equals 16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> -v <span class="m">1</span>
<span class="go">usage: prog.py [-h] [-v] square</span>
<span class="go">prog.py: error: unrecognized arguments: 1</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> -h
<span class="go">usage: prog.py [-h] [-v] square</span>
<span class="go">positional arguments:</span>
<span class="go"> square display a square of a given number</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> -v, --verbosity increase output verbosity</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> -vvv
<span class="go">16</span>
</pre></div>
</div>
<ul class="simple">
<li><p>Yes, it’s now more of a flag (similar to <code class="docutils literal notranslate"><span class="pre">action="store_true"</span></code>) in the
previous version of our script. That should explain the complaint.</p></li>
<li><p>It also behaves similar to “store_true” action.</p></li>
<li><p>Now here’s a demonstration of what the “count” action gives. You’ve probably
seen this sort of usage before.</p></li>
<li><p>And if you don’t specify the <code class="docutils literal notranslate"><span class="pre">-v</span></code> flag, that flag is considered to have
<code class="docutils literal notranslate"><span class="pre">None</span></code> value.</p></li>
<li><p>As should be expected, specifying the long form of the flag, we should get
the same output.</p></li>
<li><p>Sadly, our help output isn’t very informative on the new ability our script
has acquired, but that can always be fixed by improving the documentation for
our script (e.g. via the <code class="docutils literal notranslate"><span class="pre">help</span></code> keyword argument).</p></li>
<li><p>That last output exposes a bug in our program.</p></li>
</ul>
<p>Let’s fix:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"square"</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="s2">"display a square of a given number"</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="s2">"-v"</span><span class="p">,</span> <span class="s2">"--verbosity"</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">"count"</span><span class="p">,</span>
<span class="n">help</span><span class="o">=</span><span class="s2">"increase output verbosity"</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="c1"># bugfix: replace == with >=</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="nb">print</span><span class="p">(</span><span class="s2">"the square of </span><span class="si">{}</span><span class="s2"> equals </span><span class="si">{}</span><span class="s2">"</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="nb">print</span><span class="p">(</span><span class="s2">"</span><span class="si">{}</span><span class="s2">^2 == </span><span class="si">{}</span><span class="s2">"</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="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>
</pre></div>
</div>
<p>And this is what it gives:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py <span class="m">4</span> -vvv
<span class="go">the square of 4 equals 16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> -vvvv
<span class="go">the square of 4 equals 16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span>
<span class="go">Traceback (most recent call last):</span>
<span class="go"> File "prog.py", line 11, in <module></span>
<span class="go"> if args.verbosity >= 2:</span>
<span class="go">TypeError: '>=' not supported between instances of 'NoneType' and 'int'</span>
</pre></div>
</div>
<ul class="simple">
<li><p>First output went well, and fixes the bug we had before.
That is, we want any value >= 2 to be as verbose as possible.</p></li>
<li><p>Third output not so good.</p></li>
</ul>
<p>Let’s fix that bug:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"square"</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="s2">"display a square of a given number"</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="s2">"-v"</span><span class="p">,</span> <span class="s2">"--verbosity"</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">"count"</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="s2">"increase output verbosity"</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="nb">print</span><span class="p">(</span><span class="s2">"the square of </span><span class="si">{}</span><span class="s2"> equals </span><span class="si">{}</span><span class="s2">"</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="nb">print</span><span class="p">(</span><span class="s2">"</span><span class="si">{}</span><span class="s2">^2 == </span><span class="si">{}</span><span class="s2">"</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="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>
</pre></div>
</div>
<p>We’ve just introduced yet another keyword, <code class="docutils literal notranslate"><span class="pre">default</span></code>.
We’ve set it to <code class="docutils literal notranslate"><span class="pre">0</span></code> in order to make it comparable to the other int values.
Remember that by default,
if an optional argument isn’t specified,
it gets the <code class="docutils literal notranslate"><span class="pre">None</span></code> value, and that cannot be compared to an int value
(hence the <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> exception).</p>
<p>And:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py <span class="m">4</span>
<span class="go">16</span>
</pre></div>
</div>
<p>You can go quite far just with what we’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."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></a> module is very powerful,
and we’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-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"x"</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="s2">"the base"</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="s2">"y"</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="s2">"the exponent"</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="s2">"-v"</span><span class="p">,</span> <span class="s2">"--verbosity"</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">"count"</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">>=</span> <span class="mi">2</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"</span><span class="si">{}</span><span class="s2"> to the power </span><span class="si">{}</span><span class="s2"> equals </span><span class="si">{}</span><span class="s2">"</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">>=</span> <span class="mi">1</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"</span><span class="si">{}</span><span class="s2">^</span><span class="si">{}</span><span class="s2"> == </span><span class="si">{}</span><span class="s2">"</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="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>
</pre></div>
</div>
<p>Output:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py
<span class="go">usage: prog.py [-h] [-v] x y</span>
<span class="go">prog.py: error: the following arguments are required: x, y</span>
<span class="gp">$</span> python3 prog.py -h
<span class="go">usage: prog.py [-h] [-v] x y</span>
<span class="go">positional arguments:</span>
<span class="go"> x the base</span>
<span class="go"> y the exponent</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> -v, --verbosity</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> <span class="m">2</span> -v
<span class="go">4^2 == 16</span>
</pre></div>
</div>
<p>Notice that so far we’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-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"x"</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="s2">"the base"</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="s2">"y"</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="s2">"the exponent"</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="s2">"-v"</span><span class="p">,</span> <span class="s2">"--verbosity"</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">"count"</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">>=</span> <span class="mi">2</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"Running '</span><span class="si">{}</span><span class="s2">'"</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="vm">__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">>=</span> <span class="mi">1</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"</span><span class="si">{}</span><span class="s2">^</span><span class="si">{}</span><span class="s2"> == "</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">end</span><span class="o">=</span><span class="s2">""</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">answer</span><span class="p">)</span>
</pre></div>
</div>
<p>Output:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py <span class="m">4</span> <span class="m">2</span>
<span class="go">16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> <span class="m">2</span> -v
<span class="go">4^2 == 16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> <span class="m">2</span> -vv
<span class="go">Running 'prog.py'</span>
<span class="go">4^2 == 16</span>
</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"><code class="xref py py-class docutils literal notranslate"><span class="pre">argparse.ArgumentParser</span></code></a> instance. Let’s introduce a third one,
<code class="xref py py-meth docutils literal notranslate"><span class="pre">add_mutually_exclusive_group()</span></code>. It allows for us to specify options that
conflict with each other. Let’s also change the rest of the program so that
the new functionality makes more sense:
we’ll introduce the <code class="docutils literal notranslate"><span class="pre">--quiet</span></code> option,
which will be the opposite of the <code class="docutils literal notranslate"><span class="pre">--verbose</span></code> one:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"-v"</span><span class="p">,</span> <span class="s2">"--verbose"</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">"store_true"</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="s2">"-q"</span><span class="p">,</span> <span class="s2">"--quiet"</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">"store_true"</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="s2">"x"</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="s2">"the base"</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="s2">"y"</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="s2">"the exponent"</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="nb">print</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">verbose</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"</span><span class="si">{}</span><span class="s2"> to the power </span><span class="si">{}</span><span class="s2"> equals </span><span class="si">{}</span><span class="s2">"</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="nb">print</span><span class="p">(</span><span class="s2">"</span><span class="si">{}</span><span class="s2">^</span><span class="si">{}</span><span class="s2"> == </span><span class="si">{}</span><span class="s2">"</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’ve lost some functionality for the sake of
demonstration. Anyways, here’s the output:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py <span class="m">4</span> <span class="m">2</span>
<span class="go">4^2 == 16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> <span class="m">2</span> -q
<span class="go">16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> <span class="m">2</span> -v
<span class="go">4 to the power 2 equals 16</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> <span class="m">2</span> -vq
<span class="go">usage: prog.py [-h] [-v | -q] x y</span>
<span class="go">prog.py: error: argument -q/--quiet: not allowed with argument -v/--verbose</span>
<span class="gp">$</span> python3 prog.py <span class="m">4</span> <span class="m">2</span> -v --quiet
<span class="go">usage: prog.py [-h] [-v | -q] x y</span>
<span class="go">prog.py: error: argument -q/--quiet: not allowed with argument -v/--verbose</span>
</pre></div>
</div>
<p>That should be easy to follow. I’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’t know:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><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="s2">"calculate X to the power of Y"</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="s2">"-v"</span><span class="p">,</span> <span class="s2">"--verbose"</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">"store_true"</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="s2">"-q"</span><span class="p">,</span> <span class="s2">"--quiet"</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s2">"store_true"</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="s2">"x"</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="s2">"the base"</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="s2">"y"</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="s2">"the exponent"</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="nb">print</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">verbose</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"</span><span class="si">{}</span><span class="s2"> to the power </span><span class="si">{}</span><span class="s2"> equals </span><span class="si">{}</span><span class="s2">"</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="nb">print</span><span class="p">(</span><span class="s2">"</span><span class="si">{}</span><span class="s2">^</span><span class="si">{}</span><span class="s2"> == </span><span class="si">{}</span><span class="s2">"</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 <code class="docutils literal notranslate"><span class="pre">[-v</span> <span class="pre">|</span> <span class="pre">-q]</span></code>,
which tells us that we can either use <code class="docutils literal notranslate"><span class="pre">-v</span></code> or <code class="docutils literal notranslate"><span class="pre">-q</span></code>,
but not both at the same time:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python3 prog.py --help
<span class="go">usage: prog.py [-h] [-v | -q] x y</span>
<span class="go">calculate X to the power of Y</span>
<span class="go">positional arguments:</span>
<span class="go"> x the base</span>
<span class="go"> y the exponent</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> -v, --verbose</span>
<span class="go"> -q, --quiet</span>
</pre></div>
</div>
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<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."><code class="xref py py-mod docutils literal notranslate"><span class="pre">argparse</span></code></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>
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<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>
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<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>
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