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 <div class="section" id="module-argparse">
<h1>15.4. <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal">argparse</tt></a> &#8212; Parser for command-line options, arguments and sub-commands<a class="headerlink" href="#module-argparse" title="Permalink to this headline">¶</a></h1>

New in version 2.7.
Source code: <a class="reference external" href="http://hg.python.org/cpython/file/2.7/Lib/argparse.py">Lib/argparse.py</a>
<hr class="docutils" />
<div class="sidebar">
Tutorial
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">argparse tutorial</a>.
</div>
The <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal">argparse</tt></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."><tt class="xref py py-mod docutils literal">argparse</tt></a>
will figure out how to parse those out of <a class="reference internal" href="sys.html#sys.argv" title="sys.argv"><tt class="xref py py-data docutils literal">sys.argv</tt></a>. The <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal">argparse</tt></a>
module also automatically generates help and usage messages and issues errors
when users give the program invalid arguments.
<div class="section" id="example">
<h2>15.4.1. Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2>
The following code is a Python program that takes a list of integers and
produces either the sum or the max:
<div class="highlight-python"><div class="highlight"><pre>import argparse

parser = argparse.ArgumentParser(description=&#39;Process some integers.&#39;)
parser.add_argument(&#39;integers&#39;, metavar=&#39;N&#39;, type=int, nargs=&#39;+&#39;,
 help=&#39;an integer for the accumulator&#39;)
parser.add_argument(&#39;--sum&#39;, dest=&#39;accumulate&#39;, action=&#39;store_const&#39;,
 const=sum, default=max,
 help=&#39;sum the integers (default: find the max)&#39;)

args = parser.parse_args()
print args.accumulate(args.integers)
</pre></div>
</div>
Assuming the Python code above is saved into a file called <tt class="docutils literal">prog.py</tt>, it can
be run at the command line and provides useful help messages:
<div class="highlight-python"><pre>$ prog.py -h
usage: prog.py [-h] [--sum] N [N ...]

Process some integers.

positional arguments:
 N           an integer for the accumulator

optional arguments:
 -h, --help show this help message and exit
 --sum sum the integers (default: find the max)</pre>
</div>
When run with the appropriate arguments, it prints either the sum or the max of
the command-line integers:
<div class="highlight-python"><pre>$ prog.py 1 2 3 4
4

$ prog.py 1 2 3 4 --sum
10</pre>
</div>
If invalid arguments are passed in, it will issue an error:
<div class="highlight-python"><pre>$ prog.py a b c
usage: prog.py [-h] [--sum] N [N ...]
prog.py: error: argument N: invalid int value: 'a'</pre>
</div>
The following sections walk you through this example.
<div class="section" id="creating-a-parser">
<h3>15.4.1.1. Creating a parser<a class="headerlink" href="#creating-a-parser" title="Permalink to this headline">¶</a></h3>
The first step in using the <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal">argparse</tt></a> is creating an
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> object:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(description=&#39;Process some integers.&#39;)
</pre></div>
</div>
The <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> object will hold all the information necessary to
parse the command line into Python data types.
</div>
<div class="section" id="adding-arguments">
<h3>15.4.1.2. Adding arguments<a class="headerlink" href="#adding-arguments" title="Permalink to this headline">¶</a></h3>
Filling an <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></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"><tt class="xref py py-meth docutils literal">add_argument()</tt></a> method.
Generally, these calls tell the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></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"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> is called. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser.add_argument(&#39;integers&#39;, metavar=&#39;N&#39;, type=int, nargs=&#39;+&#39;,
... help=&#39;an integer for the accumulator&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--sum&#39;, dest=&#39;accumulate&#39;, action=&#39;store_const&#39;,
... const=sum, default=max,
... help=&#39;sum the integers (default: find the max)&#39;)
</pre></div>
</div>
Later, calling <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> will return an object with
two attributes, <tt class="docutils literal">integers</tt> and <tt class="docutils literal">accumulate</tt>. The <tt class="docutils literal">integers</tt> attribute
will be a list of one or more ints, and the <tt class="docutils literal">accumulate</tt> attribute will be
either the <a class="reference internal" href="functions.html#sum" title="sum"><tt class="xref py py-func docutils literal">sum()</tt></a> function, if <tt class="docutils literal">--sum</tt> was specified at the command line,
or the <a class="reference internal" href="functions.html#max" title="max"><tt class="xref py py-func docutils literal">max()</tt></a> function if it was not.
</div>
<div class="section" id="parsing-arguments">
<h3>15.4.1.3. Parsing arguments<a class="headerlink" href="#parsing-arguments" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> parses arguments through the
<a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></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"><tt class="xref py py-class docutils literal">Namespace</tt></a> object will be built up from
attributes parsed out of the command line:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser.parse_args([&#39;--sum&#39;, &#39;7&#39;, &#39;-1&#39;, &#39;42&#39;])
Namespace(accumulate=&lt;built-in function sum&gt;, integers=[7, -1, 42])
</pre></div>
</div>
In a script, <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> will typically be called with no
arguments, and the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> will automatically determine the
command-line arguments from <a class="reference internal" href="sys.html#sys.argv" title="sys.argv"><tt class="xref py py-data docutils literal">sys.argv</tt></a>.
</div>
</div>
<div class="section" id="argumentparser-objects">
<h2>15.4.2. ArgumentParser objects<a class="headerlink" href="#argumentparser-objects" title="Permalink to this headline">¶</a></h2>
<dl class="class">
<dt id="argparse.ArgumentParser">
class <tt class="descclassname">argparse.</tt><tt class="descname">ArgumentParser</tt><big>(</big>prog=None, usage=None, description=None, epilog=None, parents=[], formatter_class=argparse.HelpFormatter, prefix_chars='-', fromfile_prefix_chars=None, argument_default=None, conflict_handler='error', add_help=True<big>)</big><a class="headerlink" href="#argparse.ArgumentParser" title="Permalink to this definition">¶</a></dt>
<dd>Create a new <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> object. All parameters should be passed
as keyword arguments. Each parameter has its own more detailed description
below, but in short they are:
<ul class="simple">
<li><a class="reference internal" href="#prog">prog</a> - The name of the program (default: <tt class="docutils literal">sys.argv[0]</tt>)</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"><tt class="xref py py-class docutils literal">ArgumentParser</tt></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: &#8216;-&#8216;)</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: <tt class="docutils literal">None</tt>)</li>
<li><a class="reference internal" href="#argument-default">argument_default</a> - The global default value for arguments
(default: <tt class="docutils literal">None</tt>)</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 -h/&#8211;help option to the parser (default: <tt class="docutils literal">True</tt>)</li>
</ul>
</dd></dl>

The following sections describe how each of these are used.
<div class="section" id="prog">
<h3>15.4.2.1. prog<a class="headerlink" href="#prog" title="Permalink to this headline">¶</a></h3>
By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> objects uses <tt class="docutils literal">sys.argv[0]</tt> 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
<tt class="docutils literal">myprogram.py</tt> with the following code:
<div class="highlight-python"><div class="highlight"><pre>import argparse
parser = argparse.ArgumentParser()
parser.add_argument(&#39;--foo&#39;, help=&#39;foo help&#39;)
args = parser.parse_args()
</pre></div>
</div>
The help for this program will display <tt class="docutils literal">myprogram.py</tt> as the program name
(regardless of where the program was invoked from):
<div class="highlight-python"><pre>$ python myprogram.py --help
usage: myprogram.py [-h] [--foo FOO]

optional arguments:
 -h, --help  show this help message and exit
 --foo FOO   foo help
$ cd ..
$ python subdir\myprogram.py --help
usage: myprogram.py [-h] [--foo FOO]

optional arguments:
 -h, --help show this help message and exit
 --foo FOO foo help</pre>
</div>
To change this default behavior, another value can be supplied using the
<tt class="docutils literal">prog=</tt> argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;myprogram&#39;)
&gt;&gt;&gt; parser.print_help()
usage: myprogram [-h]

optional arguments:
 -h, --help show this help message and exit
</pre></div>
</div>
Note that the program name, whether determined from <tt class="docutils literal">sys.argv[0]</tt> or from the
<tt class="docutils literal">prog=</tt> argument, is available to help messages using the <tt class="docutils literal">%(prog)s</tt> format
specifier.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;myprogram&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, help=&#39;foo of the %(prog)s program&#39;)
&gt;&gt;&gt; parser.print_help()
usage: myprogram [-h] [--foo FOO]

optional arguments:
 -h, --help show this help message and exit
 --foo FOO foo of the myprogram program
</pre></div>
</div>
</div>
<div class="section" id="usage">
<h3>15.4.2.2. usage<a class="headerlink" href="#usage" title="Permalink to this headline">¶</a></h3>
By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> calculates the usage message from the
arguments it contains:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, nargs=&#39;?&#39;, help=&#39;foo help&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;, nargs=&#39;+&#39;, help=&#39;bar help&#39;)
&gt;&gt;&gt; parser.print_help()
usage: PROG [-h] [--foo [FOO]] bar [bar ...]

positional arguments:
 bar bar help

optional arguments:
 -h, --help show this help message and exit
 --foo [FOO] foo help
</pre></div>
</div>
The default message can be overridden with the <tt class="docutils literal">usage=</tt> keyword argument:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;, usage=&#39;%(prog)s [options]&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, nargs=&#39;?&#39;, help=&#39;foo help&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;, nargs=&#39;+&#39;, help=&#39;bar help&#39;)
&gt;&gt;&gt; parser.print_help()
usage: PROG [options]

positional arguments:
 bar bar help

optional arguments:
 -h, --help show this help message and exit
 --foo [FOO] foo help
</pre></div>
</div>
The <tt class="docutils literal">%(prog)s</tt> format specifier is available to fill in the program name in
your usage messages.
</div>
<div class="section" id="description">
<h3>15.4.2.3. description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h3>
Most calls to the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> constructor will use the
<tt class="docutils literal">description=</tt> 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:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(description=&#39;A foo that bars&#39;)
&gt;&gt;&gt; parser.print_help()
usage: argparse.py [-h]

A foo that bars

optional arguments:
 -h, --help show this help message and exit
</pre></div>
</div>
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.
</div>
<div class="section" id="epilog">
<h3>15.4.2.4. epilog<a class="headerlink" href="#epilog" title="Permalink to this headline">¶</a></h3>
Some programs like to display additional description of the program after the
description of the arguments. Such text can be specified using the <tt class="docutils literal">epilog=</tt>
argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(
... description=&#39;A foo that bars&#39;,
... epilog=&quot;And that&#39;s how you&#39;d foo a bar&quot;)
&gt;&gt;&gt; parser.print_help()
usage: argparse.py [-h]

A foo that bars

optional arguments:
 -h, --help show this help message and exit

And that&#39;s how you&#39;d foo a bar
</pre></div>
</div>
As with the <a class="reference internal" href="#description">description</a> argument, the <tt class="docutils literal">epilog=</tt> 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"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a>.
</div>
<div class="section" id="parents">
<h3>15.4.2.5. parents<a class="headerlink" href="#parents" title="Permalink to this headline">¶</a></h3>
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 <tt class="docutils literal">parents=</tt> argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a>
can be used. The <tt class="docutils literal">parents=</tt> argument takes a list of <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></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"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> object being constructed:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parent_parser = argparse.ArgumentParser(add_help=False)
&gt;&gt;&gt; parent_parser.add_argument(&#39;--parent&#39;, type=int)

&gt;&gt;&gt; foo_parser = argparse.ArgumentParser(parents=[parent_parser])
&gt;&gt;&gt; foo_parser.add_argument(&#39;foo&#39;)
&gt;&gt;&gt; foo_parser.parse_args([&#39;--parent&#39;, &#39;2&#39;, &#39;XXX&#39;])
Namespace(foo=&#39;XXX&#39;, parent=2)

&gt;&gt;&gt; bar_parser = argparse.ArgumentParser(parents=[parent_parser])
&gt;&gt;&gt; bar_parser.add_argument(&#39;--bar&#39;)
&gt;&gt;&gt; bar_parser.parse_args([&#39;--bar&#39;, &#39;YYY&#39;])
Namespace(bar=&#39;YYY&#39;, parent=None)
</pre></div>
</div>
Note that most parent parsers will specify <tt class="docutils literal">add_help=False</tt>. Otherwise, the
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> will see two <tt class="docutils literal">-h/--help</tt> options (one in the parent
and one in the child) and raise an error.
<div class="admonition note">
Note
You must fully initialize the parsers before passing them via <tt class="docutils literal">parents=</tt>.
If you change the parent parsers after the child parser, those changes will
not be reflected in the child.
</div>
</div>
<div class="section" id="formatter-class">
<h3>15.4.2.6. formatter_class<a class="headerlink" href="#formatter-class" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> objects allow the help formatting to be customized by
specifying an alternate formatting class. Currently, there are three such
classes:
<dl class="class">
<dt id="argparse.RawDescriptionHelpFormatter">
class <tt class="descclassname">argparse.</tt><tt class="descname">RawDescriptionHelpFormatter</tt><a class="headerlink" href="#argparse.RawDescriptionHelpFormatter" title="Permalink to this definition">¶</a></dt>
<dt id="argparse.RawTextHelpFormatter">
class <tt class="descclassname">argparse.</tt><tt class="descname">RawTextHelpFormatter</tt><a class="headerlink" href="#argparse.RawTextHelpFormatter" title="Permalink to this definition">¶</a></dt>
<dt id="argparse.ArgumentDefaultsHelpFormatter">
class <tt class="descclassname">argparse.</tt><tt class="descname">ArgumentDefaultsHelpFormatter</tt><a class="headerlink" href="#argparse.ArgumentDefaultsHelpFormatter" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>

The first two allow more control over how textual descriptions are displayed,
while the last automatically adds information about argument default values.
By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></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:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(
... prog=&#39;PROG&#39;,
... description=&#39;&#39;&#39;this description
... was indented weird
... but that is okay&#39;&#39;&#39;,
... epilog=&#39;&#39;&#39;
... likewise for this epilog whose whitespace will
... be cleaned up and whose words will be wrapped
... across a couple lines&#39;&#39;&#39;)
&gt;&gt;&gt; parser.print_help()
usage: PROG [-h]

this description was indented weird but that is okay

optional arguments:
 -h, --help show this help message and exit

likewise for this epilog whose whitespace will be cleaned up and whose words
will be wrapped across a couple lines
</pre></div>
</div>
Passing <a class="reference internal" href="#argparse.RawDescriptionHelpFormatter" title="argparse.RawDescriptionHelpFormatter"><tt class="xref py py-class docutils literal">RawDescriptionHelpFormatter</tt></a> as <tt class="docutils literal">formatter_class=</tt>
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:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(
... prog=&#39;PROG&#39;,
... formatter_class=argparse.RawDescriptionHelpFormatter,
... description=textwrap.dedent(&#39;&#39;&#39;\
... Please do not mess up this text!
... --------------------------------
... I have indented it
... exactly the way
... I want it
... &#39;&#39;&#39;))
&gt;&gt;&gt; parser.print_help()
usage: PROG [-h]

Please do not mess up this text!
--------------------------------
 I have indented it
 exactly the way
 I want it

optional arguments:
 -h, --help show this help message and exit
</pre></div>
</div>
<a class="reference internal" href="#argparse.RawTextHelpFormatter" title="argparse.RawTextHelpFormatter"><tt class="xref py py-class docutils literal">RawTextHelpFormatter</tt></a> maintains whitespace for all sorts of help text,
including argument descriptions.
The other formatter class available, <a class="reference internal" href="#argparse.ArgumentDefaultsHelpFormatter" title="argparse.ArgumentDefaultsHelpFormatter"><tt class="xref py py-class docutils literal">ArgumentDefaultsHelpFormatter</tt></a>,
will add information about the default value of each of the arguments:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(
... prog=&#39;PROG&#39;,
... formatter_class=argparse.ArgumentDefaultsHelpFormatter)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, type=int, default=42, help=&#39;FOO!&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;, nargs=&#39;*&#39;, default=[1, 2, 3], help=&#39;BAR!&#39;)
&gt;&gt;&gt; parser.print_help()
usage: PROG [-h] [--foo FOO] [bar [bar ...]]

positional arguments:
 bar BAR! (default: [1, 2, 3])

optional arguments:
 -h, --help show this help message and exit
 --foo FOO FOO! (default: 42)
</pre></div>
</div>
</div>
<div class="section" id="prefix-chars">
<h3>15.4.2.7. prefix_chars<a class="headerlink" href="#prefix-chars" title="Permalink to this headline">¶</a></h3>
Most command-line options will use <tt class="docutils literal">-</tt> as the prefix, e.g. <tt class="docutils literal">-f/--foo</tt>.
Parsers that need to support different or additional prefix
characters, e.g. for options
like <tt class="docutils literal">+f</tt> or <tt class="docutils literal">/foo</tt>, may specify them using the <tt class="docutils literal">prefix_chars=</tt> argument
to the ArgumentParser constructor:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;, prefix_chars=&#39;-+&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;+f&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;++bar&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;+f X ++bar Y&#39;.split())
Namespace(bar=&#39;Y&#39;, f=&#39;X&#39;)
</pre></div>
</div>
The <tt class="docutils literal">prefix_chars=</tt> argument defaults to <tt class="docutils literal">'-'</tt>. Supplying a set of
characters that does not include <tt class="docutils literal">-</tt> will cause <tt class="docutils literal">-f/--foo</tt> options to be
disallowed.
</div>
<div class="section" id="fromfile-prefix-chars">
<h3>15.4.2.8. fromfile_prefix_chars<a class="headerlink" href="#fromfile-prefix-chars" title="Permalink to this headline">¶</a></h3>
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 <tt class="docutils literal">fromfile_prefix_chars=</tt> argument is given to the
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></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:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; with open(&#39;args.txt&#39;, &#39;w&#39;) as fp:
... fp.write(&#39;-f\nbar&#39;)
&gt;&gt;&gt; parser = argparse.ArgumentParser(fromfile_prefix_chars=&#39;@&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;-f&#39;)
&gt;&gt;&gt; parser.parse_args([&#39;-f&#39;, &#39;foo&#39;, &#39;@args.txt&#39;])
Namespace(f=&#39;bar&#39;)
</pre></div>
</div>
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"><tt class="xref py py-meth docutils literal">convert_arg_line_to_args()</tt></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 <tt class="docutils literal">['-f', 'foo', '&#64;args.txt']</tt>
is considered equivalent to the expression <tt class="docutils literal">['-f', 'foo', '-f', 'bar']</tt>.
The <tt class="docutils literal">fromfile_prefix_chars=</tt> argument defaults to <tt class="docutils literal">None</tt>, meaning that
arguments will never be treated as file references.
</div>
<div class="section" id="argument-default">
<h3>15.4.2.9. argument_default<a class="headerlink" href="#argument-default" title="Permalink to this headline">¶</a></h3>
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"><tt class="xref py py-meth docutils literal">add_argument()</tt></a> or by calling the
<a class="reference internal" href="#argparse.ArgumentParser.set_defaults" title="argparse.ArgumentParser.set_defaults"><tt class="xref py py-meth docutils literal">set_defaults()</tt></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
<tt class="docutils literal">argument_default=</tt> keyword argument to <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a>. For example,
to globally suppress attribute creation on <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a>
calls, we supply <tt class="docutils literal">argument_default=SUPPRESS</tt>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(argument_default=argparse.SUPPRESS)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;, nargs=&#39;?&#39;)
&gt;&gt;&gt; parser.parse_args([&#39;--foo&#39;, &#39;1&#39;, &#39;BAR&#39;])
Namespace(bar=&#39;BAR&#39;, foo=&#39;1&#39;)
&gt;&gt;&gt; parser.parse_args([])
Namespace()
</pre></div>
</div>
</div>
<div class="section" id="conflict-handler">
<h3>15.4.2.10. conflict_handler<a class="headerlink" href="#conflict-handler" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> objects do not allow two actions with the same option
string. By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> objects raises an exception if an
attempt is made to create an argument with an option string that is already in
use:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;-f&#39;, &#39;--foo&#39;, help=&#39;old foo help&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, help=&#39;new foo help&#39;)
Traceback (most recent call last):
 ..
ArgumentError: argument --foo: conflicting option string(s): --foo
</pre></div>
</div>
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
<tt class="docutils literal">'resolve'</tt> can be supplied to the <tt class="docutils literal">conflict_handler=</tt> argument of
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;, conflict_handler=&#39;resolve&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;-f&#39;, &#39;--foo&#39;, help=&#39;old foo help&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, help=&#39;new foo help&#39;)
&gt;&gt;&gt; parser.print_help()
usage: PROG [-h] [-f FOO] [--foo FOO]

optional arguments:
 -h, --help show this help message and exit
 -f FOO old foo help
 --foo FOO new foo help
</pre></div>
</div>
Note that <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> objects only remove an action if all of its
option strings are overridden. So, in the example above, the old <tt class="docutils literal">-f/--foo</tt>
action is retained as the <tt class="docutils literal">-f</tt> action, because only the <tt class="docutils literal">--foo</tt> option
string was overridden.
</div>
<div class="section" id="add-help">
<h3>15.4.2.11. add_help<a class="headerlink" href="#add-help" title="Permalink to this headline">¶</a></h3>
By default, ArgumentParser objects add an option which simply displays
the parser&#8217;s help message. For example, consider a file named
<tt class="docutils literal">myprogram.py</tt> containing the following code:
<div class="highlight-python"><div class="highlight"><pre>import argparse
parser = argparse.ArgumentParser()
parser.add_argument(&#39;--foo&#39;, help=&#39;foo help&#39;)
args = parser.parse_args()
</pre></div>
</div>
If <tt class="docutils literal">-h</tt> or <tt class="docutils literal">--help</tt> is supplied at the command line, the ArgumentParser
help will be printed:
<div class="highlight-python"><pre>$ python myprogram.py --help
usage: myprogram.py [-h] [--foo FOO]

optional arguments:
 -h, --help show this help message and exit
 --foo FOO foo help</pre>
</div>
Occasionally, it may be useful to disable the addition of this help option.
This can be achieved by passing <tt class="docutils literal">False</tt> as the <tt class="docutils literal">add_help=</tt> argument to
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;, add_help=False)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, help=&#39;foo help&#39;)
&gt;&gt;&gt; parser.print_help()
usage: PROG [--foo FOO]

optional arguments:
 --foo FOO foo help
</pre></div>
</div>
The help option is typically <tt class="docutils literal">-h/--help</tt>. The exception to this is
if the <tt class="docutils literal">prefix_chars=</tt> is specified and does not include <tt class="docutils literal">-</tt>, in
which case <tt class="docutils literal">-h</tt> and <tt class="docutils literal">--help</tt> are not valid options. In
this case, the first character in <tt class="docutils literal">prefix_chars</tt> is used to prefix
the help options:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;, prefix_chars=&#39;+/&#39;)
&gt;&gt;&gt; parser.print_help()
usage: PROG [-h]

optional arguments:
 -h, --help show this help message and exit
</pre></div>
</div>
</div>
</div>
<div class="section" id="the-add-argument-method">
<h2>15.4.3. The add_argument() method<a class="headerlink" href="#the-add-argument-method" title="Permalink to this headline">¶</a></h2>
<dl class="method">
<dt id="argparse.ArgumentParser.add_argument">
<tt class="descclassname">ArgumentParser.</tt><tt class="descname">add_argument</tt><big>(</big>name or flags...[, action][, nargs][, const][, default][, type][, choices][, required][, help][, metavar][, dest]<big>)</big><a class="headerlink" href="#argparse.ArgumentParser.add_argument" title="Permalink to this definition">¶</a></dt>
<dd>Define how a single command-line argument should be parsed. Each parameter
has its own more detailed description below, but in short they are:
<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. <tt class="docutils literal">foo</tt>
or <tt class="docutils literal">-f, --foo</tt>.</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"><tt class="xref py py-meth docutils literal">parse_args()</tt></a>.</li>
</ul>
</dd></dl>

The following sections describe how each of these are used.
<div class="section" id="name-or-flags">
<h3>15.4.3.1. name or flags<a class="headerlink" href="#name-or-flags" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a> method must know whether an optional
argument, like <tt class="docutils literal">-f</tt> or <tt class="docutils literal">--foo</tt>, 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"><tt class="xref py py-meth docutils literal">add_argument()</tt></a> must therefore be either a series of
flags, or a simple argument name. For example, an optional argument could
be created like:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser.add_argument(&#39;-f&#39;, &#39;--foo&#39;)
</pre></div>
</div>
while a positional argument could be created like:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;)
</pre></div>
</div>
When <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> is called, optional arguments will be
identified by the <tt class="docutils literal">-</tt> prefix, and the remaining arguments will be assumed to
be positional:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;-f&#39;, &#39;--foo&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;)
&gt;&gt;&gt; parser.parse_args([&#39;BAR&#39;])
Namespace(bar=&#39;BAR&#39;, foo=None)
&gt;&gt;&gt; parser.parse_args([&#39;BAR&#39;, &#39;--foo&#39;, &#39;FOO&#39;])
Namespace(bar=&#39;BAR&#39;, foo=&#39;FOO&#39;)
&gt;&gt;&gt; parser.parse_args([&#39;--foo&#39;, &#39;FOO&#39;])
usage: PROG [-h] [-f FOO] bar
PROG: error: too few arguments
</pre></div>
</div>
</div>
<div class="section" id="action">
<h3>15.4.3.2. action<a class="headerlink" href="#action" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></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"><tt class="xref py py-meth docutils literal">parse_args()</tt></a>. The <tt class="docutils literal">action</tt> keyword argument specifies
how the command-line arguments should be handled. The supported actions are:
<ul>
<li><tt class="docutils literal">'store'</tt> - This just stores the argument&#8217;s value. This is the default
action. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;--foo 1&#39;.split())
Namespace(foo=&#39;1&#39;)
</pre></div>
</div>
</li>
<li><tt class="docutils literal">'store_const'</tt> - This stores the value specified by the <a class="reference internal" href="#const">const</a> keyword
argument. (Note that the <a class="reference internal" href="#const">const</a> keyword argument defaults to the rather
unhelpful <tt class="docutils literal">None</tt>.) The <tt class="docutils literal">'store_const'</tt> action is most commonly used with
optional arguments that specify some sort of flag. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, action=&#39;store_const&#39;, const=42)
&gt;&gt;&gt; parser.parse_args(&#39;--foo&#39;.split())
Namespace(foo=42)
</pre></div>
</div>
</li>
<li><tt class="docutils literal">'store_true'</tt> and <tt class="docutils literal">'store_false'</tt> - These are special cases of
<tt class="docutils literal">'store_const'</tt> using for storing the values <tt class="docutils literal">True</tt> and <tt class="docutils literal">False</tt>
respectively. In addition, they create default values of False and True
respectively. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, action=&#39;store_true&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--bar&#39;, action=&#39;store_false&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--baz&#39;, action=&#39;store_false&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;--foo --bar&#39;.split())
Namespace(bar=False, baz=True, foo=True)
</pre></div>
</div>
</li>
<li><tt class="docutils literal">'append'</tt> - 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:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, action=&#39;append&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;--foo 1 --foo 2&#39;.split())
Namespace(foo=[&#39;1&#39;, &#39;2&#39;])
</pre></div>
</div>
</li>
<li><tt class="docutils literal">'append_const'</tt> - 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 <tt class="docutils literal">None</tt>.) The <tt class="docutils literal">'append_const'</tt> action is typically
useful when multiple arguments need to store constants to the same list. For
example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--str&#39;, dest=&#39;types&#39;, action=&#39;append_const&#39;, const=str)
&gt;&gt;&gt; parser.add_argument(&#39;--int&#39;, dest=&#39;types&#39;, action=&#39;append_const&#39;, const=int)
&gt;&gt;&gt; parser.parse_args(&#39;--str --int&#39;.split())
Namespace(types=[&lt;type &#39;str&#39;&gt;, &lt;type &#39;int&#39;&gt;])
</pre></div>
</div>
</li>
<li><tt class="docutils literal">'count'</tt> - This counts the number of times a keyword argument occurs. For
example, this is useful for increasing verbosity levels:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--verbose&#39;, &#39;-v&#39;, action=&#39;count&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;-vvv&#39;.split())
Namespace(verbose=3)
</pre></div>
</div>
</li>
<li><tt class="docutils literal">'help'</tt> - 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"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> for details of how the
output is created.
</li>
<li><tt class="docutils literal">'version'</tt> - This expects a <tt class="docutils literal">version=</tt> keyword argument in the
<a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a> call, and prints version information
and exits when invoked:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; import argparse
&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--version&#39;, action=&#39;version&#39;, version=&#39;%(prog)s 2.0&#39;)
&gt;&gt;&gt; parser.parse_args([&#39;--version&#39;])
PROG 2.0
</pre></div>
</div>
</li>
</ul>
You can also specify an arbitrary action by passing an object that implements
the Action API. The easiest way to do this is to extend
<tt class="xref py py-class docutils literal">argparse.Action</tt>, supplying an appropriate <tt class="docutils literal">__call__</tt> method. The
<tt class="docutils literal">__call__</tt> method should accept four parameters:
<ul class="simple">
<li><tt class="docutils literal">parser</tt> - The ArgumentParser object which contains this action.</li>
<li><tt class="docutils literal">namespace</tt> - The <a class="reference internal" href="#argparse.Namespace" title="argparse.Namespace"><tt class="xref py py-class docutils literal">Namespace</tt></a> object that will be returned by
<a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a>. Most actions add an attribute to this
object.</li>
<li><tt class="docutils literal">values</tt> - The associated command-line arguments, with any type conversions
applied. (Type conversions are specified with the <a class="reference internal" href="#type">type</a> keyword argument to
<a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a>.)</li>
<li><tt class="docutils literal">option_string</tt> - The option string that was used to invoke this action.
The <tt class="docutils literal">option_string</tt> argument is optional, and will be absent if the action
is associated with a positional argument.</li>
</ul>
An example of a custom action:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; class FooAction(argparse.Action):
... def __call__(self, parser, namespace, values, option_string=None):
... print &#39;%r %r %r&#39; % (namespace, values, option_string)
... setattr(namespace, self.dest, values)
...
&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, action=FooAction)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;, action=FooAction)
&gt;&gt;&gt; args = parser.parse_args(&#39;1 --foo 2&#39;.split())
Namespace(bar=None, foo=None) &#39;1&#39; None
Namespace(bar=&#39;1&#39;, foo=None) &#39;2&#39; &#39;--foo&#39;
&gt;&gt;&gt; args
Namespace(bar=&#39;1&#39;, foo=&#39;2&#39;)
</pre></div>
</div>
</div>
<div class="section" id="nargs">
<h3>15.4.3.3. nargs<a class="headerlink" href="#nargs" title="Permalink to this headline">¶</a></h3>
ArgumentParser objects usually associate a single command-line argument with a
single action to be taken. The <tt class="docutils literal">nargs</tt> keyword argument associates a
different number of command-line arguments with a single action. The supported
values are:
<ul>
<li><tt class="docutils literal">N</tt> (an integer). <tt class="docutils literal">N</tt> arguments from the command line will be gathered
together into a list. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, nargs=2)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;, nargs=1)
&gt;&gt;&gt; parser.parse_args(&#39;c --foo a b&#39;.split())
Namespace(bar=[&#39;c&#39;], foo=[&#39;a&#39;, &#39;b&#39;])
</pre></div>
</div>
Note that <tt class="docutils literal">nargs=1</tt> produces a list of one item. This is different from
the default, in which the item is produced by itself.
</li>
<li><tt class="docutils literal">'?'</tt>. 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:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, nargs=&#39;?&#39;, const=&#39;c&#39;, default=&#39;d&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;, nargs=&#39;?&#39;, default=&#39;d&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;XX --foo YY&#39;.split())
Namespace(bar=&#39;XX&#39;, foo=&#39;YY&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;XX --foo&#39;.split())
Namespace(bar=&#39;XX&#39;, foo=&#39;c&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;&#39;.split())
Namespace(bar=&#39;d&#39;, foo=&#39;d&#39;)
</pre></div>
</div>
One of the more common uses of <tt class="docutils literal">nargs='?'</tt> is to allow optional input and
output files:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;infile&#39;, nargs=&#39;?&#39;, type=argparse.FileType(&#39;r&#39;),
... default=sys.stdin)
&gt;&gt;&gt; parser.add_argument(&#39;outfile&#39;, nargs=&#39;?&#39;, type=argparse.FileType(&#39;w&#39;),
... default=sys.stdout)
&gt;&gt;&gt; parser.parse_args([&#39;input.txt&#39;, &#39;output.txt&#39;])
Namespace(infile=&lt;open file &#39;input.txt&#39;, mode &#39;r&#39; at 0x...&gt;,
 outfile=&lt;open file &#39;output.txt&#39;, mode &#39;w&#39; at 0x...&gt;)
&gt;&gt;&gt; parser.parse_args([])
Namespace(infile=&lt;open file &#39;&lt;stdin&gt;&#39;, mode &#39;r&#39; at 0x...&gt;,
 outfile=&lt;open file &#39;&lt;stdout&gt;&#39;, mode &#39;w&#39; at 0x...&gt;)
</pre></div>
</div>
</li>
<li><tt class="docutils literal">'*'</tt>. All command-line arguments present are gathered into a list. Note that
it generally doesn&#8217;t make much sense to have more than one positional argument
with <tt class="docutils literal">nargs='*'</tt>, but multiple optional arguments with <tt class="docutils literal">nargs='*'</tt> is
possible. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, nargs=&#39;*&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--bar&#39;, nargs=&#39;*&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;baz&#39;, nargs=&#39;*&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;a b --foo x y --bar 1 2&#39;.split())
Namespace(bar=[&#39;1&#39;, &#39;2&#39;], baz=[&#39;a&#39;, &#39;b&#39;], foo=[&#39;x&#39;, &#39;y&#39;])
</pre></div>
</div>
</li>
<li><tt class="docutils literal">'+'</tt>. Just like <tt class="docutils literal">'*'</tt>, all command-line args present are gathered into a
list. Additionally, an error message will be generated if there wasn&#8217;t at
least one command-line argument present. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;foo&#39;, nargs=&#39;+&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;a b&#39;.split())
Namespace(foo=[&#39;a&#39;, &#39;b&#39;])
&gt;&gt;&gt; parser.parse_args(&#39;&#39;.split())
usage: PROG [-h] foo [foo ...]
PROG: error: too few arguments
</pre></div>
</div>
</li>
<li><tt class="docutils literal">argparse.REMAINDER</tt>. 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:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;command&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;args&#39;, nargs=argparse.REMAINDER)
&gt;&gt;&gt; print parser.parse_args(&#39;--foo B cmd --arg1 XX ZZ&#39;.split())
Namespace(args=[&#39;--arg1&#39;, &#39;XX&#39;, &#39;ZZ&#39;], command=&#39;cmd&#39;, foo=&#39;B&#39;)
</pre></div>
</div>
</li>
</ul>
If the <tt class="docutils literal">nargs</tt> 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.
</div>
<div class="section" id="const">
<h3>15.4.3.4. const<a class="headerlink" href="#const" title="Permalink to this headline">¶</a></h3>
The <tt class="docutils literal">const</tt> argument of <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></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"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> actions. The two most common uses of it are:
<ul class="simple">
<li>When <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a> is called with
<tt class="docutils literal">action='store_const'</tt> or <tt class="docutils literal">action='append_const'</tt>. These actions add the
<tt class="docutils literal">const</tt> 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"><tt class="xref py py-meth docutils literal">parse_args()</tt></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"><tt class="xref py py-meth docutils literal">add_argument()</tt></a> is called with option strings
(like <tt class="docutils literal">-f</tt> or <tt class="docutils literal">--foo</tt>) and <tt class="docutils literal">nargs='?'</tt>. 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 <tt class="docutils literal">const</tt> will be assumed instead.
See the <a class="reference internal" href="#nargs">nargs</a> description for examples.</li>
</ul>
The <tt class="docutils literal">const</tt> keyword argument defaults to <tt class="docutils literal">None</tt>.
</div>
<div class="section" id="default">
<h3>15.4.3.5. default<a class="headerlink" href="#default" title="Permalink to this headline">¶</a></h3>
All optional arguments and some positional arguments may be omitted at the
command line. The <tt class="docutils literal">default</tt> keyword argument of
<a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a>, whose value defaults to <tt class="docutils literal">None</tt>,
specifies what value should be used if the command-line argument is not present.
For optional arguments, the <tt class="docutils literal">default</tt> value is used when the option string
was not present at the command line:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, default=42)
&gt;&gt;&gt; parser.parse_args(&#39;--foo 2&#39;.split())
Namespace(foo=&#39;2&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;&#39;.split())
Namespace(foo=42)
</pre></div>
</div>
If the <tt class="docutils literal">default</tt> 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"><tt class="xref py py-class docutils literal">Namespace</tt></a> return value. Otherwise, the parser uses the value as is:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--length&#39;, default=&#39;10&#39;, type=int)
&gt;&gt;&gt; parser.add_argument(&#39;--width&#39;, default=10.5, type=int)
&gt;&gt;&gt; parser.parse_args()
Namespace(length=10, width=10.5)
</pre></div>
</div>
For positional arguments with <a class="reference internal" href="#nargs">nargs</a> equal to <tt class="docutils literal">?</tt> or <tt class="docutils literal">*</tt>, the <tt class="docutils literal">default</tt> value
is used when no command-line argument was present:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;foo&#39;, nargs=&#39;?&#39;, default=42)
&gt;&gt;&gt; parser.parse_args(&#39;a&#39;.split())
Namespace(foo=&#39;a&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;&#39;.split())
Namespace(foo=42)
</pre></div>
</div>
Providing <tt class="docutils literal">default=argparse.SUPPRESS</tt> causes no attribute to be added if the
command-line argument was not present.:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, default=argparse.SUPPRESS)
&gt;&gt;&gt; parser.parse_args([])
Namespace()
&gt;&gt;&gt; parser.parse_args([&#39;--foo&#39;, &#39;1&#39;])
Namespace(foo=&#39;1&#39;)
</pre></div>
</div>
</div>
<div class="section" id="type">
<h3>15.4.3.6. type<a class="headerlink" href="#type" title="Permalink to this headline">¶</a></h3>
By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> objects read command-line arguments in as simple
strings. However, quite often the command-line string should instead be
interpreted as another type, like a <a class="reference internal" href="functions.html#float" title="float"><tt class="xref py py-class docutils literal">float</tt></a> or <a class="reference internal" href="functions.html#int" title="int"><tt class="xref py py-class docutils literal">int</tt></a>. The
<tt class="docutils literal">type</tt> keyword argument of <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a> allows any
necessary type-checking and type conversions to be performed. Common built-in
types and functions can be used directly as the value of the <tt class="docutils literal">type</tt> argument:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;foo&#39;, type=int)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;, type=file)
&gt;&gt;&gt; parser.parse_args(&#39;2 temp.txt&#39;.split())
Namespace(bar=&lt;open file &#39;temp.txt&#39;, mode &#39;r&#39; at 0x...&gt;, foo=2)
</pre></div>
</div>
See the section on the <a class="reference internal" href="#default">default</a> keyword argument for information on when the
<tt class="docutils literal">type</tt> argument is applied to default arguments.
To ease the use of various types of files, the argparse module provides the
factory FileType which takes the <tt class="docutils literal">mode=</tt> and <tt class="docutils literal">bufsize=</tt> arguments of the
<tt class="docutils literal">file</tt> object. For example, <tt class="docutils literal">FileType('w')</tt> can be used to create a
writable file:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;, type=argparse.FileType(&#39;w&#39;))
&gt;&gt;&gt; parser.parse_args([&#39;out.txt&#39;])
Namespace(bar=&lt;open file &#39;out.txt&#39;, mode &#39;w&#39; at 0x...&gt;)
</pre></div>
</div>
<tt class="docutils literal">type=</tt> can take any callable that takes a single string argument and returns
the converted value:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def perfect_square(string):
... value = int(string)
... sqrt = math.sqrt(value)
... if sqrt != int(sqrt):
... msg = &quot;%r is not a perfect square&quot; % string
... raise argparse.ArgumentTypeError(msg)
... return value
...
&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;foo&#39;, type=perfect_square)
&gt;&gt;&gt; parser.parse_args(&#39;9&#39;.split())
Namespace(foo=9)
&gt;&gt;&gt; parser.parse_args(&#39;7&#39;.split())
usage: PROG [-h] foo
PROG: error: argument foo: &#39;7&#39; is not a perfect square
</pre></div>
</div>
The <a class="reference internal" href="#choices">choices</a> keyword argument may be more convenient for type checkers that
simply check against a range of values:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;foo&#39;, type=int, choices=xrange(5, 10))
&gt;&gt;&gt; parser.parse_args(&#39;7&#39;.split())
Namespace(foo=7)
&gt;&gt;&gt; parser.parse_args(&#39;11&#39;.split())
usage: PROG [-h] {5,6,7,8,9}
PROG: error: argument foo: invalid choice: 11 (choose from 5, 6, 7, 8, 9)
</pre></div>
</div>
See the <a class="reference internal" href="#choices">choices</a> section for more details.
</div>
<div class="section" id="choices">
<h3>15.4.3.7. choices<a class="headerlink" href="#choices" title="Permalink to this headline">¶</a></h3>
Some command-line arguments should be selected from a restricted set of values.
These can be handled by passing a container object as the choices keyword
argument to <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a>. When the command line is
parsed, argument values will be checked, and an error message will be displayed
if the argument was not one of the acceptable values:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;game.py&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;move&#39;, choices=[&#39;rock&#39;, &#39;paper&#39;, &#39;scissors&#39;])
&gt;&gt;&gt; parser.parse_args([&#39;rock&#39;])
Namespace(move=&#39;rock&#39;)
&gt;&gt;&gt; parser.parse_args([&#39;fire&#39;])
usage: game.py [-h] {rock,paper,scissors}
game.py: error: argument move: invalid choice: &#39;fire&#39; (choose from &#39;rock&#39;,
&#39;paper&#39;, &#39;scissors&#39;)
</pre></div>
</div>
Note that inclusion in the choices container is checked after any <a class="reference internal" href="#type">type</a>
conversions have been performed, so the type of the objects in the choices
container should match the <a class="reference internal" href="#type">type</a> specified:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;doors.py&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;door&#39;, type=int, choices=range(1, 4))
&gt;&gt;&gt; print(parser.parse_args([&#39;3&#39;]))
Namespace(door=3)
&gt;&gt;&gt; parser.parse_args([&#39;4&#39;])
usage: doors.py [-h] {1,2,3}
doors.py: error: argument door: invalid choice: 4 (choose from 1, 2, 3)
</pre></div>
</div>
Any object that supports the <tt class="docutils literal">in</tt> operator can be passed as the choices
value, so <a class="reference internal" href="stdtypes.html#dict" title="dict"><tt class="xref py py-class docutils literal">dict</tt></a> objects, <a class="reference internal" href="stdtypes.html#set" title="set"><tt class="xref py py-class docutils literal">set</tt></a> objects, custom containers,
etc. are all supported.
</div>
<div class="section" id="required">
<h3>15.4.3.8. required<a class="headerlink" href="#required" title="Permalink to this headline">¶</a></h3>
In general, the <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal">argparse</tt></a> module assumes that flags like <tt class="docutils literal">-f</tt> and <tt class="docutils literal">--bar</tt>
indicate optional arguments, which can always be omitted at the command line.
To make an option required, <tt class="docutils literal">True</tt> can be specified for the <tt class="docutils literal">required=</tt>
keyword argument to <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, required=True)
&gt;&gt;&gt; parser.parse_args([&#39;--foo&#39;, &#39;BAR&#39;])
Namespace(foo=&#39;BAR&#39;)
&gt;&gt;&gt; parser.parse_args([])
usage: argparse.py [-h] [--foo FOO]
argparse.py: error: option --foo is required
</pre></div>
</div>
As the example shows, if an option is marked as <tt class="docutils literal">required</tt>,
<a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> will report an error if that option is not
present at the command line.
<div class="admonition note">
Note
Required options are generally considered bad form because users expect
options to be optional, and thus they should be avoided when possible.
</div>
</div>
<div class="section" id="help">
<h3>15.4.3.9. help<a class="headerlink" href="#help" title="Permalink to this headline">¶</a></h3>
The <tt class="docutils literal">help</tt> value is a string containing a brief description of the argument.
When a user requests help (usually by using <tt class="docutils literal">-h</tt> or <tt class="docutils literal">--help</tt> at the
command line), these <tt class="docutils literal">help</tt> descriptions will be displayed with each
argument:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;frobble&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, action=&#39;store_true&#39;,
... help=&#39;foo the bars before frobbling&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;, nargs=&#39;+&#39;,
... help=&#39;one of the bars to be frobbled&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;-h&#39;.split())
usage: frobble [-h] [--foo] bar [bar ...]

positional arguments:
 bar one of the bars to be frobbled

optional arguments:
 -h, --help show this help message and exit
 --foo foo the bars before frobbling
</pre></div>
</div>
The <tt class="docutils literal">help</tt> strings can include various format specifiers to avoid repetition
of things like the program name or the argument <a class="reference internal" href="#default">default</a>. The available
specifiers include the program name, <tt class="docutils literal">%(prog)s</tt> and most keyword arguments to
<a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a>, e.g. <tt class="docutils literal">%(default)s</tt>, <tt class="docutils literal">%(type)s</tt>, etc.:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;frobble&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;, nargs=&#39;?&#39;, type=int, default=42,
... help=&#39;the bar to %(prog)s (default: %(default)s)&#39;)
&gt;&gt;&gt; parser.print_help()
usage: frobble [-h] [bar]

positional arguments:
 bar the bar to frobble (default: 42)

optional arguments:
 -h, --help show this help message and exit
</pre></div>
</div>
<a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal">argparse</tt></a> supports silencing the help entry for certain options, by
setting the <tt class="docutils literal">help</tt> value to <tt class="docutils literal">argparse.SUPPRESS</tt>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;frobble&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, help=argparse.SUPPRESS)
&gt;&gt;&gt; parser.print_help()
usage: frobble [-h]

optional arguments:
 -h, --help show this help message and exit
</pre></div>
</div>
</div>
<div class="section" id="metavar">
<h3>15.4.3.10. metavar<a class="headerlink" href="#metavar" title="Permalink to this headline">¶</a></h3>
When <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> generates help messages, it needs some way to refer
to each expected argument. By default, ArgumentParser objects use the <a class="reference internal" href="#dest">dest</a>
value as the &#8220;name&#8221; of each object. By default, for positional argument
actions, the <a class="reference internal" href="#dest">dest</a> value is used directly, and for optional argument actions,
the <a class="reference internal" href="#dest">dest</a> value is uppercased. So, a single positional argument with
<tt class="docutils literal">dest='bar'</tt> will be referred to as <tt class="docutils literal">bar</tt>. A single
optional argument <tt class="docutils literal">--foo</tt> that should be followed by a single command-line argument
will be referred to as <tt class="docutils literal">FOO</tt>. An example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;X --foo Y&#39;.split())
Namespace(bar=&#39;X&#39;, foo=&#39;Y&#39;)
&gt;&gt;&gt; parser.print_help()
usage: [-h] [--foo FOO] bar

positional arguments:
 bar

optional arguments:
 -h, --help show this help message and exit
 --foo FOO
</pre></div>
</div>
An alternative name can be specified with <tt class="docutils literal">metavar</tt>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, metavar=&#39;YYY&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;, metavar=&#39;XXX&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;X --foo Y&#39;.split())
Namespace(bar=&#39;X&#39;, foo=&#39;Y&#39;)
&gt;&gt;&gt; parser.print_help()
usage: [-h] [--foo YYY] XXX

positional arguments:
 XXX

optional arguments:
 -h, --help show this help message and exit
 --foo YYY
</pre></div>
</div>
Note that <tt class="docutils literal">metavar</tt> only changes the displayed name - the name of the
attribute on the <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> object is still determined
by the <a class="reference internal" href="#dest">dest</a> value.
Different values of <tt class="docutils literal">nargs</tt> may cause the metavar to be used multiple times.
Providing a tuple to <tt class="docutils literal">metavar</tt> specifies a different display for each of the
arguments:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;-x&#39;, nargs=2)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, nargs=2, metavar=(&#39;bar&#39;, &#39;baz&#39;))
&gt;&gt;&gt; parser.print_help()
usage: PROG [-h] [-x X X] [--foo bar baz]

optional arguments:
 -h, --help show this help message and exit
 -x X X
 --foo bar baz
</pre></div>
</div>
</div>
<div class="section" id="dest">
<h3>15.4.3.11. dest<a class="headerlink" href="#dest" title="Permalink to this headline">¶</a></h3>
Most <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> actions add some value as an attribute of the
object returned by <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a>. The name of this
attribute is determined by the <tt class="docutils literal">dest</tt> keyword argument of
<a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a>. For positional argument actions,
<tt class="docutils literal">dest</tt> is normally supplied as the first argument to
<a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;XXX&#39;.split())
Namespace(bar=&#39;XXX&#39;)
</pre></div>
</div>
For optional argument actions, the value of <tt class="docutils literal">dest</tt> is normally inferred from
the option strings. <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> generates the value of <tt class="docutils literal">dest</tt> by
taking the first long option string and stripping away the initial <tt class="docutils literal">--</tt>
string. If no long option strings were supplied, <tt class="docutils literal">dest</tt> will be derived from
the first short option string by stripping the initial <tt class="docutils literal">-</tt> character. Any
internal <tt class="docutils literal">-</tt> characters will be converted to <tt class="docutils literal">_</tt> characters to make sure
the string is a valid attribute name. The examples below illustrate this
behavior:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;-f&#39;, &#39;--foo-bar&#39;, &#39;--foo&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;-x&#39;, &#39;-y&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;-f 1 -x 2&#39;.split())
Namespace(foo_bar=&#39;1&#39;, x=&#39;2&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;--foo 1 -y 2&#39;.split())
Namespace(foo_bar=&#39;1&#39;, x=&#39;2&#39;)
</pre></div>
</div>
<tt class="docutils literal">dest</tt> allows a custom attribute name to be provided:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, dest=&#39;bar&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;--foo XXX&#39;.split())
Namespace(bar=&#39;XXX&#39;)
</pre></div>
</div>
</div>
</div>
<div class="section" id="the-parse-args-method">
<h2>15.4.4. The parse_args() method<a class="headerlink" href="#the-parse-args-method" title="Permalink to this headline">¶</a></h2>
<dl class="method">
<dt id="argparse.ArgumentParser.parse_args">
<tt class="descclassname">ArgumentParser.</tt><tt class="descname">parse_args</tt><big>(</big>args=None, namespace=None<big>)</big><a class="headerlink" href="#argparse.ArgumentParser.parse_args" title="Permalink to this definition">¶</a></dt>
<dd>Convert argument strings to objects and assign them as attributes of the
namespace. Return the populated namespace.
Previous calls to <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a> determine exactly what objects are
created and how they are assigned. See the documentation for
<a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a> for details.
By default, the argument strings are taken from <a class="reference internal" href="sys.html#sys.argv" title="sys.argv"><tt class="xref py py-data docutils literal">sys.argv</tt></a>, and a new empty
<a class="reference internal" href="#argparse.Namespace" title="argparse.Namespace"><tt class="xref py py-class docutils literal">Namespace</tt></a> object is created for the attributes.
</dd></dl>

<div class="section" id="option-value-syntax">
<h3>15.4.4.1. Option value syntax<a class="headerlink" href="#option-value-syntax" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> method supports several ways of
specifying the value of an option (if it takes one). In the simplest case, the
option and its value are passed as two separate arguments:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;-x&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;-x X&#39;.split())
Namespace(foo=None, x=&#39;X&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;--foo FOO&#39;.split())
Namespace(foo=&#39;FOO&#39;, x=None)
</pre></div>
</div>
For long options (options with names longer than a single character), the option
and value can also be passed as a single command-line argument, using <tt class="docutils literal">=</tt> to
separate them:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser.parse_args(&#39;--foo=FOO&#39;.split())
Namespace(foo=&#39;FOO&#39;, x=None)
</pre></div>
</div>
For short options (options only one character long), the option and its value
can be concatenated:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser.parse_args(&#39;-xX&#39;.split())
Namespace(foo=None, x=&#39;X&#39;)
</pre></div>
</div>
Several short options can be joined together, using only a single <tt class="docutils literal">-</tt> prefix,
as long as only the last option (or none of them) requires a value:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;-x&#39;, action=&#39;store_true&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;-y&#39;, action=&#39;store_true&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;-z&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;-xyzZ&#39;.split())
Namespace(x=True, y=True, z=&#39;Z&#39;)
</pre></div>
</div>
</div>
<div class="section" id="invalid-arguments">
<h3>15.4.4.2. Invalid arguments<a class="headerlink" href="#invalid-arguments" title="Permalink to this headline">¶</a></h3>
While parsing the command line, <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> checks for a
variety of errors, including ambiguous options, invalid types, invalid options,
wrong number of positional arguments, etc. When it encounters such an error,
it exits and prints the error along with a usage message:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, type=int)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;, nargs=&#39;?&#39;)

&gt;&gt;&gt; # invalid type
&gt;&gt;&gt; parser.parse_args([&#39;--foo&#39;, &#39;spam&#39;])
usage: PROG [-h] [--foo FOO] [bar]
PROG: error: argument --foo: invalid int value: &#39;spam&#39;

&gt;&gt;&gt; # invalid option
&gt;&gt;&gt; parser.parse_args([&#39;--bar&#39;])
usage: PROG [-h] [--foo FOO] [bar]
PROG: error: no such option: --bar

&gt;&gt;&gt; # wrong number of arguments
&gt;&gt;&gt; parser.parse_args([&#39;spam&#39;, &#39;badger&#39;])
usage: PROG [-h] [--foo FOO] [bar]
PROG: error: extra arguments found: badger
</pre></div>
</div>
</div>
<div class="section" id="arguments-containing">
<h3>15.4.4.3. Arguments containing <tt class="docutils literal">-</tt><a class="headerlink" href="#arguments-containing" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> method attempts to give errors whenever
the user has clearly made a mistake, but some situations are inherently
ambiguous. For example, the command-line argument <tt class="docutils literal">-1</tt> could either be an
attempt to specify an option or an attempt to provide a positional argument.
The <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> method is cautious here: positional
arguments may only begin with <tt class="docutils literal">-</tt> if they look like negative numbers and
there are no options in the parser that look like negative numbers:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;-x&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;foo&#39;, nargs=&#39;?&#39;)

&gt;&gt;&gt; # no negative number options, so -1 is a positional argument
&gt;&gt;&gt; parser.parse_args([&#39;-x&#39;, &#39;-1&#39;])
Namespace(foo=None, x=&#39;-1&#39;)

&gt;&gt;&gt; # no negative number options, so -1 and -5 are positional arguments
&gt;&gt;&gt; parser.parse_args([&#39;-x&#39;, &#39;-1&#39;, &#39;-5&#39;])
Namespace(foo=&#39;-5&#39;, x=&#39;-1&#39;)

&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;-1&#39;, dest=&#39;one&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;foo&#39;, nargs=&#39;?&#39;)

&gt;&gt;&gt; # negative number options present, so -1 is an option
&gt;&gt;&gt; parser.parse_args([&#39;-1&#39;, &#39;X&#39;])
Namespace(foo=None, one=&#39;X&#39;)

&gt;&gt;&gt; # negative number options present, so -2 is an option
&gt;&gt;&gt; parser.parse_args([&#39;-2&#39;])
usage: PROG [-h] [-1 ONE] [foo]
PROG: error: no such option: -2

&gt;&gt;&gt; # negative number options present, so both -1s are options
&gt;&gt;&gt; parser.parse_args([&#39;-1&#39;, &#39;-1&#39;])
usage: PROG [-h] [-1 ONE] [foo]
PROG: error: argument -1: expected one argument
</pre></div>
</div>
If you have positional arguments that must begin with <tt class="docutils literal">-</tt> and don&#8217;t look
like negative numbers, you can insert the pseudo-argument <tt class="docutils literal">'--'</tt> which tells
<a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> that everything after that is a positional
argument:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser.parse_args([&#39;--&#39;, &#39;-f&#39;])
Namespace(foo=&#39;-f&#39;, one=None)
</pre></div>
</div>
</div>
<div class="section" id="argument-abbreviations">
<h3>15.4.4.4. Argument abbreviations<a class="headerlink" href="#argument-abbreviations" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> method allows long options to be
abbreviated if the abbreviation is unambiguous:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;-bacon&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;-badger&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;-bac MMM&#39;.split())
Namespace(bacon=&#39;MMM&#39;, badger=None)
&gt;&gt;&gt; parser.parse_args(&#39;-bad WOOD&#39;.split())
Namespace(bacon=None, badger=&#39;WOOD&#39;)
&gt;&gt;&gt; parser.parse_args(&#39;-ba BA&#39;.split())
usage: PROG [-h] [-bacon BACON] [-badger BADGER]
PROG: error: ambiguous option: -ba could match -badger, -bacon
</pre></div>
</div>
An error is produced for arguments that could produce more than one options.
</div>
<div class="section" id="beyond-sys-argv">
<h3>15.4.4.5. Beyond <tt class="docutils literal">sys.argv</tt><a class="headerlink" href="#beyond-sys-argv" title="Permalink to this headline">¶</a></h3>
Sometimes it may be useful to have an ArgumentParser parse arguments other than those
of <a class="reference internal" href="sys.html#sys.argv" title="sys.argv"><tt class="xref py py-data docutils literal">sys.argv</tt></a>. This can be accomplished by passing a list of strings to
<a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a>. This is useful for testing at the
interactive prompt:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(
... &#39;integers&#39;, metavar=&#39;int&#39;, type=int, choices=xrange(10),
... nargs=&#39;+&#39;, help=&#39;an integer in the range 0..9&#39;)
&gt;&gt;&gt; parser.add_argument(
... &#39;--sum&#39;, dest=&#39;accumulate&#39;, action=&#39;store_const&#39;, const=sum,
... default=max, help=&#39;sum the integers (default: find the max)&#39;)
&gt;&gt;&gt; parser.parse_args([&#39;1&#39;, &#39;2&#39;, &#39;3&#39;, &#39;4&#39;])
Namespace(accumulate=&lt;built-in function max&gt;, integers=[1, 2, 3, 4])
&gt;&gt;&gt; parser.parse_args(&#39;1 2 3 4 --sum&#39;.split())
Namespace(accumulate=&lt;built-in function sum&gt;, integers=[1, 2, 3, 4])
</pre></div>
</div>
</div>
<div class="section" id="the-namespace-object">
<h3>15.4.4.6. The Namespace object<a class="headerlink" href="#the-namespace-object" title="Permalink to this headline">¶</a></h3>
<dl class="class">
<dt id="argparse.Namespace">
class <tt class="descclassname">argparse.</tt><tt class="descname">Namespace</tt><a class="headerlink" href="#argparse.Namespace" title="Permalink to this definition">¶</a></dt>
<dd>Simple class used by default by <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> to create
an object holding attributes and return it.
</dd></dl>

This class is deliberately simple, just an <a class="reference internal" href="functions.html#object" title="object"><tt class="xref py py-class docutils literal">object</tt></a> subclass with a
readable string representation. If you prefer to have dict-like view of the
attributes, you can use the standard Python idiom, <a class="reference internal" href="functions.html#vars" title="vars"><tt class="xref py py-func docutils literal">vars()</tt></a>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;)
&gt;&gt;&gt; args = parser.parse_args([&#39;--foo&#39;, &#39;BAR&#39;])
&gt;&gt;&gt; vars(args)
{&#39;foo&#39;: &#39;BAR&#39;}
</pre></div>
</div>
It may also be useful to have an <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> assign attributes to an
already existing object, rather than a new <a class="reference internal" href="#argparse.Namespace" title="argparse.Namespace"><tt class="xref py py-class docutils literal">Namespace</tt></a> object. This can
be achieved by specifying the <tt class="docutils literal">namespace=</tt> keyword argument:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; class C(object):
... pass
...
&gt;&gt;&gt; c = C()
&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;)
&gt;&gt;&gt; parser.parse_args(args=[&#39;--foo&#39;, &#39;BAR&#39;], namespace=c)
&gt;&gt;&gt; c.foo
&#39;BAR&#39;
</pre></div>
</div>
</div>
</div>
<div class="section" id="other-utilities">
<h2>15.4.5. Other utilities<a class="headerlink" href="#other-utilities" title="Permalink to this headline">¶</a></h2>
<div class="section" id="sub-commands">
<h3>15.4.5.1. Sub-commands<a class="headerlink" href="#sub-commands" title="Permalink to this headline">¶</a></h3>
<dl class="method">
<dt id="argparse.ArgumentParser.add_subparsers">
<tt class="descclassname">ArgumentParser.</tt><tt class="descname">add_subparsers</tt><big>(</big><big>)</big><a class="headerlink" href="#argparse.ArgumentParser.add_subparsers" title="Permalink to this definition">¶</a></dt>
<dd>Many programs split up their functionality into a number of sub-commands,
for example, the <tt class="docutils literal">svn</tt> program can invoke sub-commands like <tt class="docutils literal">svn
checkout</tt>, <tt class="docutils literal">svn update</tt>, and <tt class="docutils literal">svn commit</tt>. Splitting up functionality
this way can be a particularly good idea when a program performs several
different functions which require different kinds of command-line arguments.
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> supports the creation of such sub-commands with the
<a class="reference internal" href="#argparse.ArgumentParser.add_subparsers" title="argparse.ArgumentParser.add_subparsers"><tt class="xref py py-meth docutils literal">add_subparsers()</tt></a> method. The <a class="reference internal" href="#argparse.ArgumentParser.add_subparsers" title="argparse.ArgumentParser.add_subparsers"><tt class="xref py py-meth docutils literal">add_subparsers()</tt></a> method is normally
called with no arguments and returns a special action object. This object
has a single method, <tt class="xref py py-meth docutils literal">add_parser()</tt>, which takes a
command name and any <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> constructor arguments, and
returns an <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> object that can be modified as usual.
Some example usage:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; # create the top-level parser
&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, action=&#39;store_true&#39;, help=&#39;foo help&#39;)
&gt;&gt;&gt; subparsers = parser.add_subparsers(help=&#39;sub-command help&#39;)
&gt;&gt;&gt;
&gt;&gt;&gt; # create the parser for the &quot;a&quot; command
&gt;&gt;&gt; parser_a = subparsers.add_parser(&#39;a&#39;, help=&#39;a help&#39;)
&gt;&gt;&gt; parser_a.add_argument(&#39;bar&#39;, type=int, help=&#39;bar help&#39;)
&gt;&gt;&gt;
&gt;&gt;&gt; # create the parser for the &quot;b&quot; command
&gt;&gt;&gt; parser_b = subparsers.add_parser(&#39;b&#39;, help=&#39;b help&#39;)
&gt;&gt;&gt; parser_b.add_argument(&#39;--baz&#39;, choices=&#39;XYZ&#39;, help=&#39;baz help&#39;)
&gt;&gt;&gt;
&gt;&gt;&gt; # parse some argument lists
&gt;&gt;&gt; parser.parse_args([&#39;a&#39;, &#39;12&#39;])
Namespace(bar=12, foo=False)
&gt;&gt;&gt; parser.parse_args([&#39;--foo&#39;, &#39;b&#39;, &#39;--baz&#39;, &#39;Z&#39;])
Namespace(baz=&#39;Z&#39;, foo=True)
</pre></div>
</div>
Note that the object returned by <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> will only contain
attributes for the main parser and the subparser that was selected by the
command line (and not any other subparsers). So in the example above, when
the <tt class="docutils literal">a</tt> command is specified, only the <tt class="docutils literal">foo</tt> and <tt class="docutils literal">bar</tt> attributes are
present, and when the <tt class="docutils literal">b</tt> command is specified, only the <tt class="docutils literal">foo</tt> and
<tt class="docutils literal">baz</tt> attributes are present.
Similarly, when a help message is requested from a subparser, only the help
for that particular parser will be printed. The help message will not
include parent parser or sibling parser messages. (A help message for each
subparser command, however, can be given by supplying the <tt class="docutils literal">help=</tt> argument
to <tt class="xref py py-meth docutils literal">add_parser()</tt> as above.)
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser.parse_args([&#39;--help&#39;])
usage: PROG [-h] [--foo] {a,b} ...

positional arguments:
 {a,b} sub-command help
 a a help
 b b help

optional arguments:
 -h, --help show this help message and exit
 --foo foo help

&gt;&gt;&gt; parser.parse_args([&#39;a&#39;, &#39;--help&#39;])
usage: PROG a [-h] bar

positional arguments:
 bar bar help

optional arguments:
 -h, --help show this help message and exit

&gt;&gt;&gt; parser.parse_args([&#39;b&#39;, &#39;--help&#39;])
usage: PROG b [-h] [--baz {X,Y,Z}]

optional arguments:
 -h, --help show this help message and exit
 --baz {X,Y,Z} baz help
</pre></div>
</div>
The <a class="reference internal" href="#argparse.ArgumentParser.add_subparsers" title="argparse.ArgumentParser.add_subparsers"><tt class="xref py py-meth docutils literal">add_subparsers()</tt></a> method also supports <tt class="docutils literal">title</tt> and <tt class="docutils literal">description</tt>
keyword arguments. When either is present, the subparser&#8217;s commands will
appear in their own group in the help output. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; subparsers = parser.add_subparsers(title=&#39;subcommands&#39;,
... description=&#39;valid subcommands&#39;,
... help=&#39;additional help&#39;)
&gt;&gt;&gt; subparsers.add_parser(&#39;foo&#39;)
&gt;&gt;&gt; subparsers.add_parser(&#39;bar&#39;)
&gt;&gt;&gt; parser.parse_args([&#39;-h&#39;])
usage: [-h] {foo,bar} ...

optional arguments:
 -h, --help show this help message and exit

subcommands:
 valid subcommands

{foo,bar} additional help
</pre></div>
</div>
One particularly effective way of handling sub-commands is to combine the use
of the <a class="reference internal" href="#argparse.ArgumentParser.add_subparsers" title="argparse.ArgumentParser.add_subparsers"><tt class="xref py py-meth docutils literal">add_subparsers()</tt></a> method with calls to <a class="reference internal" href="#argparse.ArgumentParser.set_defaults" title="argparse.ArgumentParser.set_defaults"><tt class="xref py py-meth docutils literal">set_defaults()</tt></a> so
that each subparser knows which Python function it should execute. For
example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; # sub-command functions
&gt;&gt;&gt; def foo(args):
... print args.x * args.y
...
&gt;&gt;&gt; def bar(args):
... print &#39;((%s))&#39; % args.z
...
&gt;&gt;&gt; # create the top-level parser
&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; subparsers = parser.add_subparsers()
&gt;&gt;&gt;
&gt;&gt;&gt; # create the parser for the &quot;foo&quot; command
&gt;&gt;&gt; parser_foo = subparsers.add_parser(&#39;foo&#39;)
&gt;&gt;&gt; parser_foo.add_argument(&#39;-x&#39;, type=int, default=1)
&gt;&gt;&gt; parser_foo.add_argument(&#39;y&#39;, type=float)
&gt;&gt;&gt; parser_foo.set_defaults(func=foo)
&gt;&gt;&gt;
&gt;&gt;&gt; # create the parser for the &quot;bar&quot; command
&gt;&gt;&gt; parser_bar = subparsers.add_parser(&#39;bar&#39;)
&gt;&gt;&gt; parser_bar.add_argument(&#39;z&#39;)
&gt;&gt;&gt; parser_bar.set_defaults(func=bar)
&gt;&gt;&gt;
&gt;&gt;&gt; # parse the args and call whatever function was selected
&gt;&gt;&gt; args = parser.parse_args(&#39;foo 1 -x 2&#39;.split())
&gt;&gt;&gt; args.func(args)
2.0
&gt;&gt;&gt;
&gt;&gt;&gt; # parse the args and call whatever function was selected
&gt;&gt;&gt; args = parser.parse_args(&#39;bar XYZYX&#39;.split())
&gt;&gt;&gt; args.func(args)
((XYZYX))
</pre></div>
</div>
This way, you can let <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> do the job of calling the
appropriate function after argument parsing is complete. Associating
functions with actions like this is typically the easiest way to handle the
different actions for each of your subparsers. However, if it is necessary
to check the name of the subparser that was invoked, the <tt class="docutils literal">dest</tt> keyword
argument to the <a class="reference internal" href="#argparse.ArgumentParser.add_subparsers" title="argparse.ArgumentParser.add_subparsers"><tt class="xref py py-meth docutils literal">add_subparsers()</tt></a> call will work:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; subparsers = parser.add_subparsers(dest=&#39;subparser_name&#39;)
&gt;&gt;&gt; subparser1 = subparsers.add_parser(&#39;1&#39;)
&gt;&gt;&gt; subparser1.add_argument(&#39;-x&#39;)
&gt;&gt;&gt; subparser2 = subparsers.add_parser(&#39;2&#39;)
&gt;&gt;&gt; subparser2.add_argument(&#39;y&#39;)
&gt;&gt;&gt; parser.parse_args([&#39;2&#39;, &#39;frobble&#39;])
Namespace(subparser_name=&#39;2&#39;, y=&#39;frobble&#39;)
</pre></div>
</div>
</dd></dl>

</div>
<div class="section" id="filetype-objects">
<h3>15.4.5.2. FileType objects<a class="headerlink" href="#filetype-objects" title="Permalink to this headline">¶</a></h3>
<dl class="class">
<dt id="argparse.FileType">
class <tt class="descclassname">argparse.</tt><tt class="descname">FileType</tt><big>(</big>mode='r', bufsize=None<big>)</big><a class="headerlink" href="#argparse.FileType" title="Permalink to this definition">¶</a></dt>
<dd>The <a class="reference internal" href="#argparse.FileType" title="argparse.FileType"><tt class="xref py py-class docutils literal">FileType</tt></a> factory creates objects that can be passed to the type
argument of <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">ArgumentParser.add_argument()</tt></a>. Arguments that have
<a class="reference internal" href="#argparse.FileType" title="argparse.FileType"><tt class="xref py py-class docutils literal">FileType</tt></a> objects as their type will open command-line arguments as files
with the requested modes and buffer sizes:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--output&#39;, type=argparse.FileType(&#39;wb&#39;, 0))
&gt;&gt;&gt; parser.parse_args([&#39;--output&#39;, &#39;out&#39;])
Namespace(output=&lt;open file &#39;out&#39;, mode &#39;wb&#39; at 0x...&gt;)
</pre></div>
</div>
FileType objects understand the pseudo-argument <tt class="docutils literal">'-'</tt> and automatically
convert this into <tt class="docutils literal">sys.stdin</tt> for readable <a class="reference internal" href="#argparse.FileType" title="argparse.FileType"><tt class="xref py py-class docutils literal">FileType</tt></a> objects and
<tt class="docutils literal">sys.stdout</tt> for writable <a class="reference internal" href="#argparse.FileType" title="argparse.FileType"><tt class="xref py py-class docutils literal">FileType</tt></a> objects:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;infile&#39;, type=argparse.FileType(&#39;r&#39;))
&gt;&gt;&gt; parser.parse_args([&#39;-&#39;])
Namespace(infile=&lt;open file &#39;&lt;stdin&gt;&#39;, mode &#39;r&#39; at 0x...&gt;)
</pre></div>
</div>
</dd></dl>

</div>
<div class="section" id="argument-groups">
<h3>15.4.5.3. Argument groups<a class="headerlink" href="#argument-groups" title="Permalink to this headline">¶</a></h3>
<dl class="method">
<dt id="argparse.ArgumentParser.add_argument_group">
<tt class="descclassname">ArgumentParser.</tt><tt class="descname">add_argument_group</tt><big>(</big>title=None, description=None<big>)</big><a class="headerlink" href="#argparse.ArgumentParser.add_argument_group" title="Permalink to this definition">¶</a></dt>
<dd>By default, <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> groups command-line arguments into
&#8220;positional arguments&#8221; and &#8220;optional arguments&#8221; when displaying help
messages. When there is a better conceptual grouping of arguments than this
default one, appropriate groups can be created using the
<a class="reference internal" href="#argparse.ArgumentParser.add_argument_group" title="argparse.ArgumentParser.add_argument_group"><tt class="xref py py-meth docutils literal">add_argument_group()</tt></a> method:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;, add_help=False)
&gt;&gt;&gt; group = parser.add_argument_group(&#39;group&#39;)
&gt;&gt;&gt; group.add_argument(&#39;--foo&#39;, help=&#39;foo help&#39;)
&gt;&gt;&gt; group.add_argument(&#39;bar&#39;, help=&#39;bar help&#39;)
&gt;&gt;&gt; parser.print_help()
usage: PROG [--foo FOO] bar

group:
 bar bar help
 --foo FOO foo help
</pre></div>
</div>
The <a class="reference internal" href="#argparse.ArgumentParser.add_argument_group" title="argparse.ArgumentParser.add_argument_group"><tt class="xref py py-meth docutils literal">add_argument_group()</tt></a> method returns an argument group object which
has an <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a> method just like a regular
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a>. When an argument is added to the group, the parser
treats it just like a normal argument, but displays the argument in a
separate group for help messages. The <a class="reference internal" href="#argparse.ArgumentParser.add_argument_group" title="argparse.ArgumentParser.add_argument_group"><tt class="xref py py-meth docutils literal">add_argument_group()</tt></a> method
accepts title and description arguments which can be used to
customize this display:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;, add_help=False)
&gt;&gt;&gt; group1 = parser.add_argument_group(&#39;group1&#39;, &#39;group1 description&#39;)
&gt;&gt;&gt; group1.add_argument(&#39;foo&#39;, help=&#39;foo help&#39;)
&gt;&gt;&gt; group2 = parser.add_argument_group(&#39;group2&#39;, &#39;group2 description&#39;)
&gt;&gt;&gt; group2.add_argument(&#39;--bar&#39;, help=&#39;bar help&#39;)
&gt;&gt;&gt; parser.print_help()
usage: PROG [--bar BAR] foo

group1:
 group1 description

foo foo help

group2:
 group2 description

--bar BAR bar help
</pre></div>
</div>
Note that any arguments not in your user-defined groups will end up back
in the usual &#8220;positional arguments&#8221; and &#8220;optional arguments&#8221; sections.
</dd></dl>

</div>
<div class="section" id="mutual-exclusion">
<h3>15.4.5.4. Mutual exclusion<a class="headerlink" href="#mutual-exclusion" title="Permalink to this headline">¶</a></h3>
<dl class="method">
<dt id="argparse.add_mutually_exclusive_group">
<tt class="descclassname">argparse.</tt><tt class="descname">add_mutually_exclusive_group</tt><big>(</big>required=False<big>)</big><a class="headerlink" href="#argparse.add_mutually_exclusive_group" title="Permalink to this definition">¶</a></dt>
<dd>Create a mutually exclusive group. <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal">argparse</tt></a> will make sure that only
one of the arguments in the mutually exclusive group was present on the
command line:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; group = parser.add_mutually_exclusive_group()
&gt;&gt;&gt; group.add_argument(&#39;--foo&#39;, action=&#39;store_true&#39;)
&gt;&gt;&gt; group.add_argument(&#39;--bar&#39;, action=&#39;store_false&#39;)
&gt;&gt;&gt; parser.parse_args([&#39;--foo&#39;])
Namespace(bar=True, foo=True)
&gt;&gt;&gt; parser.parse_args([&#39;--bar&#39;])
Namespace(bar=False, foo=False)
&gt;&gt;&gt; parser.parse_args([&#39;--foo&#39;, &#39;--bar&#39;])
usage: PROG [-h] [--foo | --bar]
PROG: error: argument --bar: not allowed with argument --foo
</pre></div>
</div>
The <a class="reference internal" href="#argparse.add_mutually_exclusive_group" title="argparse.add_mutually_exclusive_group"><tt class="xref py py-meth docutils literal">add_mutually_exclusive_group()</tt></a> method also accepts a required
argument, to indicate that at least one of the mutually exclusive arguments
is required:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser(prog=&#39;PROG&#39;)
&gt;&gt;&gt; group = parser.add_mutually_exclusive_group(required=True)
&gt;&gt;&gt; group.add_argument(&#39;--foo&#39;, action=&#39;store_true&#39;)
&gt;&gt;&gt; group.add_argument(&#39;--bar&#39;, action=&#39;store_false&#39;)
&gt;&gt;&gt; parser.parse_args([])
usage: PROG [-h] (--foo | --bar)
PROG: error: one of the arguments --foo --bar is required
</pre></div>
</div>
Note that currently mutually exclusive argument groups do not support the
title and description arguments of
<a class="reference internal" href="#argparse.ArgumentParser.add_argument_group" title="argparse.ArgumentParser.add_argument_group"><tt class="xref py py-meth docutils literal">add_argument_group()</tt></a>.
</dd></dl>

</div>
<div class="section" id="parser-defaults">
<h3>15.4.5.5. Parser defaults<a class="headerlink" href="#parser-defaults" title="Permalink to this headline">¶</a></h3>
<dl class="method">
<dt id="argparse.ArgumentParser.set_defaults">
<tt class="descclassname">ArgumentParser.</tt><tt class="descname">set_defaults</tt><big>(</big>**kwargs<big>)</big><a class="headerlink" href="#argparse.ArgumentParser.set_defaults" title="Permalink to this definition">¶</a></dt>
<dd>Most of the time, the attributes of the object returned by <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a>
will be fully determined by inspecting the command-line arguments and the argument
actions. <a class="reference internal" href="#argparse.ArgumentParser.set_defaults" title="argparse.ArgumentParser.set_defaults"><tt class="xref py py-meth docutils literal">set_defaults()</tt></a> allows some additional
attributes that are determined without any inspection of the command line to
be added:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;foo&#39;, type=int)
&gt;&gt;&gt; parser.set_defaults(bar=42, baz=&#39;badger&#39;)
&gt;&gt;&gt; parser.parse_args([&#39;736&#39;])
Namespace(bar=42, baz=&#39;badger&#39;, foo=736)
</pre></div>
</div>
Note that parser-level defaults always override argument-level defaults:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, default=&#39;bar&#39;)
&gt;&gt;&gt; parser.set_defaults(foo=&#39;spam&#39;)
&gt;&gt;&gt; parser.parse_args([])
Namespace(foo=&#39;spam&#39;)
</pre></div>
</div>
Parser-level defaults can be particularly useful when working with multiple
parsers. See the <a class="reference internal" href="#argparse.ArgumentParser.add_subparsers" title="argparse.ArgumentParser.add_subparsers"><tt class="xref py py-meth docutils literal">add_subparsers()</tt></a> method for an
example of this type.
</dd></dl>

<dl class="method">
<dt id="argparse.ArgumentParser.get_default">
<tt class="descclassname">ArgumentParser.</tt><tt class="descname">get_default</tt><big>(</big>dest<big>)</big><a class="headerlink" href="#argparse.ArgumentParser.get_default" title="Permalink to this definition">¶</a></dt>
<dd>Get the default value for a namespace attribute, as set by either
<a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">add_argument()</tt></a> or by
<a class="reference internal" href="#argparse.ArgumentParser.set_defaults" title="argparse.ArgumentParser.set_defaults"><tt class="xref py py-meth docutils literal">set_defaults()</tt></a>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, default=&#39;badger&#39;)
&gt;&gt;&gt; parser.get_default(&#39;foo&#39;)
&#39;badger&#39;
</pre></div>
</div>
</dd></dl>

</div>
<div class="section" id="printing-help">
<h3>15.4.5.6. Printing help<a class="headerlink" href="#printing-help" title="Permalink to this headline">¶</a></h3>
In most typical applications, <a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> will take
care of formatting and printing any usage or error messages. However, several
formatting methods are available:
<dl class="method">
<dt id="argparse.ArgumentParser.print_usage">
<tt class="descclassname">ArgumentParser.</tt><tt class="descname">print_usage</tt><big>(</big>file=None<big>)</big><a class="headerlink" href="#argparse.ArgumentParser.print_usage" title="Permalink to this definition">¶</a></dt>
<dd>Print a brief description of how the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> should be
invoked on the command line. If file is <tt class="docutils literal">None</tt>, <a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><tt class="xref py py-data docutils literal">sys.stdout</tt></a> is
assumed.
</dd></dl>

<dl class="method">
<dt id="argparse.ArgumentParser.print_help">
<tt class="descclassname">ArgumentParser.</tt><tt class="descname">print_help</tt><big>(</big>file=None<big>)</big><a class="headerlink" href="#argparse.ArgumentParser.print_help" title="Permalink to this definition">¶</a></dt>
<dd>Print a help message, including the program usage and information about the
arguments registered with the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a>. If file is
<tt class="docutils literal">None</tt>, <a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><tt class="xref py py-data docutils literal">sys.stdout</tt></a> is assumed.
</dd></dl>

There are also variants of these methods that simply return a string instead of
printing it:
<dl class="method">
<dt id="argparse.ArgumentParser.format_usage">
<tt class="descclassname">ArgumentParser.</tt><tt class="descname">format_usage</tt><big>(</big><big>)</big><a class="headerlink" href="#argparse.ArgumentParser.format_usage" title="Permalink to this definition">¶</a></dt>
<dd>Return a string containing a brief description of how the
<a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> should be invoked on the command line.
</dd></dl>

<dl class="method">
<dt id="argparse.ArgumentParser.format_help">
<tt class="descclassname">ArgumentParser.</tt><tt class="descname">format_help</tt><big>(</big><big>)</big><a class="headerlink" href="#argparse.ArgumentParser.format_help" title="Permalink to this definition">¶</a></dt>
<dd>Return a string containing a help message, including the program usage and
information about the arguments registered with the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a>.
</dd></dl>

</div>
<div class="section" id="partial-parsing">
<h3>15.4.5.7. Partial parsing<a class="headerlink" href="#partial-parsing" title="Permalink to this headline">¶</a></h3>
<dl class="method">
<dt id="argparse.ArgumentParser.parse_known_args">
<tt class="descclassname">ArgumentParser.</tt><tt class="descname">parse_known_args</tt><big>(</big>args=None, namespace=None<big>)</big><a class="headerlink" href="#argparse.ArgumentParser.parse_known_args" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>

Sometimes a script may only parse a few of the command-line arguments, passing
the remaining arguments on to another script or program. In these cases, the
<a class="reference internal" href="#argparse.ArgumentParser.parse_known_args" title="argparse.ArgumentParser.parse_known_args"><tt class="xref py py-meth docutils literal">parse_known_args()</tt></a> method can be useful. It works much like
<a class="reference internal" href="#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><tt class="xref py py-meth docutils literal">parse_args()</tt></a> except that it does not produce an error when
extra arguments are present. Instead, it returns a two item tuple containing
the populated namespace and the list of remaining argument strings.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; parser = argparse.ArgumentParser()
&gt;&gt;&gt; parser.add_argument(&#39;--foo&#39;, action=&#39;store_true&#39;)
&gt;&gt;&gt; parser.add_argument(&#39;bar&#39;)
&gt;&gt;&gt; parser.parse_known_args([&#39;--foo&#39;, &#39;--badger&#39;, &#39;BAR&#39;, &#39;spam&#39;])
(Namespace(bar=&#39;BAR&#39;, foo=True), [&#39;--badger&#39;, &#39;spam&#39;])
</pre></div>
</div>
</div>
<div class="section" id="customizing-file-parsing">
<h3>15.4.5.8. Customizing file parsing<a class="headerlink" href="#customizing-file-parsing" title="Permalink to this headline">¶</a></h3>
<dl class="method">
<dt id="argparse.ArgumentParser.convert_arg_line_to_args">
<tt class="descclassname">ArgumentParser.</tt><tt class="descname">convert_arg_line_to_args</tt><big>(</big>arg_line<big>)</big><a class="headerlink" href="#argparse.ArgumentParser.convert_arg_line_to_args" title="Permalink to this definition">¶</a></dt>
<dd>Arguments that are read from a file (see the fromfile_prefix_chars
keyword argument to the <a class="reference internal" href="#argparse.ArgumentParser" title="argparse.ArgumentParser"><tt class="xref py py-class docutils literal">ArgumentParser</tt></a> constructor) are read one
argument per line. <a class="reference internal" href="#argparse.ArgumentParser.convert_arg_line_to_args" title="argparse.ArgumentParser.convert_arg_line_to_args"><tt class="xref py py-meth docutils literal">convert_arg_line_to_args()</tt></a> can be overriden for
fancier reading.
This method takes a single argument arg_line which is a string read from
the argument file. It returns a list of arguments parsed from this string.
The method is called once per line read from the argument file, in order.
A useful override of this method is one that treats each space-separated word
as an argument:
<div class="highlight-python"><div class="highlight"><pre>def convert_arg_line_to_args(self, arg_line):
 for arg in arg_line.split():
 if not arg.strip():
 continue
 yield arg
</pre></div>
</div>
</dd></dl>

</div>
<div class="section" id="exiting-methods">
<h3>15.4.5.9. Exiting methods<a class="headerlink" href="#exiting-methods" title="Permalink to this headline">¶</a></h3>
<dl class="method">
<dt id="argparse.ArgumentParser.exit">
<tt class="descclassname">ArgumentParser.</tt><tt class="descname">exit</tt><big>(</big>status=0, message=None<big>)</big><a class="headerlink" href="#argparse.ArgumentParser.exit" title="Permalink to this definition">¶</a></dt>
<dd>This method terminates the program, exiting with the specified status
and, if given, it prints a message before that.
</dd></dl>

<dl class="method">
<dt id="argparse.ArgumentParser.error">
<tt class="descclassname">ArgumentParser.</tt><tt class="descname">error</tt><big>(</big>message<big>)</big><a class="headerlink" href="#argparse.ArgumentParser.error" title="Permalink to this definition">¶</a></dt>
<dd>This method prints a usage message including the message to the
standard error and terminates the program with a status code of 2.
</dd></dl>

</div>
</div>
<div class="section" id="upgrading-optparse-code">
<h2>15.4.6. Upgrading optparse code<a class="headerlink" href="#upgrading-optparse-code" title="Permalink to this headline">¶</a></h2>
Originally, the <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal">argparse</tt></a> module had attempted to maintain compatibility
with <a class="reference internal" href="optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><tt class="xref py py-mod docutils literal">optparse</tt></a>. However, <a class="reference internal" href="optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><tt class="xref py py-mod docutils literal">optparse</tt></a> was difficult to extend
transparently, particularly with the changes required to support the new
<tt class="docutils literal">nargs=</tt> specifiers and better usage messages. When most everything in
<a class="reference internal" href="optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><tt class="xref py py-mod docutils literal">optparse</tt></a> had either been copy-pasted over or monkey-patched, it no
longer seemed practical to try to maintain the backwards compatibility.
A partial upgrade path from <a class="reference internal" href="optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><tt class="xref py py-mod docutils literal">optparse</tt></a> to <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal">argparse</tt></a>:
<ul class="simple">
<li>Replace all <a class="reference internal" href="optparse.html#optparse.OptionParser.add_option" title="optparse.OptionParser.add_option"><tt class="xref py py-meth docutils literal">optparse.OptionParser.add_option()</tt></a> calls with
<a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">ArgumentParser.add_argument()</tt></a> calls.</li>
<li>Replace <tt class="docutils literal">(options, args) = parser.parse_args()</tt> with <tt class="docutils literal">args =
parser.parse_args()</tt> and add additional <a class="reference internal" href="#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><tt class="xref py py-meth docutils literal">ArgumentParser.add_argument()</tt></a>
calls for the positional arguments. Keep in mind that what was previously
called <tt class="docutils literal">options</tt>, now in <a class="reference internal" href="#module-argparse" title="argparse: Command-line option and argument parsing library."><tt class="xref py py-mod docutils literal">argparse</tt></a> context is called <tt class="docutils literal">args</tt>.</li>
<li>Replace callback actions and the <tt class="docutils literal">callback_*</tt> keyword arguments with
<tt class="docutils literal">type</tt> or <tt class="docutils literal">action</tt> arguments.</li>
<li>Replace string names for <tt class="docutils literal">type</tt> keyword arguments with the corresponding
type objects (e.g. int, float, complex, etc).</li>
<li>Replace <tt class="xref py py-class docutils literal">optparse.Values</tt> with <a class="reference internal" href="#argparse.Namespace" title="argparse.Namespace"><tt class="xref py py-class docutils literal">Namespace</tt></a> and
<tt class="xref py py-exc docutils literal">optparse.OptionError</tt> and <tt class="xref py py-exc docutils literal">optparse.OptionValueError</tt> with
<tt class="xref py py-exc docutils literal">ArgumentError</tt>.</li>
<li>Replace strings with implicit arguments such as <tt class="docutils literal">%default</tt> or <tt class="docutils literal">%prog</tt> with
the standard Python syntax to use dictionaries to format strings, that is,
<tt class="docutils literal">%(default)s</tt> and <tt class="docutils literal">%(prog)s</tt>.</li>
<li>Replace the OptionParser constructor <tt class="docutils literal">version</tt> argument with a call to
<tt class="docutils literal">parser.add_argument('--version', action='version', version='&lt;the version&gt;')</tt></li>
</ul>
</div>
</div>

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 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">15.4. <tt class="docutils literal">argparse</tt> &#8212; Parser for command-line options, arguments and sub-commands</a><ul>
<li><a class="reference internal" href="#example">15.4.1. Example</a><ul>
<li><a class="reference internal" href="#creating-a-parser">15.4.1.1. Creating a parser</a></li>
<li><a class="reference internal" href="#adding-arguments">15.4.1.2. Adding arguments</a></li>
<li><a class="reference internal" href="#parsing-arguments">15.4.1.3. Parsing arguments</a></li>
</ul>
</li>
<li><a class="reference internal" href="#argumentparser-objects">15.4.2. ArgumentParser objects</a><ul>
<li><a class="reference internal" href="#prog">15.4.2.1. prog</a></li>
<li><a class="reference internal" href="#usage">15.4.2.2. usage</a></li>
<li><a class="reference internal" href="#description">15.4.2.3. description</a></li>
<li><a class="reference internal" href="#epilog">15.4.2.4. epilog</a></li>
<li><a class="reference internal" href="#parents">15.4.2.5. parents</a></li>
<li><a class="reference internal" href="#formatter-class">15.4.2.6. formatter_class</a></li>
<li><a class="reference internal" href="#prefix-chars">15.4.2.7. prefix_chars</a></li>
<li><a class="reference internal" href="#fromfile-prefix-chars">15.4.2.8. fromfile_prefix_chars</a></li>
<li><a class="reference internal" href="#argument-default">15.4.2.9. argument_default</a></li>
<li><a class="reference internal" href="#conflict-handler">15.4.2.10. conflict_handler</a></li>
<li><a class="reference internal" href="#add-help">15.4.2.11. add_help</a></li>
</ul>
</li>
<li><a class="reference internal" href="#the-add-argument-method">15.4.3. The add_argument() method</a><ul>
<li><a class="reference internal" href="#name-or-flags">15.4.3.1. name or flags</a></li>
<li><a class="reference internal" href="#action">15.4.3.2. action</a></li>
<li><a class="reference internal" href="#nargs">15.4.3.3. nargs</a></li>
<li><a class="reference internal" href="#const">15.4.3.4. const</a></li>
<li><a class="reference internal" href="#default">15.4.3.5. default</a></li>
<li><a class="reference internal" href="#type">15.4.3.6. type</a></li>
<li><a class="reference internal" href="#choices">15.4.3.7. choices</a></li>
<li><a class="reference internal" href="#required">15.4.3.8. required</a></li>
<li><a class="reference internal" href="#help">15.4.3.9. help</a></li>
<li><a class="reference internal" href="#metavar">15.4.3.10. metavar</a></li>
<li><a class="reference internal" href="#dest">15.4.3.11. dest</a></li>
</ul>
</li>
<li><a class="reference internal" href="#the-parse-args-method">15.4.4. The parse_args() method</a><ul>
<li><a class="reference internal" href="#option-value-syntax">15.4.4.1. Option value syntax</a></li>
<li><a class="reference internal" href="#invalid-arguments">15.4.4.2. Invalid arguments</a></li>
<li><a class="reference internal" href="#arguments-containing">15.4.4.3. Arguments containing <tt class="docutils literal">-</tt></a></li>
<li><a class="reference internal" href="#argument-abbreviations">15.4.4.4. Argument abbreviations</a></li>
<li><a class="reference internal" href="#beyond-sys-argv">15.4.4.5. Beyond <tt class="docutils literal">sys.argv</tt></a></li>
<li><a class="reference internal" href="#the-namespace-object">15.4.4.6. The Namespace object</a></li>
</ul>
</li>
<li><a class="reference internal" href="#other-utilities">15.4.5. Other utilities</a><ul>
<li><a class="reference internal" href="#sub-commands">15.4.5.1. Sub-commands</a></li>
<li><a class="reference internal" href="#filetype-objects">15.4.5.2. FileType objects</a></li>
<li><a class="reference internal" href="#argument-groups">15.4.5.3. Argument groups</a></li>
<li><a class="reference internal" href="#mutual-exclusion">15.4.5.4. Mutual exclusion</a></li>
<li><a class="reference internal" href="#parser-defaults">15.4.5.5. Parser defaults</a></li>
<li><a class="reference internal" href="#printing-help">15.4.5.6. Printing help</a></li>
<li><a class="reference internal" href="#partial-parsing">15.4.5.7. Partial parsing</a></li>
<li><a class="reference internal" href="#customizing-file-parsing">15.4.5.8. Customizing file parsing</a></li>
<li><a class="reference internal" href="#exiting-methods">15.4.5.9. Exiting methods</a></li>
</ul>
</li>
<li><a class="reference internal" href="#upgrading-optparse-code">15.4.6. Upgrading optparse code</a></li>
</ul>
</li>
</ul>

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