cpython/Doc/library/argparse.rst

:mod:`argparse` -- Parser for command line options, arguments and sub-commands
==============================================================================

.. module:: argparse
   :synopsis: Command-line option and argument parsing library.
.. moduleauthor:: Steven Bethard <steven.bethard@gmail.com>
.. versionadded:: 2.7
.. sectionauthor:: Steven Bethard <steven.bethard@gmail.com>


The :mod:`argparse` module makes it easy to write user friendly command line
interfaces. You define what arguments your program requires, and
:mod:`argparse` will figure out how to parse those out of ``sys.argv``. The
:mod:`argparse` module also automatically generates help and usage messages
based on the arguments you have defined, and issues errors when users give your
program invalid arguments.

Example
-------

As an example, the following code is a Python program that takes a list of
integers and produces either the sum or the max::

   import argparse

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

   args = parser.parse_args()
   print args.accumulate(args.integers)

Assuming the Python code above is saved into a file called ``prog.py``, it can
be run at the command line and provides useful help messages::

   $ 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)

When run with the appropriate arguments, it prints either the sum or the max of
the command-line integers::

   $ prog.py 1 2 3 4
   4

   $ prog.py 1 2 3 4 --sum
   10

If invalid arguments are passed in, it will issue an error::

   $ prog.py a b c
   usage: prog.py [-h] [--sum] N [N ...]
   prog.py: error: argument N: invalid int value: 'a'

The following sections walk you through this example.

Creating a parser
^^^^^^^^^^^^^^^^^

Pretty much every script that uses the :mod:`argparse` module will start out by
creating an :class:`ArgumentParser` object::

   >>> parser = argparse.ArgumentParser(description='Process some integers.')

The :class:`ArgumentParser` object will hold all the information necessary to
parse the command line into a more manageable form for your program.


Adding arguments
^^^^^^^^^^^^^^^^

Once you've created an :class:`ArgumentParser`, you'll want to fill it with
information about your program arguments. You typically do this by making calls
to the :meth:`add_argument` method.  Generally, these calls tell the
:class:`ArgumentParser` how to take the strings on the command line and turn
them into objects for you. This information is stored and used when
:meth:`parse_args` is called. For example, if we add some arguments like this::

   >>> parser.add_argument('integers', metavar='N', type=int, nargs='+',
   ...                     help='an integer for the accumulator')
   >>> parser.add_argument('--sum', dest='accumulate', action='store_const',
   ...                     const=sum, default=max,
   ...                     help='sum the integers (default: find the max)')

when we later call :meth:`parse_args`, we can expect it to return an object
with two attributes, ``integers`` and ``accumulate``.  The ``integers``
attribute will be a list of one or more ints, and the ``accumulate`` attribute
will be either the ``sum`` function, if ``--sum`` was specified at the command
line, or the ``max`` function if it was not.

Parsing arguments
^^^^^^^^^^^^^^^^^

Once an :class:`ArgumentParser` has been initialized with appropriate calls to
:meth:`add_argument`, it can be instructed to parse the command-line args by
calling the :meth:`parse_args` method.  This will inspect the command-line,
convert each arg to the appropriate type and then invoke the appropriate
action.  In most cases, this means a simple namespace object will be built up
from attributes parsed out of the command-line::

   >>> parser.parse_args(['--sum', '7', '-1', '42'])
   Namespace(accumulate=<built-in function sum>, integers=[7, -1, 42])

In a script, :meth:`parse_args` will typically be called with no arguments, and
the :class:`ArgumentParser` will automatically determine the command-line args
from ``sys.argv``. That's pretty much it. You're now ready to go write some
command line interfaces!


ArgumentParser objects
----------------------

.. class:: ArgumentParser([description], [epilog], [prog], [usage], [add_help], [argument_default], [parents], [prefix_chars], [conflict_handler], [formatter_class])

   Create a new :class:`ArgumentParser` object. Each parameter has its own more
   detailed description below, but in short they are:

   * description_ - Text to display before the argument help.

   * epilog_ - Text to display after the argument help.

   * add_help_ - Add a -h/--help option to the parser. (default: True)

   * argument_default_ - Set the global default value for arguments.
     (default: None)

   * parents_ - A list of :class:ArgumentParser objects whose arguments should
     also be included.

   * prefix_chars_ - The set of characters that prefix optional arguments.
     (default: '-')

   * fromfile_prefix_chars_ - The set of characters that prefix files from
     which additional arguments should be read. (default: None)

   * formatter_class_ - A class for customizing the help output.

   * conflict_handler_ - Usually unnecessary, defines strategy for resolving
     conflicting optionals.

   * prog_ - Usually unnecessary, the name of the program
     (default: ``sys.argv[0]``)

   * usage_ - Usually unnecessary, the string describing the program usage
     (default: generated)

   The following sections describe how each of these are used.


description
^^^^^^^^^^^

Most calls to the ArgumentParser constructor will use the ``description=``
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::

   >>> parser = argparse.ArgumentParser(description='A foo that bars')
   >>> parser.print_help()
   usage: argparse.py [-h]

   A foo that bars

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

By default, the description will be line-wrapped so that it fits within the
given space. To change this behavior, see the formatter_class_ argument.


epilog
^^^^^^

Some programs like to display additional description of the program after the
description of the arguments. Such text can be specified using the ``epilog=``
argument to ArgumentParser::

   >>> parser = argparse.ArgumentParser(
   ...     description='A foo that bars',
   ...     epilog="And that's how you'd foo a bar")
   >>> parser.print_help()
   usage: argparse.py [-h]

   A foo that bars

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

   And that's how you'd foo a bar

As with the description_ argument, the ``epilog=`` text is by default
line-wrapped, but this behavior can be adjusted with the formatter_class_
argument to ArgumentParser.


add_help
^^^^^^^^

By default, ArgumentParser objects add a ``-h/--help`` option which simply
displays the parser's help message. For example, consider a file named
``myprogram.py`` containing the following code::

   import argparse
   parser = argparse.ArgumentParser()
   parser.add_argument('--foo', help='foo help')
   args = parser.parse_args()

If ``-h`` or ``--help`` is supplied is at the command-line, the ArgumentParser
help will be printed::

   $ python myprogram.py --help
   usage: myprogram.py [-h] [--foo FOO]

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

Occasionally, it may be useful to disable the addition of this help option.
This can be achieved by passing ``False`` as the ``add_help=`` argument to
ArgumentParser::

   >>> parser = argparse.ArgumentParser(prog='PROG', add_help=False)
   >>> parser.add_argument('--foo', help='foo help')
   >>> parser.print_help()
   usage: PROG [--foo FOO]

   optional arguments:
    --foo FOO  foo help


prefix_chars
^^^^^^^^^^^^

Most command-line options will use ``'-'`` as the prefix, e.g. ``-f/--foo``.
Parsers that need to support additional prefix characters, e.g. for options
like ``+f`` or ``/foo``, may specify them using the ``prefix_chars=`` argument
to the ArgumentParser constructor::

   >>> parser = argparse.ArgumentParser(prog='PROG', prefix_chars='-+')
   >>> parser.add_argument('+f')
   >>> parser.add_argument('++bar')
   >>> parser.parse_args('+f X ++bar Y'.split())
   Namespace(bar='Y', f='X')

The ``prefix_chars=`` argument defaults to ``'-'``. Supplying a set of
characters that does not include ``'-'`` will cause ``-f/--foo`` options to be
disallowed.


fromfile_prefix_chars
^^^^^^^^^^^^^^^^^^^^^

Sometimes, e.g. for 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 ``fromfile_prefix_chars=`` argument is given to the ArgumentParser
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::

   >>> open('args.txt', 'w').write('-f\nbar')
   >>> parser = argparse.ArgumentParser(fromfile_prefix_chars='@')
   >>> parser.add_argument('-f')
   >>> parser.parse_args(['-f', 'foo', '@args.txt'])
   Namespace(f='bar')

Arguments read from a file must by default be one per line (but see also
:meth:`convert_arg_line_to_args`) 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 ``['-f', 'foo', '@args.txt']`` is considered
equivalent to the expression ``['-f', 'foo', '-f', 'bar']``.

The ``fromfile_prefix_chars=`` argument defaults to ``None``, meaning that
arguments will never be treated as file references.

argument_default
^^^^^^^^^^^^^^^^

Generally, argument defaults are specified either by passing a default to
:meth:`add_argument` or by calling the :meth:`set_defaults` 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 ``argument_default=`` keyword argument to ArgumentParser. For
example, to globally suppress attribute creation on :meth:`parse_args` calls,
we supply ``argument_default=SUPPRESS``::

   >>> parser = argparse.ArgumentParser(argument_default=argparse.SUPPRESS)
   >>> parser.add_argument('--foo')
   >>> parser.add_argument('bar', nargs='?')
   >>> parser.parse_args(['--foo', '1', 'BAR'])
   Namespace(bar='BAR', foo='1')
   >>> parser.parse_args([])
   Namespace()


parents
^^^^^^^

Sometimes, several parsers share a common set of arguments. Rather than
repeating the definitions of these arguments, you can define a single parser
with all the shared arguments and then use the ``parents=`` argument to
ArgumentParser to have these "inherited". The ``parents=`` argument takes a
list of ArgumentParser objects, collects all the positional and optional
actions from them, and adds these actions to the ArgumentParser object being
constructed::

   >>> parent_parser = argparse.ArgumentParser(add_help=False)
   >>> parent_parser.add_argument('--parent', type=int)

   >>> foo_parser = argparse.ArgumentParser(parents=[parent_parser])
   >>> foo_parser.add_argument('foo')
   >>> foo_parser.parse_args(['--parent', '2', 'XXX'])
   Namespace(foo='XXX', parent=2)

   >>> bar_parser = argparse.ArgumentParser(parents=[parent_parser])
   >>> bar_parser.add_argument('--bar')
   >>> bar_parser.parse_args(['--bar', 'YYY'])
   Namespace(bar='YYY', parent=None)

Note that most parent parsers will specify ``add_help=False``. Otherwise, the
ArgumentParser will see two ``-h/--help`` options (one in the parent and one in
the child) and raise an error.


formatter_class
^^^^^^^^^^^^^^^

ArgumentParser objects allow the help formatting to be customized by specifying
an alternate formatting class. Currently, there are three such classes:
``argparse.RawDescriptionHelpFormatter``, ``argparse.RawTextHelpFormatter`` and
``argparse.ArgumentDefaultsHelpFormatter``. The first two allow more control
over how textual descriptions are displayed, while the last automatically adds
information about argument default values.

By default, ArgumentParser objects line-wrap the description_ and epilog_ texts
in command-line help messages::

   >>> parser = argparse.ArgumentParser(
   ...     prog='PROG',
   ...     description='''this description
   ...         was indented weird
   ...             but that is okay''',
   ...     epilog='''
   ...             likewise for this epilog whose whitespace will
   ...         be cleaned up and whose words will be wrapped
   ...         across a couple lines''')
   >>> 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

When you have description_ and epilog_ that is already correctly formatted and
should not be line-wrapped, you can indicate this by passing
``argparse.RawDescriptionHelpFormatter`` as the ``formatter_class=`` argument
to ArgumentParser::

   >>> parser = argparse.ArgumentParser(
   ...     prog='PROG',
   ...     formatter_class=argparse.RawDescriptionHelpFormatter,
   ...     description=textwrap.dedent('''\
   ...         Please do not mess up this text!
   ...         --------------------------------
   ...             I have indented it
   ...             exactly the way
   ...             I want it
   ...         '''))
   >>> 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

If you want to maintain whitespace for all sorts of help text (including
argument descriptions), you can use ``argparse.RawTextHelpFormatter``.

The other formatter class available,
``argparse.ArgumentDefaultsHelpFormatter``, will add information about the
default value of each of the arguments::

   >>> parser = argparse.ArgumentParser(
   ...     prog='PROG',
   ...     formatter_class=argparse.ArgumentDefaultsHelpFormatter)
   >>> parser.add_argument('--foo', type=int, default=42, help='FOO!')
   >>> parser.add_argument('bar', nargs='*', default=[1, 2, 3], help='BAR!')
   >>> 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)


conflict_handler
^^^^^^^^^^^^^^^^

ArgumentParser objects do not allow two actions with the same option string.
By default, ArgumentParser objects will raise an exception if you try to create
an argument with an option string that is already in use::

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-f', '--foo', help='old foo help')
   >>> parser.add_argument('--foo', help='new foo help')
   Traceback (most recent call last):
    ..
   ArgumentError: argument --foo: conflicting option string(s): --foo

Sometimes (e.g. when using parents_) it may be useful to simply override any
older arguments with the same option string. To get this behavior, the value
``'resolve'`` can be supplied to the ``conflict_handler=`` argument of
ArgumentParser::

   >>> parser = argparse.ArgumentParser(prog='PROG', conflict_handler='resolve')
   >>> parser.add_argument('-f', '--foo', help='old foo help')
   >>> parser.add_argument('--foo', help='new foo help')
   >>> 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

Note that ArgumentParser objects only remove an action if all of its option
strings are overridden. So, in the example above, the old ``-f/--foo`` action
is retained as the ``-f`` action, because only the ``--foo`` option string was
overridden.


prog
^^^^

By default, ArgumentParser objects use ``sys.argv[0]`` to determine how to
display the name of the program in help messages. This default is almost always
what you want because it will make the help messages match what your users have
typed at the command line. For example, consider a file named ``myprogram.py``
with the following code::

   import argparse
   parser = argparse.ArgumentParser()
   parser.add_argument('--foo', help='foo help')
   args = parser.parse_args()

The help for this program will display ``myprogram.py`` as the program name
(regardless of where the program was invoked from)::

   $ 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

To change this default behavior, another value can be supplied using the
``prog=`` argument to ArgumentParser::

   >>> parser = argparse.ArgumentParser(prog='myprogram')
   >>> parser.print_help()
   usage: myprogram [-h]

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

Note that the program name, whether determined from ``sys.argv[0]`` or from the
``prog=`` argument, is available to help messages using the ``%(prog)s`` format
specifier.

::

   >>> parser = argparse.ArgumentParser(prog='myprogram')
   >>> parser.add_argument('--foo', help='foo of the %(prog)s program')
   >>> 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


usage
^^^^^

By default, ArgumentParser objects calculate the usage message from the
arguments it contains::

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('--foo', nargs='?', help='foo help')
   >>> parser.add_argument('bar', nargs='+', help='bar help')
   >>> 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

If the default usage message is not appropriate for your application, you can
supply your own usage message using the ``usage=`` keyword argument to
ArgumentParser::

   >>> parser = argparse.ArgumentParser(prog='PROG', usage='%(prog)s [options]')
   >>> parser.add_argument('--foo', nargs='?', help='foo help')
   >>> parser.add_argument('bar', nargs='+', help='bar help')
   >>> 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

Note you can use the ``%(prog)s`` format specifier to fill in the program name
in your usage messages.


The add_argument() method
-------------------------

.. method:: add_argument(name or flags..., [action], [nargs], [const], [default], [type], [choices], [required], [help], [metavar], [dest])

   Define how a single command line argument should be parsed. Each parameter
   has its own more detailed description below, but in short they are:

   * `name or flags`_ - Either a name or a list of option strings, e.g. ``foo``
     or ``-f, --foo``

   * action_ - The basic type of action to be taken when this argument is
     encountered at the command-line.

   * nargs_ - The number of command-line arguments that should be consumed.

   * const_ - A constant value required by some action_ and nargs_ selections.

   * default_ - The value produced if the argument is absent from the
     command-line.

   * type_ - The type to which the command-line arg should be converted.

   * choices_ - A container of the allowable values for the argument.

   * required_ - Whether or not the command-line option may be omitted
     (optionals only).

   * help_ - A brief description of what the argument does.

   * metavar_ - A name for the argument in usage messages.

   * dest_ - The name of the attribute to be added to the object returned by
     :meth:`parse_args`.

   The following sections describe how each of these are used.

name or flags
^^^^^^^^^^^^^

The :meth:`add_argument` method needs to know whether you're expecting an
optional argument, e.g. ``-f`` or ``--foo``, or a positional argument, e.g. a
list of filenames. The first arguments passed to :meth:`add_argument` must
therefore be either a series of flags, or a simple argument name. For example,
an optional argument could be created like::

   >>> parser.add_argument('-f', '--foo')

while a positional argument could be created like::

   >>> parser.add_argument('bar')

When :meth:`parse_args` is called, optional arguments will be identified by the
``-`` prefix, and the remaining arguments will be assumed to be positional::

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-f', '--foo')
   >>> parser.add_argument('bar')
   >>> parser.parse_args(['BAR'])
   Namespace(bar='BAR', foo=None)
   >>> parser.parse_args(['BAR', '--foo', 'FOO'])
   Namespace(bar='BAR', foo='FOO')
   >>> parser.parse_args(['--foo', 'FOO'])
   usage: PROG [-h] [-f FOO] bar
   PROG: error: too few arguments

action
^^^^^^

:class:`ArgumentParser` objects associate command-line args with actions. These
actions can do just about anything with the command-line args associated with
them, though most actions simply add an attribute to the object returned by
:meth:`parse_args`.  When you specify a new argument using the
:meth:`add_argument` method, you can indicate how the command-line args should
be handled by specifying the ``action`` keyword argument. The supported actions
are:

* ``'store'`` - This just stores the argument's value. This is the default
   action. For example::

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--foo')
    >>> parser.parse_args('--foo 1'.split())
    Namespace(foo='1')

* ``'store_const'`` - This stores the value specified by the const_ keyword
   argument. Note that the const_ keyword argument defaults to ``None``, so
   you'll almost always need to provide a value for it. The ``'store_const'``
   action is most commonly used with optional arguments that specify some sort
   of flag.  For example::

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--foo', action='store_const', const=42)
    >>> parser.parse_args('--foo'.split())
    Namespace(foo=42)

* ``'store_true'`` and ``'store_false'`` - These store the values ``True`` and
  ``False`` respectively.  These are basically special cases of
  ``'store_const'``.  For example::

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--foo', action='store_true')
    >>> parser.add_argument('--bar', action='store_false')
    >>> parser.parse_args('--foo --bar'.split())
    Namespace(bar=False, foo=True)

* ``'append'`` - This stores a list, and appends each argument value to the
  list.  This is useful when you want to allow an option to be specified
  multiple times.  Example usage::

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--foo', action='append')
    >>> parser.parse_args('--foo 1 --foo 2'.split())
    Namespace(foo=['1', '2'])

* ``'append_const'`` - This stores a list, and appends the value specified by
  the const_ keyword argument to the list.  Note that the const_ keyword
  argument defaults to ``None``, so you'll almost always need to provide a
  value for it.  The ``'append_const'`` action is typically useful when you
  want multiple arguments to store constants to the same list, for example::

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--str', dest='types', action='append_const', const=str)
    >>> parser.add_argument('--int', dest='types', action='append_const', const=int)
    >>> parser.parse_args('--str --int'.split())
    Namespace(types=[<type 'str'>, <type 'int'>])

* ``'version'`` - This expects a ``version=`` keyword argument in the
  :meth:`add_argument` call, and prints version information and exits when
  invoked.

    >>> import argparse
    >>> parser = argparse.ArgumentParser(prog='PROG')
    >>> parser.add_argument('-v', '--version', action='version', version='%(prog)s 2.0')
    >>> parser.parse_args(['-v'])
    PROG 2.0

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 ``argparse.Action``,
supplying an appropriate ``__call__`` method.  The ``__call__`` method accepts
four parameters:

* ``parser`` - The ArgumentParser object which contains this action.

* ``namespace`` - The namespace object that will be returned by
  :meth:`parse_args`. Most actions add an attribute to this object.

* ``values`` - The associated command-line args, with any type-conversions
  applied.  (Type-conversions are specified with the type_ keyword argument to
  :meth:`add_argument`.

* ``option_string`` - The option string that was used to invoke this action.
  The ``option_string`` argument is optional, and will be absent if the action
  is associated with a positional argument.

So for example::

   >>> class FooAction(argparse.Action):
   ...     def __call__(self, parser, namespace, values, option_string=None):
   ...     print '%r %r %r' % (namespace, values, option_string)
   ...     setattr(namespace, self.dest, values)
   ...
   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', action=FooAction)
   >>> parser.add_argument('bar', action=FooAction)
   >>> args = parser.parse_args('1 --foo 2'.split())
   Namespace(bar=None, foo=None) '1' None
   Namespace(bar='1', foo=None) '2' '--foo'
   >>> args
   Namespace(bar='1', foo='2')


nargs
^^^^^

ArgumentParser objects usually associate a single command-line argument with a
single action to be taken.  In the situations where you'd like to associate a
different number of command-line arguments with a single action, you can use
the ``nargs`` keyword argument to :meth:`add_argument`. The supported values
are:

* N (an integer). N args from the command-line will be gathered together into
  a list.  For example::

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--foo', nargs=2)
    >>> parser.add_argument('bar', nargs=1)
    >>> parser.parse_args('c --foo a b'.split())
    Namespace(bar=['c'], foo=['a', 'b'])

   Note that ``nargs=1`` produces a list of one item.  This is different from
   the default, in which the item is produced by itself.

* ``'?'``. One arg will be consumed from the command-line if possible, and
  produced as a single item.  If no command-line arg is present, the value from
  default_ 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 arg.  In this case the value from const_ will be produced.  Some
  examples to illustrate this::

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--foo', nargs='?', const='c', default='d')
    >>> parser.add_argument('bar', nargs='?', default='d')
    >>> parser.parse_args('XX --foo YY'.split())
    Namespace(bar='XX', foo='YY')
    >>> parser.parse_args('XX --foo'.split())
    Namespace(bar='XX', foo='c')
    >>> parser.parse_args(''.split())
    Namespace(bar='d', foo='d')

   One of the more common uses of ``nargs='?'`` is to allow optional input and
   output files::

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('infile', nargs='?', type=argparse.FileType('r'), default=sys.stdin)
    >>> parser.add_argument('outfile', nargs='?', type=argparse.FileType('w'), default=sys.stdout)
    >>> parser.parse_args(['input.txt', 'output.txt'])
    Namespace(infile=<open file 'input.txt', mode 'r' at 0x...>, outfile=<open file 'output.txt', mode 'w' at 0x...>)
    >>> parser.parse_args([])
    Namespace(infile=<open file '<stdin>', mode 'r' at 0x...>, outfile=<open file '<stdout>', mode 'w' at 0x...>)

* ``'*'``. All command-line args present are gathered into a list. Note that
  it generally doesn't make much sense to have more than one positional
  argument with ``nargs='*'``, but multiple optional arguments with
  ``nargs='*'`` is possible.  For example::

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--foo', nargs='*')
    >>> parser.add_argument('--bar', nargs='*')
    >>> parser.add_argument('baz', nargs='*')
    >>> parser.parse_args('a b --foo x y --bar 1 2'.split())
    Namespace(bar=['1', '2'], baz=['a', 'b'], foo=['x', 'y'])

* ``'+'``. Just like ``'*'``, all command-line args present are gathered into a
  list.  Additionally, an error message will be generated if there wasn't at
  least one command-line arg present.  For example::

    >>> parser = argparse.ArgumentParser(prog='PROG')
    >>> parser.add_argument('foo', nargs='+')
    >>> parser.parse_args('a b'.split())
    Namespace(foo=['a', 'b'])
    >>> parser.parse_args(''.split())
    usage: PROG [-h] foo [foo ...]
    PROG: error: too few arguments

If the ``nargs`` keyword argument is not provided, the number of args consumed
is determined by the action_. Generally this means a single command-line arg
will be consumed and a single item (not a list) will be produced.


const
^^^^^

The ``const`` argument of :meth:`add_argument` is used to hold constant values
that are not read from the command line but are required for the various
ArgumentParser actions.  The two most common uses of it are:

* When :meth:`add_argument` is called with ``action='store_const'`` or
  ``action='append_const'``.  These actions add the ``const`` value to one of
  the attributes of the object returned by :meth:`parse_args`.  See the action_
  description for examples.

* When :meth:`add_argument` is called with option strings (like ``-f`` or
  ``--foo``) and ``nargs='?'``. This creates an optional argument that can be
  followed by zero or one command-line args.  When parsing the command-line, if
  the option string is encountered with no command-line arg following it, the
  value of ``const`` will be assumed instead. See the nargs_ description for
  examples.

The ``const`` keyword argument defaults to ``None``.


default
^^^^^^^

All optional arguments and some positional arguments may be omitted at the
command-line.  The ``default`` keyword argument of :meth:`add_argument`, whose
value defaults to ``None``, specifies what value should be used if the
command-line arg is not present.  For optional arguments, the ``default`` value
is used when the option string was not present at the command line::

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', default=42)
   >>> parser.parse_args('--foo 2'.split())
   Namespace(foo='2')
   >>> parser.parse_args(''.split())
   Namespace(foo=42)

For positional arguments with nargs_ ``='?'`` or ``'*'``, the ``default`` value
is used when no command-line arg was present::

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('foo', nargs='?', default=42)
   >>> parser.parse_args('a'.split())
   Namespace(foo='a')
   >>> parser.parse_args(''.split())
   Namespace(foo=42)


If you don't want to see an attribute when an option was not present at the
command line, you can supply ``default=argparse.SUPPRESS``::

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', default=argparse.SUPPRESS)
   >>> parser.parse_args([])
   Namespace()
   >>> parser.parse_args(['--foo', '1'])
   Namespace(foo='1')


type
^^^^

By default, ArgumentParser objects read command-line args in as simple strings.
However, quite often the command-line string should instead be interpreted as
another type, e.g. ``float``, ``int`` or ``file``. The ``type`` keyword
argument of :meth:`add_argument` allows any necessary type-checking and
type-conversions to be performed.  Many common builtin types can be used
directly as the value of the ``type`` argument::

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('foo', type=int)
   >>> parser.add_argument('bar', type=file)
   >>> parser.parse_args('2 temp.txt'.split())
   Namespace(bar=<open file 'temp.txt', mode 'r' at 0x...>, foo=2)

To ease the use of various types of files, the argparse module provides the
factory FileType which takes the ``mode=`` and ``bufsize=`` arguments of the
``file`` object. For example, ``FileType('w')`` can be used to create a
writable file::

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('bar', type=argparse.FileType('w'))
   >>> parser.parse_args(['out.txt'])
   Namespace(bar=<open file 'out.txt', mode 'w' at 0x...>)

If you need to do some special type-checking or type-conversions, you can
provide your own types by passing to ``type=`` a callable that takes a single
string argument and returns the type-converted value::

   >>> def perfect_square(string):
   ...     value = int(string)
   ...     sqrt = math.sqrt(value)
   ...     if sqrt != int(sqrt):
   ...         msg = "%r is not a perfect square" % string
   ...         raise argparse.ArgumentTypeError(msg)
   ...     return value
   ...
   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('foo', type=perfect_square)
   >>> parser.parse_args('9'.split())
   Namespace(foo=9)
   >>> parser.parse_args('7'.split())
   usage: PROG [-h] foo
   PROG: error: argument foo: '7' is not a perfect square

Note that if your type-checking function is just checking for a particular set
of values, it may be more convenient to use the choices_ keyword argument::

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('foo', type=int, choices=xrange(5, 10))
   >>> parser.parse_args('7'.split())
   Namespace(foo=7)
   >>> parser.parse_args('11'.split())
   usage: PROG [-h] {5,6,7,8,9}
   PROG: error: argument foo: invalid choice: 11 (choose from 5, 6, 7, 8, 9)

See the choices_ section for more details.


choices
^^^^^^^

Some command-line args should be selected from a restricted set of values.
ArgumentParser objects can be told about such sets of values by passing a
container object as the ``choices`` keyword argument to :meth:`add_argument`.
When the command-line is parsed with :meth:`parse_args`, arg values will be
checked, and an error message will be displayed if the arg was not one of the
acceptable values::

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('foo', choices='abc')
   >>> parser.parse_args('c'.split())
   Namespace(foo='c')
   >>> parser.parse_args('X'.split())
   usage: PROG [-h] {a,b,c}
   PROG: error: argument foo: invalid choice: 'X' (choose from 'a', 'b', 'c')

Note that inclusion in the ``choices`` container is checked after any type_
conversions have been performed, so the type of the objects in the ``choices``
container should match the type_ specified::

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('foo', type=complex, choices=[1, 1j])
   >>> parser.parse_args('1j'.split())
   Namespace(foo=1j)
   >>> parser.parse_args('-- -4'.split())
   usage: PROG [-h] {1,1j}
   PROG: error: argument foo: invalid choice: (-4+0j) (choose from 1, 1j)

Any object that supports the ``in`` operator can be passed as the ``choices``
value, so ``dict`` objects, ``set`` objects, custom containers, etc. are all
supported.


required
^^^^^^^^

In general, the argparse module assumes that flags like ``-f`` and ``--bar``
indicate *optional* arguments, which can always be omitted at the command-line.
To change this behavior, i.e. to make an option *required*, the value ``True``
should be specified for the ``required=`` keyword argument to
:meth:`add_argument`::

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', required=True)
   >>> parser.parse_args(['--foo', 'BAR'])
   Namespace(foo='BAR')
   >>> parser.parse_args([])
   usage: argparse.py [-h] [--foo FOO]
   argparse.py: error: option --foo is required

As the example shows, if an option is marked as ``required``, :meth:`parse_args`
will report an error if that option is not present at the command line.

**Warning:** Required options are generally considered bad form - normal users
expect *options* to be *optional*. You should avoid the use of required options
whenever possible.


help
^^^^

A great command-line interface isn't worth anything if your users can't figure
out which option does what.  So for the end-users, ``help`` is probably the
most important argument to include in your :meth:`add_argument` calls.  The
``help`` value should be a string containing a brief description of what the
argument specifies.  When a user requests help (usually by using ``-h`` or
``--help`` at the command-line), these ``help`` descriptions will be displayed
with each argument::

   >>> parser = argparse.ArgumentParser(prog='frobble')
   >>> parser.add_argument('--foo', action='store_true',
   ...         help='foo the bars before frobbling')
   >>> parser.add_argument('bar', nargs='+',
   ...         help='one of the bars to be frobbled')
   >>> parser.parse_args('-h'.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

The ``help`` strings can include various format specifiers to avoid repetition
of things like the program name or the argument default_.  The available
specifiers include the program name, ``%(prog)s`` and most keyword arguments to
:meth:`add_argument`, e.g. ``%(default)s``, ``%(type)s``, etc.::

   >>> parser = argparse.ArgumentParser(prog='frobble')
   >>> parser.add_argument('bar', nargs='?', type=int, default=42,
   ...         help='the bar to %(prog)s (default: %(default)s)')
   >>> 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


metavar
^^^^^^^

When ArgumentParser objects generate help messages, they need some way to refer
to each expected argument. By default, ArgumentParser objects use the dest_
value as the "name" of each object.  By default, for positional argument
actions, the dest_ value is used directly, and for optional argument actions,
the dest_ value is uppercased.  So if we have a single positional argument with
``dest='bar'``, that argument will be referred to as ``bar``.  And if we have a
single optional argument ``--foo`` that should be followed by a single
command-line arg, that arg will be referred to as ``FOO``.  You can see this
behavior in the example below::

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo')
   >>> parser.add_argument('bar')
   >>> parser.parse_args('X --foo Y'.split())
   Namespace(bar='X', foo='Y')
   >>> parser.print_help()
   usage:  [-h] [--foo FOO] bar

   positional arguments:
    bar

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

If you would like to provide a different name for your argument in help
messages, you can supply a value for the ``metavar`` keyword argument to
:meth:`add_argument`::

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', metavar='YYY')
   >>> parser.add_argument('bar', metavar='XXX')
   >>> parser.parse_args('X --foo Y'.split())
   Namespace(bar='X', foo='Y')
   >>> parser.print_help()
   usage:  [-h] [--foo YYY] XXX

   positional arguments:
    XXX

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

Note that ``metavar`` only changes the *displayed* name - the name of the
attribute on the :meth:`parse_args` object is still determined by the dest_
value.

Different values of ``nargs`` may cause the metavar to be used multiple times.
If you'd like to specify a different display name for each of the arguments,
you can provide a tuple to ``metavar``::

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-x', nargs=2)
   >>> parser.add_argument('--foo', nargs=2, metavar=('bar', 'baz'))
   >>> 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


dest
^^^^

Most ArgumentParser actions add some value as an attribute of the object
returned by :meth:`parse_args`. The name of this attribute is determined by the
``dest`` keyword argument of :meth:`add_argument`. For positional argument
actions, ``dest`` is normally supplied as the first argument to
:meth:`add_argument`::

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('bar')
   >>> parser.parse_args('XXX'.split())
   Namespace(bar='XXX')

For optional argument actions, the value of ``dest`` is normally inferred from
the option strings. ArgumentParser objects generate the value of ``dest`` by
taking the first long option string and stripping away the initial ``'--'``
string.  If no long option strings were supplied, ``dest`` will be derived from
the first short option string by stripping the initial ``'-'`` character.  Any
internal ``'-'`` characters will be converted to ``'_'`` characters to make
sure the string is a valid attribute name. The examples below illustrate this
behavior::

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('-f', '--foo-bar', '--foo')
   >>> parser.add_argument('-x', '-y')
   >>> parser.parse_args('-f 1 -x 2'.split())
   Namespace(foo_bar='1', x='2')
   >>> parser.parse_args('--foo 1 -y 2'.split())
   Namespace(foo_bar='1', x='2')

If you would like to use a different attribute name from the one automatically
inferred by the ArgumentParser, you can supply it with an explicit ``dest``
parameter::

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', dest='bar')
   >>> parser.parse_args('--foo XXX'.split())
   Namespace(bar='XXX')


The parse_args() method
-----------------------

.. method:: parse_args([args], [namespace])

   Convert the strings to objects and assign them as attributes of the
   namespace. Return the populated namespace.

   Previous calls to :meth:`add_argument` determine exactly what objects are
   created and how they are assigned. See the documentation for
   :meth:`add_argument` for details.

   By default, the arg strings are taken from ``sys.argv``, and a new empty
   ``Namespace`` object is created for the attributes.

Option value syntax
^^^^^^^^^^^^^^^^^^^

The :meth:`parse_args` 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::

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-x')
   >>> parser.add_argument('--foo')
   >>> parser.parse_args('-x X'.split())
   Namespace(foo=None, x='X')
   >>> parser.parse_args('--foo FOO'.split())
   Namespace(foo='FOO', x=None)

For long options (options with names longer than a single character), you may
also pass the option and value as a single command line argument, using ``=``
to separate them::

   >>> parser.parse_args('--foo=FOO'.split())
   Namespace(foo='FOO', x=None)

For short options (options only one character long), you may simply concatenate
the option and its value::

   >>> parser.parse_args('-xX'.split())
   Namespace(foo=None, x='X')

You can also combine several short options together, using only a single ``-``
prefix, as long as only the last option (or none of them) requires a value::

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-x', action='store_true')
   >>> parser.add_argument('-y', action='store_true')
   >>> parser.add_argument('-z')
   >>> parser.parse_args('-xyzZ'.split())
   Namespace(x=True, y=True, z='Z')


Invalid arguments
^^^^^^^^^^^^^^^^^

While parsing the command-line, ``parse_args`` 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::

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('--foo', type=int)
   >>> parser.add_argument('bar', nargs='?')

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

   >>> # invalid option
   >>> parser.parse_args(['--bar'])
   usage: PROG [-h] [--foo FOO] [bar]
   PROG: error: no such option: --bar

   >>> # wrong number of arguments
   >>> parser.parse_args(['spam', 'badger'])
   usage: PROG [-h] [--foo FOO] [bar]
   PROG: error: extra arguments found: badger


Arguments containing ``"-"``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The ``parse_args`` method attempts to give errors whenever the user has clearly
made a mistake, but some situations are inherently ambiguous. For example, the
command-line arg ``'-1'`` could either be an attempt to specify an option or an
attempt to provide a positional argument. The ``parse_args`` method is cautious
here: positional arguments may only begin with ``'-'`` if they look like
negative numbers and there are no options in the parser that look like negative
numbers::

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-x')
   >>> parser.add_argument('foo', nargs='?')

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

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

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-1', dest='one')
   >>> parser.add_argument('foo', nargs='?')

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

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

   >>> # negative number options present, so both -1s are options
   >>> parser.parse_args(['-1', '-1'])
   usage: PROG [-h] [-1 ONE] [foo]
   PROG: error: argument -1: expected one argument

If you have positional arguments that must begin with ``'-'`` and don't look
like negative numbers, you can insert the pseudo-argument ``'--'`` which tells
``parse_args`` that everything after that is a positional argument::

   >>> parser.parse_args(['--', '-f'])
   Namespace(foo='-f', one=None)


Argument abbreviations
^^^^^^^^^^^^^^^^^^^^^^

The :meth:`parse_args` method allows you to abbreviate long options if the
abbreviation is unambiguous::

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-bacon')
   >>> parser.add_argument('-badger')
   >>> parser.parse_args('-bac MMM'.split())
   Namespace(bacon='MMM', badger=None)
   >>> parser.parse_args('-bad WOOD'.split())
   Namespace(bacon=None, badger='WOOD')
   >>> parser.parse_args('-ba BA'.split())
   usage: PROG [-h] [-bacon BACON] [-badger BADGER]
   PROG: error: ambiguous option: -ba could match -badger, -bacon

As you can see above, you will get an error if you pick a prefix that could
refer to more than one option.


Beyond ``sys.argv``
^^^^^^^^^^^^^^^^^^^

Sometimes it may be useful to have an ArgumentParser parse args other than
those of ``sys.argv``.  This can be accomplished by passing a list of strings
to ``parse_args``.  You may have noticed that the examples in the argparse
documentation have made heavy use of this calling style - it is much easier
to use at the interactive prompt::

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument(
   ...     'integers', metavar='int', type=int, choices=xrange(10),
   ...  nargs='+', help='an integer in the range 0..9')
   >>> parser.add_argument(
   ...     '--sum', dest='accumulate', action='store_const', const=sum,
   ...   default=max, help='sum the integers (default: find the max)')
   >>> parser.parse_args(['1', '2', '3', '4'])
   Namespace(accumulate=<built-in function max>, integers=[1, 2, 3, 4])
   >>> parser.parse_args('1 2 3 4 --sum'.split())
   Namespace(accumulate=<built-in function sum>, integers=[1, 2, 3, 4])


Custom namespaces
^^^^^^^^^^^^^^^^^

It may also be useful to have an ArgumentParser assign attributes to an already
existing object, rather than the newly-created Namespace object that is
normally used. This can be achieved by specifying the ``namespace=`` keyword
argument::

   >>> class C(object):
   ...     pass
   ...
   >>> c = C()
   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo')
   >>> parser.parse_args(args=['--foo', 'BAR'], namespace=c)
   >>> c.foo
   'BAR'


Other utilities
---------------

Sub-commands
^^^^^^^^^^^^

.. method:: add_subparsers()

   A lot of programs split up their functionality into a number of
   sub-commands, for example, the ``svn`` program can invoke sub-commands like
   ``svn checkout``, ``svn update``, ``svn commit``, etc.  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.  ArgumentParser objects support the creation of such
   sub-commands with the :meth:`add_subparsers` method.  The
   :meth:`add_subparsers` method is normally called with no arguments and
   returns an special action object.  This object has a single method,
   ``add_parser``, which takes a command name and any ArgumentParser
   constructor arguments, and returns an ArgumentParser object that can be
   modified as usual.

   Some example usage::

     >>> # create the top-level parser
     >>> parser = argparse.ArgumentParser(prog='PROG')
     >>> parser.add_argument('--foo', action='store_true', help='foo help')
     >>> subparsers = parser.add_subparsers(help='sub-command help')
     >>>
     >>> # create the parser for the "a" command
     >>> parser_a = subparsers.add_parser('a', help='a help')
     >>> parser_a.add_argument('bar', type=int, help='bar help')
     >>>
     >>> # create the parser for the "b" command
     >>> parser_b = subparsers.add_parser('b', help='b help')
     >>> parser_b.add_argument('--baz', choices='XYZ', help='baz help')
     >>>
     >>> # parse some arg lists
     >>> parser.parse_args(['a', '12'])
     Namespace(bar=12, foo=False)
     >>> parser.parse_args(['--foo', 'b', '--baz', 'Z'])
     Namespace(baz='Z', foo=True)

   Note that the object returned by :meth:`parse_args` 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 ``"a"`` command is specified, only the ``foo`` and ``bar`` attributes
   are present, and when the ``"b"`` command is specified, only the ``foo`` and
   ``baz`` 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. (You can however supply a
   help message for each subparser command by suppling the ``help=`` argument
   to ``add_parser`` as above.)

   ::

     >>> parser.parse_args(['--help'])
     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

     >>> parser.parse_args(['a', '--help'])
     usage: PROG a [-h] bar

     positional arguments:
       bar     bar help

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

     >>> parser.parse_args(['b', '--help'])
     usage: PROG b [-h] [--baz {X,Y,Z}]

     optional arguments:
       -h, --help     show this help message and exit
       --baz {X,Y,Z}  baz help

   The :meth:`add_subparsers` method also supports ``title`` and
   ``description`` keyword arguments.  When either is present, the subparser's
   commands will appear in their own group in the help output.  For example::

     >>> parser = argparse.ArgumentParser()
     >>> subparsers = parser.add_subparsers(title='subcommands',
     ...                                    description='valid subcommands',
     ...                                    help='additional help')
     >>> subparsers.add_parser('foo')
     >>> subparsers.add_parser('bar')
     >>> parser.parse_args(['-h'])
     usage:  [-h] {foo,bar} ...

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

     subcommands:
       valid subcommands

       {foo,bar}   additional help


   One particularly effective way of handling sub-commands is to combine the
   use of the :meth:`add_subparsers` method with calls to :meth:`set_defaults`
   so that each subparser knows which Python function it should execute.  For
   example::

     >>> # sub-command functions
     >>> def foo(args):
     ...     print args.x * args.y
     ...
     >>> def bar(args):
     ...     print '((%s))' % args.z
     ...
     >>> # create the top-level parser
     >>> parser = argparse.ArgumentParser()
     >>> subparsers = parser.add_subparsers()
     >>>
     >>> # create the parser for the "foo" command
     >>> parser_foo = subparsers.add_parser('foo')
     >>> parser_foo.add_argument('-x', type=int, default=1)
     >>> parser_foo.add_argument('y', type=float)
     >>> parser_foo.set_defaults(func=foo)
     >>>
     >>> # create the parser for the "bar" command
     >>> parser_bar = subparsers.add_parser('bar')
     >>> parser_bar.add_argument('z')
     >>> parser_bar.set_defaults(func=bar)
     >>>
     >>> # parse the args and call whatever function was selected
     >>> args = parser.parse_args('foo 1 -x 2'.split())
     >>> args.func(args)
     2.0
     >>>
     >>> # parse the args and call whatever function was selected
     >>> args = parser.parse_args('bar XYZYX'.split())
     >>> args.func(args)
     ((XYZYX))

   This way, you can let :meth:`parse_args` do all the work for you, and then
   just call the appropriate function after the 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 you
   find it necessary to check the name of the subparser that was invoked, you
   can always provide a ``dest`` keyword argument to the :meth:`add_subparsers`
   call::

     >>> parser = argparse.ArgumentParser()
     >>> subparsers = parser.add_subparsers(dest='subparser_name')
     >>> subparser1 = subparsers.add_parser('1')
     >>> subparser1.add_argument('-x')
     >>> subparser2 = subparsers.add_parser('2')
     >>> subparser2.add_argument('y')
     >>> parser.parse_args(['2', 'frobble'])
     Namespace(subparser_name='2', y='frobble')


FileType objects
^^^^^^^^^^^^^^^^

.. class:: FileType(mode='r', bufsize=None)

   The :class:`FileType` factory creates objects that can be passed to the type
   argument of :meth:`add_argument`. Arguments that have :class:`FileType`
   objects as their type will open command-line args as files with the
   requested modes and buffer sizes:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--output', type=argparse.FileType('wb', 0))
   >>> parser.parse_args(['--output', 'out'])
   Namespace(output=<open file 'out', mode 'wb' at 0x...>)

   FileType objects understand the pseudo-argument ``'-'`` and automatically
   convert this into ``sys.stdin`` for readable :class:`FileType` objects and
   ``sys.stdout`` for writable :class:`FileType` objects:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('infile', type=argparse.FileType('r'))
   >>> parser.parse_args(['-'])
   Namespace(infile=<open file '<stdin>', mode 'r' at 0x...>)


Argument groups
^^^^^^^^^^^^^^^

.. method:: add_argument_group([title], [description])

   By default, ArgumentParser objects group command-line arguments into
   "positional arguments" and "optional arguments" when displaying help
   messages. When there is a better conceptual grouping of arguments than this
   default one, appropriate groups can be created using the
   :meth:`add_argument_group` method::

     >>> parser = argparse.ArgumentParser(prog='PROG', add_help=False)
     >>> group = parser.add_argument_group('group')
     >>> group.add_argument('--foo', help='foo help')
     >>> group.add_argument('bar', help='bar help')
     >>> parser.print_help()
     usage: PROG [--foo FOO] bar

     group:
       bar    bar help
       --foo FOO  foo help

   The :meth:`add_argument_group` method returns an argument group object which
   has an :meth:`add_argument` method just like a regular ArgumentParser
   objects. 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 :meth:`add_argument_group` method accepts ``title`` and
   ``description`` arguments which can be used to customize this display::

     >>> parser = argparse.ArgumentParser(prog='PROG', add_help=False)
     >>> group1 = parser.add_argument_group('group1', 'group1 description')
     >>> group1.add_argument('foo', help='foo help')
     >>> group2 = parser.add_argument_group('group2', 'group2 description')
     >>> group2.add_argument('--bar', help='bar help')
     >>> parser.print_help()
     usage: PROG [--bar BAR] foo

     group1:
       group1 description

       foo    foo help

     group2:
       group2 description

       --bar BAR  bar help

   Note that any arguments not in your user defined groups will end up back in
   the usual "positional arguments" and "optional arguments" sections.


Mutual exclusion
^^^^^^^^^^^^^^^^

.. method:: add_mutually_exclusive_group([required=False])

   Sometimes, you need to make sure that only one of a couple different options
   is specified on the command line. You can create groups of such mutually
   exclusive arguments using the :meth:`add_mutually_exclusive_group` method.
   When :func:`parse_args` is called, argparse will make sure that only one of
   the arguments in the mutually exclusive group was present on the command
   line::

     >>> parser = argparse.ArgumentParser(prog='PROG')
     >>> group = parser.add_mutually_exclusive_group()
     >>> group.add_argument('--foo', action='store_true')
     >>> group.add_argument('--bar', action='store_false')
     >>> parser.parse_args(['--foo'])
     Namespace(bar=True, foo=True)
     >>> parser.parse_args(['--bar'])
     Namespace(bar=False, foo=False)
     >>> parser.parse_args(['--foo', '--bar'])
     usage: PROG [-h] [--foo | --bar]
     PROG: error: argument --bar: not allowed with argument --foo

   The :meth:`add_mutually_exclusive_group` method also accepts a ``required``
   argument, to indicate that at least one of the mutually exclusive arguments
   is required::

     >>> parser = argparse.ArgumentParser(prog='PROG')
     >>> group = parser.add_mutually_exclusive_group(required=True)
     >>> group.add_argument('--foo', action='store_true')
     >>> group.add_argument('--bar', action='store_false')
     >>> parser.parse_args([])
     usage: PROG [-h] (--foo | --bar)
     PROG: error: one of the arguments --foo --bar is required

   Note that currently mutually exclusive argument groups do not support the
   ``title`` and ``description`` arguments of :meth:`add_argument_group`. This
   may change in the future however, so you are *strongly* recommended to
   specify ``required`` as a keyword argument if you use it.


Parser defaults
^^^^^^^^^^^^^^^

.. method:: set_defaults(**kwargs)

   Most of the time, the attributes of the object returned by
   :meth:`parse_args` will be fully determined by inspecting the command-line
   args and the argument actions described in your :meth:`add_argument` calls.
   However, sometimes it may be useful to add some additional attributes that
   are determined without any inspection of the command-line.  The
   :meth:`set_defaults` method allows you to do this::

     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('foo', type=int)
     >>> parser.set_defaults(bar=42, baz='badger')
     >>> parser.parse_args(['736'])
     Namespace(bar=42, baz='badger', foo=736)

   Note that parser-level defaults always override argument-level defaults. So
   if you set a parser-level default for a name that matches an argument, the
   old argument default will no longer be used::

     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('--foo', default='bar')
     >>> parser.set_defaults(foo='spam')
     >>> parser.parse_args([])
     Namespace(foo='spam')

   Parser-level defaults can be particularly useful when you're working with
   multiple parsers.  See the :meth:`add_subparsers` method for an example of
   this type.

.. method:: get_default(dest)

   Get the default value for a namespace attribute, as set by either
   :meth:`add_argument` or by :meth:`set_defaults`::

     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('--foo', default='badger')
     >>> parser.get_default('foo')
     'badger'


Printing help
^^^^^^^^^^^^^

In most typical applications, :meth:`parse_args` will take care of formatting
and printing any usage or error messages. However, should you want to format or
print these on your own, several methods are available:

.. method:: print_usage([file]):

   Print a brief description of how the :class:`ArgumentParser` should be
   invoked on the command line.  If ``file`` is not present, ``sys.stderr`` is
   assumed.

.. method:: print_help([file]):

   Print a help message, including the program usage and information about the
   arguments registered with the :class:`ArgumentParser`. If ``file`` is not
   present, ``sys.stderr`` is assumed.

There are also variants of these methods that simply return a string instead of
printing it:

.. method:: format_usage():

   Return a string containing a brief description of how the
   :class:`ArgumentParser` should be invoked on the command line.

.. method:: format_help():

   Return a string containing a help message, including the program usage and
   information about the arguments registered with the :class:`ArgumentParser`.



Partial parsing
^^^^^^^^^^^^^^^

.. method:: parse_known_args([args], [namespace])

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
:meth:`parse_known_args` method can be useful. It works much like
:meth:`parse_args` 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.

::

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', action='store_true')
   >>> parser.add_argument('bar')
   >>> parser.parse_known_args(['--foo', '--badger', 'BAR', 'spam'])
   (Namespace(bar='BAR', foo=True), ['--badger', 'spam'])


Customizing file parsing
^^^^^^^^^^^^^^^^^^^^^^^^

.. method:: convert_arg_line_to_args(arg_line)

   Arguments that are read from a file (see the ``fromfile_prefix_chars``
   keyword argument to the :class:`ArgumentParser` constructor) are read one
   argument per line.  If you need fancier parsing, then you can subclass the
   :class:`ArgumentParser` and override the :meth:`convert_arg_line_to_args`
   method.

   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::

    def convert_arg_line_to_args(self, arg_line):
        for arg in arg_line.split():
            if not arg.strip():
                continue
            yield arg


Upgrading optparse code
-----------------------

Originally, the argparse module had attempted to maintain compatibility with
 optparse. However, optparse was difficult to extend transparently,
 particularly with the changes required to support the new ``nargs=``
 specifiers and better usage messges. When most everything in optparse 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 optparse to argparse:

* Replace all ``add_option()`` calls with :meth:`add_argument` calls.

* Replace ``options, args = parser.parse_args()`` with
  ``args = parser.parse_args()`` and add additional :meth:`add_argument` calls
  for the positional arguments.

* Replace callback actions and the ``callback_*`` keyword arguments with
  ``type`` or ``action`` arguments.

* Replace string names for ``type`` keyword arguments with the corresponding
  type objects (e.g. int, float, complex, etc).

* Replace ``Values`` with ``Namespace`` and ``OptionError/OptionValueError``
  with ``ArgumentError``.

* Replace strings with implicit arguments such as ``%default`` or ``%prog``
  with the standard python syntax to use dictionaries to format strings, that
  is, ``%(default)s`` and ``%(prog)s``.