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<div class="section" id="module-argparse">
<span id="argparse-parser-for-command-line-options-arguments-and-sub-commands"></span><h1>16.4. <a class="reference internal" href="#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> — Parser for command-line options, arguments and sub-commands<a class="headerlink" href="#module-argparse" title="本標題的永久連結">¶</a></h1>
<div class="versionadded">
<p><span class="versionmodified">3.2 版新加入.</span></p>
</div>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.7/Lib/argparse.py">Lib/argparse.py</a></p>
<hr class="docutils" />
<div class="sidebar">
<p class="first sidebar-title">Tutorial</p>
<p class="last">This page contains the API reference information. For a more gentle
introduction to Python command-line parsing, have a look at the
<a class="reference internal" href="../howto/argparse.html#id1"><span class="std std-ref">argparse tutorial</span></a>.</p>
</div>
<p>The <a class="reference internal" href="#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 makes it easy to write user-friendly command-line
interfaces. The program defines what arguments it requires, and <a class="reference internal" href="#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>
will figure out how to parse those out of <a class="reference internal" href="sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.argv</span></code></a>. The <a class="reference internal" href="#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 also automatically generates help and usage messages and issues errors
when users give the program invalid arguments.</p>
<div class="section" id="example">
<h2>16.4.1. Example<a class="headerlink" href="#example" title="本標題的永久連結">¶</a></h2>
<p>The following code is a Python program that takes a list of integers and
produces either the sum or the max:</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="s1">'Process some integers.'</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="s1">'integers'</span><span class="p">,</span> <span class="n">metavar</span><span class="o">=</span><span class="s1">'N'</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">nargs</span><span class="o">=</span><span class="s1">'+'</span><span class="p">,</span>
<span class="n">help</span><span class="o">=</span><span class="s1">'an integer for the accumulator'</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="s1">'--sum'</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="s1">'accumulate'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_const'</span><span class="p">,</span>
<span class="n">const</span><span class="o">=</span><span class="nb">sum</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="nb">max</span><span class="p">,</span>
<span class="n">help</span><span class="o">=</span><span class="s1">'sum the integers (default: find the max)'</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">accumulate</span><span class="p">(</span><span class="n">args</span><span class="o">.</span><span class="n">integers</span><span class="p">))</span>
</pre></div>
</div>
<p>Assuming the Python code above is saved into a file called <code class="docutils literal notranslate"><span class="pre">prog.py</span></code>, it can
be run at the command line and provides useful help messages:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python prog.py -h
<span class="go">usage: prog.py [-h] [--sum] N [N ...]</span>
<span class="go">Process some integers.</span>
<span class="go">positional arguments:</span>
<span class="go"> N an integer for the accumulator</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> --sum sum the integers (default: find the max)</span>
</pre></div>
</div>
<p>When run with the appropriate arguments, it prints either the sum or the max of
the command-line integers:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python prog.py <span class="m">1</span> <span class="m">2</span> <span class="m">3</span> <span class="m">4</span>
<span class="go">4</span>
<span class="gp">$</span> python prog.py <span class="m">1</span> <span class="m">2</span> <span class="m">3</span> <span class="m">4</span> --sum
<span class="go">10</span>
</pre></div>
</div>
<p>If invalid arguments are passed in, it will issue an error:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python prog.py a b c
<span class="go">usage: prog.py [-h] [--sum] N [N ...]</span>
<span class="go">prog.py: error: argument N: invalid int value: 'a'</span>
</pre></div>
</div>
<p>The following sections walk you through this example.</p>
<div class="section" id="creating-a-parser">
<h3>16.4.1.1. Creating a parser<a class="headerlink" href="#creating-a-parser" title="本標題的永久連結">¶</a></h3>
<p>The first step in using the <a class="reference internal" href="#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 creating an
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> object:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="s1">'Process some integers.'</span><span class="p">)</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> object will hold all the information necessary to
parse the command line into Python data types.</p>
</div>
<div class="section" id="adding-arguments">
<h3>16.4.1.2. Adding arguments<a class="headerlink" href="#adding-arguments" title="本標題的永久連結">¶</a></h3>
<p>Filling an <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> with information about program arguments is
done by making calls to the <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> method.
Generally, these calls tell the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> how to take the strings
on the command line and turn them into objects. This information is stored and
used when <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> is called. For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'integers'</span><span class="p">,</span> <span class="n">metavar</span><span class="o">=</span><span class="s1">'N'</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">nargs</span><span class="o">=</span><span class="s1">'+'</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">help</span><span class="o">=</span><span class="s1">'an integer for the accumulator'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--sum'</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="s1">'accumulate'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_const'</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">const</span><span class="o">=</span><span class="nb">sum</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="nb">max</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">help</span><span class="o">=</span><span class="s1">'sum the integers (default: find the max)'</span><span class="p">)</span>
</pre></div>
</div>
<p>Later, calling <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> will return an object with
two attributes, <code class="docutils literal notranslate"><span class="pre">integers</span></code> and <code class="docutils literal notranslate"><span class="pre">accumulate</span></code>. The <code class="docutils literal notranslate"><span class="pre">integers</span></code> attribute
will be a list of one or more ints, and the <code class="docutils literal notranslate"><span class="pre">accumulate</span></code> attribute will be
either the <a class="reference internal" href="functions.html#sum" title="sum"><code class="xref py py-func docutils literal notranslate"><span class="pre">sum()</span></code></a> function, if <code class="docutils literal notranslate"><span class="pre">--sum</span></code> was specified at the command line,
or the <a class="reference internal" href="functions.html#max" title="max"><code class="xref py py-func docutils literal notranslate"><span class="pre">max()</span></code></a> function if it was not.</p>
</div>
<div class="section" id="parsing-arguments">
<h3>16.4.1.3. Parsing arguments<a class="headerlink" href="#parsing-arguments" title="本標題的永久連結">¶</a></h3>
<p><a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> parses arguments through the
<a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> method. This will inspect the command line,
convert each argument to the appropriate type and then invoke the appropriate action.
In most cases, this means a simple <a class="reference internal" href="#argparse.Namespace" title="argparse.Namespace"><code class="xref py py-class docutils literal notranslate"><span class="pre">Namespace</span></code></a> object will be built up from
attributes parsed out of the command line:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--sum'</span><span class="p">,</span> <span class="s1">'7'</span><span class="p">,</span> <span class="s1">'-1'</span><span class="p">,</span> <span class="s1">'42'</span><span class="p">])</span>
<span class="go">Namespace(accumulate=<built-in function sum>, integers=[7, -1, 42])</span>
</pre></div>
</div>
<p>In a script, <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> will typically be called with no
arguments, and the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> will automatically determine the
command-line arguments from <a class="reference internal" href="sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.argv</span></code></a>.</p>
</div>
</div>
<div class="section" id="argumentparser-objects">
<h2>16.4.2. ArgumentParser objects<a class="headerlink" href="#argumentparser-objects" title="本標題的永久連結">¶</a></h2>
<dl class="class">
<dt id="argparse.ArgumentParser">
<em class="property">class </em><code class="descclassname">argparse.</code><code class="descname">ArgumentParser</code><span class="sig-paren">(</span><em>prog=None</em>, <em>usage=None</em>, <em>description=None</em>, <em>epilog=None</em>, <em>parents=[]</em>, <em>formatter_class=argparse.HelpFormatter</em>, <em>prefix_chars='-'</em>, <em>fromfile_prefix_chars=None</em>, <em>argument_default=None</em>, <em>conflict_handler='error'</em>, <em>add_help=True</em>, <em>allow_abbrev=True</em><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser" title="本定義的永久連結">¶</a></dt>
<dd><p>Create a new <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> object. All parameters should be passed
as keyword arguments. Each parameter has its own more detailed description
below, but in short they are:</p>
<ul class="simple">
<li><a class="reference internal" href="#prog">prog</a> - The name of the program (default: <code class="docutils literal notranslate"><span class="pre">sys.argv[0]</span></code>)</li>
<li><a class="reference internal" href="#usage">usage</a> - The string describing the program usage (default: generated from
arguments added to parser)</li>
<li><a class="reference internal" href="#description">description</a> - Text to display before the argument help (default: none)</li>
<li><a class="reference internal" href="#epilog">epilog</a> - Text to display after the argument help (default: none)</li>
<li><a class="reference internal" href="#parents">parents</a> - A list of <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects whose arguments should
also be included</li>
<li><a class="reference internal" href="#formatter-class">formatter_class</a> - A class for customizing the help output</li>
<li><a class="reference internal" href="#prefix-chars">prefix_chars</a> - The set of characters that prefix optional arguments
(default: 『-『)</li>
<li><a class="reference internal" href="#fromfile-prefix-chars">fromfile_prefix_chars</a> - The set of characters that prefix files from
which additional arguments should be read (default: <code class="docutils literal notranslate"><span class="pre">None</span></code>)</li>
<li><a class="reference internal" href="#argument-default">argument_default</a> - The global default value for arguments
(default: <code class="docutils literal notranslate"><span class="pre">None</span></code>)</li>
<li><a class="reference internal" href="#conflict-handler">conflict_handler</a> - The strategy for resolving conflicting optionals
(usually unnecessary)</li>
<li><a class="reference internal" href="#add-help">add_help</a> - Add a <code class="docutils literal notranslate"><span class="pre">-h/--help</span></code> option to the parser (default: <code class="docutils literal notranslate"><span class="pre">True</span></code>)</li>
<li><a class="reference internal" href="#allow-abbrev">allow_abbrev</a> - Allows long options to be abbreviated if the
abbreviation is unambiguous. (default: <code class="docutils literal notranslate"><span class="pre">True</span></code>)</li>
</ul>
<div class="versionchanged">
<p><span class="versionmodified">3.5 版更變: </span><em>allow_abbrev</em> parameter was added.</p>
</div>
</dd></dl>
<p>The following sections describe how each of these are used.</p>
<div class="section" id="prog">
<h3>16.4.2.1. prog<a class="headerlink" href="#prog" title="本標題的永久連結">¶</a></h3>
<p>By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects use <code class="docutils literal notranslate"><span class="pre">sys.argv[0]</span></code> to determine
how to display the name of the program in help messages. This default is almost
always desirable because it will make the help messages match how the program was
invoked on the command line. For example, consider a file named
<code class="docutils literal notranslate"><span class="pre">myprogram.py</span></code> with the following 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="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo help'</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>
</pre></div>
</div>
<p>The help for this program will display <code class="docutils literal notranslate"><span class="pre">myprogram.py</span></code> as the program name
(regardless of where the program was invoked from):</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python myprogram.py --help
<span class="go">usage: myprogram.py [-h] [--foo FOO]</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> --foo FOO foo help</span>
<span class="gp">$</span> <span class="nb">cd</span> ..
<span class="gp">$</span> python subdir/myprogram.py --help
<span class="go">usage: myprogram.py [-h] [--foo FOO]</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> --foo FOO foo help</span>
</pre></div>
</div>
<p>To change this default behavior, another value can be supplied using the
<code class="docutils literal notranslate"><span class="pre">prog=</span></code> argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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">prog</span><span class="o">=</span><span class="s1">'myprogram'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span>
<span class="go">usage: myprogram [-h]</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
</pre></div>
</div>
<p>Note that the program name, whether determined from <code class="docutils literal notranslate"><span class="pre">sys.argv[0]</span></code> or from the
<code class="docutils literal notranslate"><span class="pre">prog=</span></code> argument, is available to help messages using the <code class="docutils literal notranslate"><span class="pre">%(prog)s</span></code> format
specifier.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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">prog</span><span class="o">=</span><span class="s1">'myprogram'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo of the </span><span class="si">%(prog)s</span><span class="s1"> program'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span>
<span class="go">usage: myprogram [-h] [--foo FOO]</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> --foo FOO foo of the myprogram program</span>
</pre></div>
</div>
</div>
<div class="section" id="usage">
<h3>16.4.2.2. usage<a class="headerlink" href="#usage" title="本標題的永久連結">¶</a></h3>
<p>By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> calculates the usage message from the
arguments it contains:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo help'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'+'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'bar help'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span>
<span class="go">usage: PROG [-h] [--foo [FOO]] bar [bar ...]</span>
<span class="go">positional arguments:</span>
<span class="go"> bar bar help</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> --foo [FOO] foo help</span>
</pre></div>
</div>
<p>The default message can be overridden with the <code class="docutils literal notranslate"><span class="pre">usage=</span></code> keyword argument:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="n">usage</span><span class="o">=</span><span class="s1">'</span><span class="si">%(prog)s</span><span class="s1"> [options]'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo help'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'+'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'bar help'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span>
<span class="go">usage: PROG [options]</span>
<span class="go">positional arguments:</span>
<span class="go"> bar bar help</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> --foo [FOO] foo help</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">%(prog)s</span></code> format specifier is available to fill in the program name in
your usage messages.</p>
</div>
<div class="section" id="description">
<h3>16.4.2.3. description<a class="headerlink" href="#description" title="本標題的永久連結">¶</a></h3>
<p>Most calls to the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> constructor will use the
<code class="docutils literal notranslate"><span class="pre">description=</span></code> keyword argument. This argument gives a brief description of
what the program does and how it works. In help messages, the description is
displayed between the command-line usage string and the help messages for the
various arguments:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="s1">'A foo that bars'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span>
<span class="go">usage: argparse.py [-h]</span>
<span class="go">A foo that bars</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
</pre></div>
</div>
<p>By default, the description will be line-wrapped so that it fits within the
given space. To change this behavior, see the <a class="reference internal" href="#formatter-class">formatter_class</a> argument.</p>
</div>
<div class="section" id="epilog">
<h3>16.4.2.4. epilog<a class="headerlink" href="#epilog" title="本標題的永久連結">¶</a></h3>
<p>Some programs like to display additional description of the program after the
description of the arguments. Such text can be specified using the <code class="docutils literal notranslate"><span class="pre">epilog=</span></code>
argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">... </span> <span class="n">description</span><span class="o">=</span><span class="s1">'A foo that bars'</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">epilog</span><span class="o">=</span><span class="s2">"And that's how you'd foo a bar"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span>
<span class="go">usage: argparse.py [-h]</span>
<span class="go">A foo that bars</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go">And that's how you'd foo a bar</span>
</pre></div>
</div>
<p>As with the <a class="reference internal" href="#description">description</a> argument, the <code class="docutils literal notranslate"><span class="pre">epilog=</span></code> text is by default
line-wrapped, but this behavior can be adjusted with the <a class="reference internal" href="#formatter-class">formatter_class</a>
argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>.</p>
</div>
<div class="section" id="parents">
<h3>16.4.2.5. parents<a class="headerlink" href="#parents" title="本標題的永久連結">¶</a></h3>
<p>Sometimes, several parsers share a common set of arguments. Rather than
repeating the definitions of these arguments, a single parser with all the
shared arguments and passed to <code class="docutils literal notranslate"><span class="pre">parents=</span></code> argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>
can be used. The <code class="docutils literal notranslate"><span class="pre">parents=</span></code> argument takes a list of <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>
objects, collects all the positional and optional actions from them, and adds
these actions to the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> object being constructed:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parent_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">add_help</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parent_parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--parent'</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="gp">>>> </span><span class="n">foo_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">parents</span><span class="o">=</span><span class="p">[</span><span class="n">parent_parser</span><span class="p">])</span>
<span class="gp">>>> </span><span class="n">foo_parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">foo_parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--parent'</span><span class="p">,</span> <span class="s1">'2'</span><span class="p">,</span> <span class="s1">'XXX'</span><span class="p">])</span>
<span class="go">Namespace(foo='XXX', parent=2)</span>
<span class="gp">>>> </span><span class="n">bar_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">parents</span><span class="o">=</span><span class="p">[</span><span class="n">parent_parser</span><span class="p">])</span>
<span class="gp">>>> </span><span class="n">bar_parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--bar'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">bar_parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--bar'</span><span class="p">,</span> <span class="s1">'YYY'</span><span class="p">])</span>
<span class="go">Namespace(bar='YYY', parent=None)</span>
</pre></div>
</div>
<p>Note that most parent parsers will specify <code class="docutils literal notranslate"><span class="pre">add_help=False</span></code>. Otherwise, the
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> will see two <code class="docutils literal notranslate"><span class="pre">-h/--help</span></code> options (one in the parent
and one in the child) and raise an error.</p>
<div class="admonition note">
<p class="first admonition-title">備註</p>
<p class="last">You must fully initialize the parsers before passing them via <code class="docutils literal notranslate"><span class="pre">parents=</span></code>.
If you change the parent parsers after the child parser, those changes will
not be reflected in the child.</p>
</div>
</div>
<div class="section" id="formatter-class">
<h3>16.4.2.6. formatter_class<a class="headerlink" href="#formatter-class" title="本標題的永久連結">¶</a></h3>
<p><a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects allow the help formatting to be customized by
specifying an alternate formatting class. Currently, there are four such
classes:</p>
<dl class="class">
<dt id="argparse.RawDescriptionHelpFormatter">
<em class="property">class </em><code class="descclassname">argparse.</code><code class="descname">RawDescriptionHelpFormatter</code><a class="headerlink" href="#argparse.RawDescriptionHelpFormatter" title="本定義的永久連結">¶</a></dt>
<dt id="argparse.RawTextHelpFormatter">
<em class="property">class </em><code class="descclassname">argparse.</code><code class="descname">RawTextHelpFormatter</code><a class="headerlink" href="#argparse.RawTextHelpFormatter" title="本定義的永久連結">¶</a></dt>
<dt id="argparse.ArgumentDefaultsHelpFormatter">
<em class="property">class </em><code class="descclassname">argparse.</code><code class="descname">ArgumentDefaultsHelpFormatter</code><a class="headerlink" href="#argparse.ArgumentDefaultsHelpFormatter" title="本定義的永久連結">¶</a></dt>
<dt id="argparse.MetavarTypeHelpFormatter">
<em class="property">class </em><code class="descclassname">argparse.</code><code class="descname">MetavarTypeHelpFormatter</code><a class="headerlink" href="#argparse.MetavarTypeHelpFormatter" title="本定義的永久連結">¶</a></dt>
<dd></dd></dl>
<p><a class="reference internal" href="#argparse.RawDescriptionHelpFormatter" title="argparse.RawDescriptionHelpFormatter"><code class="xref py py-class docutils literal notranslate"><span class="pre">RawDescriptionHelpFormatter</span></code></a> and <a class="reference internal" href="#argparse.RawTextHelpFormatter" title="argparse.RawTextHelpFormatter"><code class="xref py py-class docutils literal notranslate"><span class="pre">RawTextHelpFormatter</span></code></a> give
more control over how textual descriptions are displayed.
By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects line-wrap the <a class="reference internal" href="#description">description</a> and
<a class="reference internal" href="#epilog">epilog</a> texts in command-line help messages:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">... </span> <span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">description</span><span class="o">=</span><span class="s1">'''this description</span>
<span class="gp">... </span><span class="s1"> was indented weird</span>
<span class="gp">... </span><span class="s1"> but that is okay'''</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">epilog</span><span class="o">=</span><span class="s1">'''</span>
<span class="gp">... </span><span class="s1"> likewise for this epilog whose whitespace will</span>
<span class="gp">... </span><span class="s1"> be cleaned up and whose words will be wrapped</span>
<span class="gp">... </span><span class="s1"> across a couple lines'''</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span>
<span class="go">usage: PROG [-h]</span>
<span class="go">this description was indented weird but that is okay</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go">likewise for this epilog whose whitespace will be cleaned up and whose words</span>
<span class="go">will be wrapped across a couple lines</span>
</pre></div>
</div>
<p>Passing <a class="reference internal" href="#argparse.RawDescriptionHelpFormatter" title="argparse.RawDescriptionHelpFormatter"><code class="xref py py-class docutils literal notranslate"><span class="pre">RawDescriptionHelpFormatter</span></code></a> as <code class="docutils literal notranslate"><span class="pre">formatter_class=</span></code>
indicates that <a class="reference internal" href="#description">description</a> and <a class="reference internal" href="#epilog">epilog</a> are already correctly formatted and
should not be line-wrapped:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">... </span> <span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">formatter_class</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">RawDescriptionHelpFormatter</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">description</span><span class="o">=</span><span class="n">textwrap</span><span class="o">.</span><span class="n">dedent</span><span class="p">(</span><span class="s1">'''</span><span class="se">\</span>
<span class="gp">... </span><span class="s1"> Please do not mess up this text!</span>
<span class="gp">... </span><span class="s1"> --------------------------------</span>
<span class="gp">... </span><span class="s1"> I have indented it</span>
<span class="gp">... </span><span class="s1"> exactly the way</span>
<span class="gp">... </span><span class="s1"> I want it</span>
<span class="gp">... </span><span class="s1"> '''</span><span class="p">))</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span>
<span class="go">usage: PROG [-h]</span>
<span class="go">Please do not mess up this text!</span>
<span class="go">--------------------------------</span>
<span class="go"> I have indented it</span>
<span class="go"> exactly the way</span>
<span class="go"> I want it</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
</pre></div>
</div>
<p><a class="reference internal" href="#argparse.RawTextHelpFormatter" title="argparse.RawTextHelpFormatter"><code class="xref py py-class docutils literal notranslate"><span class="pre">RawTextHelpFormatter</span></code></a> maintains whitespace for all sorts of help text,
including argument descriptions. However, multiple new lines are replaced with
one. If you wish to preserve multiple blank lines, add spaces between the
newlines.</p>
<p><a class="reference internal" href="#argparse.ArgumentDefaultsHelpFormatter" title="argparse.ArgumentDefaultsHelpFormatter"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentDefaultsHelpFormatter</span></code></a> automatically adds information about
default values to each of the argument help messages:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">... </span> <span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">formatter_class</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentDefaultsHelpFormatter</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</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">default</span><span class="o">=</span><span class="mi">42</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'FOO!'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'*'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">],</span> <span class="n">help</span><span class="o">=</span><span class="s1">'BAR!'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span>
<span class="go">usage: PROG [-h] [--foo FOO] [bar [bar ...]]</span>
<span class="go">positional arguments:</span>
<span class="go"> bar BAR! (default: [1, 2, 3])</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> --foo FOO FOO! (default: 42)</span>
</pre></div>
</div>
<p><a class="reference internal" href="#argparse.MetavarTypeHelpFormatter" title="argparse.MetavarTypeHelpFormatter"><code class="xref py py-class docutils literal notranslate"><span class="pre">MetavarTypeHelpFormatter</span></code></a> uses the name of the <a class="reference internal" href="#type">type</a> argument for each
argument as the display name for its values (rather than using the <a class="reference internal" href="#dest">dest</a>
as the regular formatter does):</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">... </span> <span class="n">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">formatter_class</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">MetavarTypeHelpFormatter</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">float</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span>
<span class="go">usage: PROG [-h] [--foo int] float</span>
<span class="go">positional arguments:</span>
<span class="go"> float</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> --foo int</span>
</pre></div>
</div>
</div>
<div class="section" id="prefix-chars">
<h3>16.4.2.7. prefix_chars<a class="headerlink" href="#prefix-chars" title="本標題的永久連結">¶</a></h3>
<p>Most command-line options will use <code class="docutils literal notranslate"><span class="pre">-</span></code> as the prefix, e.g. <code class="docutils literal notranslate"><span class="pre">-f/--foo</span></code>.
Parsers that need to support different or additional prefix
characters, e.g. for options
like <code class="docutils literal notranslate"><span class="pre">+f</span></code> or <code class="docutils literal notranslate"><span class="pre">/foo</span></code>, may specify them using the <code class="docutils literal notranslate"><span class="pre">prefix_chars=</span></code> argument
to the ArgumentParser constructor:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="n">prefix_chars</span><span class="o">=</span><span class="s1">'-+'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'+f'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'++bar'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'+f X ++bar Y'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span>
<span class="go">Namespace(bar='Y', f='X')</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">prefix_chars=</span></code> argument defaults to <code class="docutils literal notranslate"><span class="pre">'-'</span></code>. Supplying a set of
characters that does not include <code class="docutils literal notranslate"><span class="pre">-</span></code> will cause <code class="docutils literal notranslate"><span class="pre">-f/--foo</span></code> options to be
disallowed.</p>
</div>
<div class="section" id="fromfile-prefix-chars">
<h3>16.4.2.8. fromfile_prefix_chars<a class="headerlink" href="#fromfile-prefix-chars" title="本標題的永久連結">¶</a></h3>
<p>Sometimes, for example when dealing with a particularly long argument lists, it
may make sense to keep the list of arguments in a file rather than typing it out
at the command line. If the <code class="docutils literal notranslate"><span class="pre">fromfile_prefix_chars=</span></code> argument is given to the
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> constructor, then arguments that start with any of the
specified characters will be treated as files, and will be replaced by the
arguments they contain. For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'args.txt'</span><span class="p">,</span> <span class="s1">'w'</span><span class="p">)</span> <span class="k">as</span> <span class="n">fp</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">fp</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s1">'-f</span><span class="se">\n</span><span class="s1">bar'</span><span class="p">)</span>
<span class="gp">>>> </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">fromfile_prefix_chars</span><span class="o">=</span><span class="s1">'@'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-f'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-f'</span><span class="p">,</span> <span class="s1">'foo'</span><span class="p">,</span> <span class="s1">'@args.txt'</span><span class="p">])</span>
<span class="go">Namespace(f='bar')</span>
</pre></div>
</div>
<p>Arguments read from a file must by default be one per line (but see also
<a class="reference internal" href="#argparse.ArgumentParser.convert_arg_line_to_args" title="argparse.ArgumentParser.convert_arg_line_to_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">convert_arg_line_to_args()</span></code></a>) and are treated as if they
were in the same place as the original file referencing argument on the command
line. So in the example above, the expression <code class="docutils literal notranslate"><span class="pre">['-f',</span> <span class="pre">'foo',</span> <span class="pre">'@args.txt']</span></code>
is considered equivalent to the expression <code class="docutils literal notranslate"><span class="pre">['-f',</span> <span class="pre">'foo',</span> <span class="pre">'-f',</span> <span class="pre">'bar']</span></code>.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">fromfile_prefix_chars=</span></code> argument defaults to <code class="docutils literal notranslate"><span class="pre">None</span></code>, meaning that
arguments will never be treated as file references.</p>
</div>
<div class="section" id="argument-default">
<h3>16.4.2.9. argument_default<a class="headerlink" href="#argument-default" title="本標題的永久連結">¶</a></h3>
<p>Generally, argument defaults are specified either by passing a default to
<a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> or by calling the
<a class="reference internal" href="#argparse.ArgumentParser.set_defaults" title="argparse.ArgumentParser.set_defaults"><code class="xref py py-meth docutils literal notranslate"><span class="pre">set_defaults()</span></code></a> methods with a specific set of name-value
pairs. Sometimes however, it may be useful to specify a single parser-wide
default for arguments. This can be accomplished by passing the
<code class="docutils literal notranslate"><span class="pre">argument_default=</span></code> keyword argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>. For example,
to globally suppress attribute creation on <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a>
calls, we supply <code class="docutils literal notranslate"><span class="pre">argument_default=SUPPRESS</span></code>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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">argument_default</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">SUPPRESS</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'1'</span><span class="p">,</span> <span class="s1">'BAR'</span><span class="p">])</span>
<span class="go">Namespace(bar='BAR', foo='1')</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span>
<span class="go">Namespace()</span>
</pre></div>
</div>
</div>
<div class="section" id="allow-abbrev">
<span id="id1"></span><h3>16.4.2.10. allow_abbrev<a class="headerlink" href="#allow-abbrev" title="本標題的永久連結">¶</a></h3>
<p>Normally, when you pass an argument list to the
<a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> method of an <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>,
it <a class="reference internal" href="#prefix-matching"><span class="std std-ref">recognizes abbreviations</span></a> of long options.</p>
<p>This feature can be disabled by setting <code class="docutils literal notranslate"><span class="pre">allow_abbrev</span></code> to <code class="docutils literal notranslate"><span class="pre">False</span></code>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="n">allow_abbrev</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foobar'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_true'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foonley'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_false'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foon'</span><span class="p">])</span>
<span class="go">usage: PROG [-h] [--foobar] [--foonley]</span>
<span class="go">PROG: error: unrecognized arguments: --foon</span>
</pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified">3.5 版新加入.</span></p>
</div>
</div>
<div class="section" id="conflict-handler">
<h3>16.4.2.11. conflict_handler<a class="headerlink" href="#conflict-handler" title="本標題的永久連結">¶</a></h3>
<p><a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects do not allow two actions with the same option
string. By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects raise an exception if an
attempt is made to create an argument with an option string that is already in
use:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-f'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'old foo help'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'new foo help'</span><span class="p">)</span>
<span class="gt">Traceback (most recent call last):</span>
<span class="gr"> ..</span>
<span class="gr">ArgumentError</span>: <span class="n">argument --foo: conflicting option string(s): --foo</span>
</pre></div>
</div>
<p>Sometimes (e.g. when using <a class="reference internal" href="#parents">parents</a>) it may be useful to simply override any
older arguments with the same option string. To get this behavior, the value
<code class="docutils literal notranslate"><span class="pre">'resolve'</span></code> can be supplied to the <code class="docutils literal notranslate"><span class="pre">conflict_handler=</span></code> argument of
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="n">conflict_handler</span><span class="o">=</span><span class="s1">'resolve'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-f'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'old foo help'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'new foo help'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span>
<span class="go">usage: PROG [-h] [-f FOO] [--foo FOO]</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> -f FOO old foo help</span>
<span class="go"> --foo FOO new foo help</span>
</pre></div>
</div>
<p>Note that <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects only remove an action if all of its
option strings are overridden. So, in the example above, the old <code class="docutils literal notranslate"><span class="pre">-f/--foo</span></code>
action is retained as the <code class="docutils literal notranslate"><span class="pre">-f</span></code> action, because only the <code class="docutils literal notranslate"><span class="pre">--foo</span></code> option
string was overridden.</p>
</div>
<div class="section" id="add-help">
<h3>16.4.2.12. add_help<a class="headerlink" href="#add-help" title="本標題的永久連結">¶</a></h3>
<p>By default, ArgumentParser objects add an option which simply displays
the parser’s help message. For example, consider a file named
<code class="docutils literal notranslate"><span class="pre">myprogram.py</span></code> containing the following 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="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo help'</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>
</pre></div>
</div>
<p>If <code class="docutils literal notranslate"><span class="pre">-h</span></code> or <code class="docutils literal notranslate"><span class="pre">--help</span></code> is supplied at the command line, the ArgumentParser
help will be printed:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> python myprogram.py --help
<span class="go">usage: myprogram.py [-h] [--foo FOO]</span>
<span class="go">optional arguments:</span>
<span class="go"> -h, --help show this help message and exit</span>
<span class="go"> --foo FOO foo help</span>
</pre></div>
</div>
<p>Occasionally, it may be useful to disable the addition of this help option.
This can be achieved by passing <code class="docutils literal notranslate"><span class="pre">False</span></code> as the <code class="docutils literal notranslate"><span class="pre">add_help=</span></code> argument to
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="n">add_help</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s1">'foo help'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span>
<span class="go">usage: PROG [--foo FOO]</span>
<span class="go">optional arguments:</span>
<span class="go"> --foo FOO foo help</span>
</pre></div>
</div>
<p>The help option is typically <code class="docutils literal notranslate"><span class="pre">-h/--help</span></code>. The exception to this is
if the <code class="docutils literal notranslate"><span class="pre">prefix_chars=</span></code> is specified and does not include <code class="docutils literal notranslate"><span class="pre">-</span></code>, in
which case <code class="docutils literal notranslate"><span class="pre">-h</span></code> and <code class="docutils literal notranslate"><span class="pre">--help</span></code> are not valid options. In
this case, the first character in <code class="docutils literal notranslate"><span class="pre">prefix_chars</span></code> is used to prefix
the help options:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">,</span> <span class="n">prefix_chars</span><span class="o">=</span><span class="s1">'+/'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">print_help</span><span class="p">()</span>
<span class="go">usage: PROG [+h]</span>
<span class="go">optional arguments:</span>
<span class="go"> +h, ++help show this help message and exit</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="the-add-argument-method">
<h2>16.4.3. The add_argument() method<a class="headerlink" href="#the-add-argument-method" title="本標題的永久連結">¶</a></h2>
<dl class="method">
<dt id="argparse.ArgumentParser.add_argument">
<code class="descclassname">ArgumentParser.</code><code class="descname">add_argument</code><span class="sig-paren">(</span><em>name or flags...</em><span class="optional">[</span>, <em>action</em><span class="optional">]</span><span class="optional">[</span>, <em>nargs</em><span class="optional">]</span><span class="optional">[</span>, <em>const</em><span class="optional">]</span><span class="optional">[</span>, <em>default</em><span class="optional">]</span><span class="optional">[</span>, <em>type</em><span class="optional">]</span><span class="optional">[</span>, <em>choices</em><span class="optional">]</span><span class="optional">[</span>, <em>required</em><span class="optional">]</span><span class="optional">[</span>, <em>help</em><span class="optional">]</span><span class="optional">[</span>, <em>metavar</em><span class="optional">]</span><span class="optional">[</span>, <em>dest</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#argparse.ArgumentParser.add_argument" title="本定義的永久連結">¶</a></dt>
<dd><p>Define how a single command-line argument should be parsed. Each parameter
has its own more detailed description below, but in short they are:</p>
<ul class="simple">
<li><a class="reference internal" href="#name-or-flags">name or flags</a> - Either a name or a list of option strings, e.g. <code class="docutils literal notranslate"><span class="pre">foo</span></code>
or <code class="docutils literal notranslate"><span class="pre">-f,</span> <span class="pre">--foo</span></code>.</li>
<li><a class="reference internal" href="#action">action</a> - The basic type of action to be taken when this argument is
encountered at the command line.</li>
<li><a class="reference internal" href="#nargs">nargs</a> - The number of command-line arguments that should be consumed.</li>
<li><a class="reference internal" href="#const">const</a> - A constant value required by some <a class="reference internal" href="#action">action</a> and <a class="reference internal" href="#nargs">nargs</a> selections.</li>
<li><a class="reference internal" href="#default">default</a> - The value produced if the argument is absent from the
command line.</li>
<li><a class="reference internal" href="#type">type</a> - The type to which the command-line argument should be converted.</li>
<li><a class="reference internal" href="#choices">choices</a> - A container of the allowable values for the argument.</li>
<li><a class="reference internal" href="#required">required</a> - Whether or not the command-line option may be omitted
(optionals only).</li>
<li><a class="reference internal" href="#help">help</a> - A brief description of what the argument does.</li>
<li><a class="reference internal" href="#metavar">metavar</a> - A name for the argument in usage messages.</li>
<li><a class="reference internal" href="#dest">dest</a> - The name of the attribute to be added to the object returned by
<a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a>.</li>
</ul>
</dd></dl>
<p>The following sections describe how each of these are used.</p>
<div class="section" id="name-or-flags">
<h3>16.4.3.1. name or flags<a class="headerlink" href="#name-or-flags" title="本標題的永久連結">¶</a></h3>
<p>The <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> method must know whether an optional
argument, like <code class="docutils literal notranslate"><span class="pre">-f</span></code> or <code class="docutils literal notranslate"><span class="pre">--foo</span></code>, or a positional argument, like a list of
filenames, is expected. The first arguments passed to
<a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> must therefore be either a series of
flags, or a simple argument name. For example, an optional argument could
be created like:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-f'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">)</span>
</pre></div>
</div>
<p>while a positional argument could be created like:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">)</span>
</pre></div>
</div>
<p>When <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a> is called, optional arguments will be
identified by the <code class="docutils literal notranslate"><span class="pre">-</span></code> prefix, and the remaining arguments will be assumed to
be positional:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'-f'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'BAR'</span><span class="p">])</span>
<span class="go">Namespace(bar='BAR', foo=None)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'BAR'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'FOO'</span><span class="p">])</span>
<span class="go">Namespace(bar='BAR', foo='FOO')</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'FOO'</span><span class="p">])</span>
<span class="go">usage: PROG [-h] [-f FOO] bar</span>
<span class="go">PROG: error: the following arguments are required: bar</span>
</pre></div>
</div>
</div>
<div class="section" id="action">
<h3>16.4.3.2. action<a class="headerlink" href="#action" title="本標題的永久連結">¶</a></h3>
<p><a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> objects associate command-line arguments with actions. These
actions can do just about anything with the command-line arguments associated with
them, though most actions simply add an attribute to the object returned by
<a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a>. The <code class="docutils literal notranslate"><span class="pre">action</span></code> keyword argument specifies
how the command-line arguments should be handled. The supplied actions are:</p>
<ul>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">'store'</span></code> - This just stores the argument’s value. This is the default
action. For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'--foo 1'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span>
<span class="go">Namespace(foo='1')</span>
</pre></div>
</div>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">'store_const'</span></code> - This stores the value specified by the <a class="reference internal" href="#const">const</a> keyword
argument. The <code class="docutils literal notranslate"><span class="pre">'store_const'</span></code> action is most commonly used with
optional arguments that specify some sort of flag. For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_const'</span><span class="p">,</span> <span class="n">const</span><span class="o">=</span><span class="mi">42</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">])</span>
<span class="go">Namespace(foo=42)</span>
</pre></div>
</div>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">'store_true'</span></code> and <code class="docutils literal notranslate"><span class="pre">'store_false'</span></code> - These are special cases of
<code class="docutils literal notranslate"><span class="pre">'store_const'</span></code> used for storing the values <code class="docutils literal notranslate"><span class="pre">True</span></code> and <code class="docutils literal notranslate"><span class="pre">False</span></code>
respectively. In addition, they create default values of <code class="docutils literal notranslate"><span class="pre">False</span></code> and
<code class="docutils literal notranslate"><span class="pre">True</span></code> respectively. For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_true'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--bar'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_false'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--baz'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'store_false'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'--foo --bar'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span>
<span class="go">Namespace(foo=True, bar=False, baz=True)</span>
</pre></div>
</div>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">'append'</span></code> - This stores a list, and appends each argument value to the
list. This is useful to allow an option to be specified multiple times.
Example usage:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'append'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'--foo 1 --foo 2'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span>
<span class="go">Namespace(foo=['1', '2'])</span>
</pre></div>
</div>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">'append_const'</span></code> - This stores a list, and appends the value specified by
the <a class="reference internal" href="#const">const</a> keyword argument to the list. (Note that the <a class="reference internal" href="#const">const</a> keyword
argument defaults to <code class="docutils literal notranslate"><span class="pre">None</span></code>.) The <code class="docutils literal notranslate"><span class="pre">'append_const'</span></code> action is typically
useful when multiple arguments need to store constants to the same list. For
example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--str'</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="s1">'types'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'append_const'</span><span class="p">,</span> <span class="n">const</span><span class="o">=</span><span class="nb">str</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--int'</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="s1">'types'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'append_const'</span><span class="p">,</span> <span class="n">const</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'--str --int'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span>
<span class="go">Namespace(types=[<class 'str'>, <class 'int'>])</span>
</pre></div>
</div>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">'count'</span></code> - This counts the number of times a keyword argument occurs. For
example, this is useful for increasing verbosity levels:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--verbose'</span><span class="p">,</span> <span class="s1">'-v'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'count'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'-vvv'</span><span class="p">])</span>
<span class="go">Namespace(verbose=3)</span>
</pre></div>
</div>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">'help'</span></code> - This prints a complete help message for all the options in the
current parser and then exits. By default a help action is automatically
added to the parser. See <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> for details of how the
output is created.</p>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">'version'</span></code> - This expects a <code class="docutils literal notranslate"><span class="pre">version=</span></code> keyword argument in the
<a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> call, and prints version information
and exits when invoked:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">argparse</span>
<span class="gp">>>> </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">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--version'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s1">'version'</span><span class="p">,</span> <span class="n">version</span><span class="o">=</span><span class="s1">'</span><span class="si">%(prog)s</span><span class="s1"> 2.0'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--version'</span><span class="p">])</span>
<span class="go">PROG 2.0</span>
</pre></div>
</div>
</li>
</ul>
<p>You may also specify an arbitrary action by passing an Action subclass or
other object that implements the same interface. The recommended way to do
this is to extend <a class="reference internal" href="#argparse.Action" title="argparse.Action"><code class="xref py py-class docutils literal notranslate"><span class="pre">Action</span></code></a>, overriding the <code class="docutils literal notranslate"><span class="pre">__call__</span></code> method
and optionally the <code class="docutils literal notranslate"><span class="pre">__init__</span></code> method.</p>
<p>An example of a custom action:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">FooAction</span><span class="p">(</span><span class="n">argparse</span><span class="o">.</span><span class="n">Action</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">option_strings</span><span class="p">,</span> <span class="n">dest</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">if</span> <span class="n">nargs</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s2">"nargs not allowed"</span><span class="p">)</span>
<span class="gp">... </span> <span class="nb">super</span><span class="p">(</span><span class="n">FooAction</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="fm">__init__</span><span class="p">(</span><span class="n">option_strings</span><span class="p">,</span> <span class="n">dest</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">__call__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">parser</span><span class="p">,</span> <span class="n">namespace</span><span class="p">,</span> <span class="n">values</span><span class="p">,</span> <span class="n">option_string</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'</span><span class="si">%r</span><span class="s1"> </span><span class="si">%r</span><span class="s1"> </span><span class="si">%r</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">namespace</span><span class="p">,</span> <span class="n">values</span><span class="p">,</span> <span class="n">option_string</span><span class="p">))</span>
<span class="gp">... </span> <span class="nb">setattr</span><span class="p">(</span><span class="n">namespace</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">dest</span><span class="p">,</span> <span class="n">values</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">>>> </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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="n">FooAction</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="n">FooAction</span><span class="p">)</span>
<span class="gp">>>> </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="s1">'1 --foo 2'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span>
<span class="go">Namespace(bar=None, foo=None) '1' None</span>
<span class="go">Namespace(bar='1', foo=None) '2' '--foo'</span>
<span class="gp">>>> </span><span class="n">args</span>
<span class="go">Namespace(bar='1', foo='2')</span>
</pre></div>
</div>
<p>For more details, see <a class="reference internal" href="#argparse.Action" title="argparse.Action"><code class="xref py py-class docutils literal notranslate"><span class="pre">Action</span></code></a>.</p>
</div>
<div class="section" id="nargs">
<h3>16.4.3.3. nargs<a class="headerlink" href="#nargs" title="本標題的永久連結">¶</a></h3>
<p>ArgumentParser objects usually associate a single command-line argument with a
single action to be taken. The <code class="docutils literal notranslate"><span class="pre">nargs</span></code> keyword argument associates a
different number of command-line arguments with a single action. The supported
values are:</p>
<ul>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">N</span></code> (an integer). <code class="docutils literal notranslate"><span class="pre">N</span></code> arguments from the command line will be gathered
together into a list. For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="mi">2</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'c --foo a b'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span>
<span class="go">Namespace(bar=['c'], foo=['a', 'b'])</span>
</pre></div>
</div>
<p>Note that <code class="docutils literal notranslate"><span class="pre">nargs=1</span></code> produces a list of one item. This is different from
the default, in which the item is produced by itself.</p>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">'?'</span></code>. One argument will be consumed from the command line if possible, and
produced as a single item. If no command-line argument is present, the value from
<a class="reference internal" href="#default">default</a> will be produced. Note that for optional arguments, there is an
additional case - the option string is present but not followed by a
command-line argument. In this case the value from <a class="reference internal" href="#const">const</a> will be produced. Some
examples to illustrate this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="n">const</span><span class="o">=</span><span class="s1">'c'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="s1">'d'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="s1">'d'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'XX'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'YY'</span><span class="p">])</span>
<span class="go">Namespace(bar='XX', foo='YY')</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'XX'</span><span class="p">,</span> <span class="s1">'--foo'</span><span class="p">])</span>
<span class="go">Namespace(bar='XX', foo='c')</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span>
<span class="go">Namespace(bar='d', foo='d')</span>
</pre></div>
</div>
<p>One of the more common uses of <code class="docutils literal notranslate"><span class="pre">nargs='?'</span></code> is to allow optional input and
output files:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'infile'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">FileType</span><span class="p">(</span><span class="s1">'r'</span><span class="p">),</span>
<span class="gp">... </span> <span class="n">default</span><span class="o">=</span><span class="n">sys</span><span class="o">.</span><span class="n">stdin</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'outfile'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">FileType</span><span class="p">(</span><span class="s1">'w'</span><span class="p">),</span>
<span class="gp">... </span> <span class="n">default</span><span class="o">=</span><span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'input.txt'</span><span class="p">,</span> <span class="s1">'output.txt'</span><span class="p">])</span>
<span class="go">Namespace(infile=<_io.TextIOWrapper name='input.txt' encoding='UTF-8'>,</span>
<span class="go"> outfile=<_io.TextIOWrapper name='output.txt' encoding='UTF-8'>)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span>
<span class="go">Namespace(infile=<_io.TextIOWrapper name='<stdin>' encoding='UTF-8'>,</span>
<span class="go"> outfile=<_io.TextIOWrapper name='<stdout>' encoding='UTF-8'>)</span>
</pre></div>
</div>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">'*'</span></code>. All command-line arguments present are gathered into a list. Note that
it generally doesn’t make much sense to have more than one positional argument
with <code class="docutils literal notranslate"><span class="pre">nargs='*'</span></code>, but multiple optional arguments with <code class="docutils literal notranslate"><span class="pre">nargs='*'</span></code> is
possible. For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'*'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'*'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'baz'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'*'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'a b --foo x y --bar 1 2'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span>
<span class="go">Namespace(bar=['1', '2'], baz=['a', 'b'], foo=['x', 'y'])</span>
</pre></div>
</div>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">'+'</span></code>. Just like <code class="docutils literal notranslate"><span class="pre">'*'</span></code>, all command-line args present are gathered into a
list. Additionally, an error message will be generated if there wasn’t at
least one command-line argument present. For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'+'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'a'</span><span class="p">,</span> <span class="s1">'b'</span><span class="p">])</span>
<span class="go">Namespace(foo=['a', 'b'])</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span>
<span class="go">usage: PROG [-h] foo [foo ...]</span>
<span class="go">PROG: error: the following arguments are required: foo</span>
</pre></div>
</div>
</li>
</ul>
<ul id="argparse-remainder">
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">argparse.REMAINDER</span></code>. All the remaining command-line arguments are gathered
into a list. This is commonly useful for command line utilities that dispatch
to other command line utilities:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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">prog</span><span class="o">=</span><span class="s1">'PROG'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'command'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'args'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="n">argparse</span><span class="o">.</span><span class="n">REMAINDER</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s1">'--foo B cmd --arg1 XX ZZ'</span><span class="o">.</span><span class="n">split</span><span class="p">()))</span>
<span class="go">Namespace(args=['--arg1', 'XX', 'ZZ'], command='cmd', foo='B')</span>
</pre></div>
</div>
</li>
</ul>
<p>If the <code class="docutils literal notranslate"><span class="pre">nargs</span></code> keyword argument is not provided, the number of arguments consumed
is determined by the <a class="reference internal" href="#action">action</a>. Generally this means a single command-line argument
will be consumed and a single item (not a list) will be produced.</p>
</div>
<div class="section" id="const">
<h3>16.4.3.4. const<a class="headerlink" href="#const" title="本標題的永久連結">¶</a></h3>
<p>The <code class="docutils literal notranslate"><span class="pre">const</span></code> argument of <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> is used to hold
constant values that are not read from the command line but are required for
the various <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><code class="xref py py-class docutils literal notranslate"><span class="pre">ArgumentParser</span></code></a> actions. The two most common uses of it are:</p>
<ul class="simple">
<li>When <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> is called with
<code class="docutils literal notranslate"><span class="pre">action='store_const'</span></code> or <code class="docutils literal notranslate"><span class="pre">action='append_const'</span></code>. These actions add the
<code class="docutils literal notranslate"><span class="pre">const</span></code> value to one of the attributes of the object returned by
<a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code class="xref py py-meth docutils literal notranslate"><span class="pre">parse_args()</span></code></a>. See the <a class="reference internal" href="#action">action</a> description for examples.</li>
<li>When <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a> is called with option strings
(like <code class="docutils literal notranslate"><span class="pre">-f</span></code> or <code class="docutils literal notranslate"><span class="pre">--foo</span></code>) and <code class="docutils literal notranslate"><span class="pre">nargs='?'</span></code>. This creates an optional
argument that can be followed by zero or one command-line arguments.
When parsing the command line, if the option string is encountered with no
command-line argument following it, the value of <code class="docutils literal notranslate"><span class="pre">const</span></code> will be assumed instead.
See the <a class="reference internal" href="#nargs">nargs</a> description for examples.</li>
</ul>
<p>With the <code class="docutils literal notranslate"><span class="pre">'store_const'</span></code> and <code class="docutils literal notranslate"><span class="pre">'append_const'</span></code> actions, the <code class="docutils literal notranslate"><span class="pre">const</span></code>
keyword argument must be given. For other actions, it defaults to <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p>
</div>
<div class="section" id="default">
<h3>16.4.3.5. default<a class="headerlink" href="#default" title="本標題的永久連結">¶</a></h3>
<p>All optional arguments and some positional arguments may be omitted at the
command line. The <code class="docutils literal notranslate"><span class="pre">default</span></code> keyword argument of
<a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code class="xref py py-meth docutils literal notranslate"><span class="pre">add_argument()</span></code></a>, whose value defaults to <code class="docutils literal notranslate"><span class="pre">None</span></code>,
specifies what value should be used if the command-line argument is not present.
For optional arguments, the <code class="docutils literal notranslate"><span class="pre">default</span></code> value is used when the option string
was not present at the command line:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">42</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'--foo'</span><span class="p">,</span> <span class="s1">'2'</span><span class="p">])</span>
<span class="go">Namespace(foo='2')</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span>
<span class="go">Namespace(foo=42)</span>
</pre></div>
</div>
<p>If the <code class="docutils literal notranslate"><span class="pre">default</span></code> value is a string, the parser parses the value as if it
were a command-line argument. In particular, the parser applies any <a class="reference internal" href="#type">type</a>
conversion argument, if provided, before setting the attribute on the
<a class="reference internal" href="#argparse.Namespace" title="argparse.Namespace"><code class="xref py py-class docutils literal notranslate"><span class="pre">Namespace</span></code></a> return value. Otherwise, the parser uses the value as is:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--length'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="s1">'10'</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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'--width'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mf">10.5</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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">()</span>
<span class="go">Namespace(length=10, width=10.5)</span>
</pre></div>
</div>
<p>For positional arguments with <a class="reference internal" href="#nargs">nargs</a> equal to <code class="docutils literal notranslate"><span class="pre">?</span></code> or <code class="docutils literal notranslate"><span class="pre">*</span></code>, the <code class="docutils literal notranslate"><span class="pre">default</span></code> value
is used when no command-line argument was present:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s1">'?'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">42</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s1">'a'</span><span class="p">])</span>
<span class="go">Namespace(foo='a')</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([])</span>
<span class="go">Namespace(foo=42)</span>
</pre></div>
</div>
<p>Providing <code class="docutils literal notranslate"><span class="pre">default=argparse.SUPPRESS</span></code> causes no attribute to be added if the
command-line argument was not present:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </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>