
``argparse`` --- Parser for command-line options, arguments and sub-commands
****************************************************************************

**Source code:** Lib/argparse.py

New in version 3.2.

======================================================================

The ``argparse`` module makes it easy to write user-friendly command-
line interfaces. The program defines what arguments it requires, and
``argparse`` will figure out how to parse those out of ``sys.argv``.
The ``argparse`` module also automatically generates help and usage
messages and issues errors when users give the program invalid
arguments.


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
-----------------

The first step in using the ``argparse`` is creating an
``ArgumentParser`` object:

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

The ``ArgumentParser`` object will hold all the information necessary
to parse the command line into Python data types.


Adding arguments
----------------

Filling an ``ArgumentParser`` with information about program arguments
is done by making calls to the ``add_argument()`` method. Generally,
these calls tell the ``ArgumentParser`` how to take the strings on the
command line and turn them into objects.  This information is stored
and used when ``parse_args()`` is called. For example:

   >>> 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)')

Later, calling ``parse_args()`` will 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
-----------------

``ArgumentParser`` parses arguments through the ``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, ``parse_args()`` will typically be called with no
arguments, and the ``ArgumentParser`` will automatically determine the
command-line arguments from ``sys.argv``.


ArgumentParser objects
======================

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

   Create a new ``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 ``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 - The name of the program (default: ``sys.argv[0]``)

   * usage - 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 an 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 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

The help option is typically ``-h/--help``. The exception to this is
if the ``prefix_chars=`` is specified and does not include ``'-'``, in
which case ``-h`` and ``--help`` are not valid options.  In this case,
the first character in ``prefix_chars`` is used to prefix the help
options:

   >>> parser = argparse.ArgumentParser(prog='PROG', prefix_chars='+/')
   >>> parser.print_help()
   usage: PROG [+h]

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


prefix_chars
------------

Most command-line options will use ``'-'`` as the prefix, e.g.
``-f/--foo``. Parsers that need to support different or 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, 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
``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:

   >>> with open('args.txt', 'w') as fp:
   ...    fp.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 ``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 ``add_argument()`` or by calling the ``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 ``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, a single parser
with all the shared arguments and passed to ``parents=`` argument to
``ArgumentParser`` can be used.  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.

Note: You must fully initialize the parsers before passing them via
  ``parents=``. If you change the parent parsers after the child
  parser, those changes will not be reflected in the child.


formatter_class
---------------

``ArgumentParser`` objects allow the help formatting to be customized
by specifying an alternate formatting class.  Currently, there are
three such classes:

class class argparse.RawDescriptionHelpFormatter
class class argparse.RawTextHelpFormatter
class class 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

Passing ``RawDescriptionHelpFormatter`` as ``formatter_class=``
indicates that description and epilog are already correctly formatted
and should not be line-wrapped:

   >>> 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

``RawTextHelpFormatter`` maintains whitespace for all sorts of help
text including argument descriptions.

The other formatter class available,
``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 raises an
exception if an attempt is made 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 uses ``sys.argv[0]`` 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 ``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`` calculates 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

The default message can be overridden with the ``usage=`` keyword
argument:

   >>> 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

The ``%(prog)s`` format specifier is available to fill in the program
name in your usage messages.


The add_argument() method
=========================

ArgumentParser.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 argument 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 ``parse_args()``.

The following sections describe how each of these are used.


name or flags
-------------

The ``add_argument()`` method must know whether an optional argument,
like ``-f`` or ``--foo``, or a positional argument, like a list of
filenames, is expected.  The first arguments passed to
``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 ``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
------

``ArgumentParser`` 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 ``parse_args()``.  The ``action``
keyword argument specifies how the command-line arguments should be
handled. 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
  the rather unhelpful ``None``.)  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 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 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``.)  The
  ``'append_const'`` action is typically useful when multiple
  arguments need 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
  ``add_argument()`` call, and prints version information and exits
  when invoked.

  >>> import argparse
  >>> parser = argparse.ArgumentParser(prog='PROG')
  >>> parser.add_argument('--version', action='version', version='%(prog)s 2.0')
  >>> parser.parse_args(['--version'])
  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 should accept four parameters:

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

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

* ``values`` - The associated command-line arguments, with any type
  conversions applied.  (Type conversions are specified with the type
  keyword argument to ``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.

An example of a custom action:

   >>> 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.  The ``nargs`` keyword
argument associates a different number of command-line arguments with
a single action.  The supported values are:

* N (an integer).  N arguments 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=<_io.TextIOWrapper name='input.txt' encoding='UTF-8'>,
               outfile=<_io.TextIOWrapper name='output.txt' encoding='UTF-8'>)
     >>> parser.parse_args([])
     Namespace(infile=<_io.TextIOWrapper name='<stdin>' encoding='UTF-8'>,
               outfile=<_io.TextIOWrapper name='<stdout>' encoding='UTF-8'>)

* ``'*'``.  All command-line arguments present are gathered into a
  list.  Note that it generally doesn't make much sense to have more
  than one positional argument with ``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
arguments 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 ``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 ``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 ``parse_args()``.
  See the action description for examples.

* When ``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 arguments. 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
``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)

Providing ``default=argparse.SUPPRESS`` causes no attribute to be
added if the command-line argument was not present.:

   >>> 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 arguments in
as simple strings. However, quite often the command-line string should
instead be interpreted as another type, like a ``float`` or ``int``.
The ``type`` keyword argument of ``add_argument()`` 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
``type`` argument:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('foo', type=int)
   >>> parser.add_argument('bar', type=open)
   >>> parser.parse_args('2 temp.txt'.split())
   Namespace(bar=<_io.TextIOWrapper name='temp.txt' encoding='UTF-8'>, 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 ``open()`` function.  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=<_io.TextIOWrapper name='out.txt' encoding='UTF-8'>)

``type=`` can take any callable that takes a single string argument
and returns the 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

The choices keyword argument may be more convenient for type checkers
that simply check against a range of values:

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('foo', type=int, choices=range(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 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 ``add_argument()``.  When the command
line is parsed, 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 make an option *required*, ``True`` can be
specified for the ``required=`` keyword argument to
``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``,
``parse_args()`` will report an error if that option is not present at
the command line.

Note: Required options are generally considered bad form because users
  expect *options* to be *optional*, and thus they should be avoided
  when possible.


help
----

The ``help`` value is a string containing a brief description of the
argument. 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 ``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`` generates help messages, it 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, a single
positional argument with ``dest='bar'`` will that argument will be
referred to as ``bar``. A single optional argument ``--foo`` that
should be followed by a single command-line arg will be referred to as
``FOO``.  An example:

   >>> 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

An alternative name can be specified with ``metavar``:

   >>> 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 ``parse_args()`` object is still determined by
the dest value.

Different values of ``nargs`` may cause the metavar to be used
multiple times. Providing a tuple to ``metavar`` specifies a different
display for each of the arguments:

   >>> 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 ``parse_args()``.  The name of this attribute is
determined by the ``dest`` keyword argument of ``add_argument()``.
For positional argument actions, ``dest`` is normally supplied as the
first argument to ``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`` generates 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')

``dest`` allows a custom attribute name to be provided:

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


The parse_args() method
=======================

ArgumentParser.parse_args(args=None, namespace=None)

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

   Previous calls to ``add_argument()`` determine exactly what objects
   are created and how they are assigned. See the documentation for
   ``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 ``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),
the option and value can also be passed 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), the option and
its value can be concatenated:

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

Several short options can be joined 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 ``parse_args()`` method allows long options to be abbreviated 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

An error is produced for arguments that could produce more than one
options.


Beyond ``sys.argv``
-------------------

Sometimes it may be useful to have an ArgumentParser parse arguments
other than those of ``sys.argv``.  This can be accomplished by passing
a list of strings to ``parse_args()``.  This is useful for testing at
the interactive prompt:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument(
   ...     'integers', metavar='int', type=int, choices=range(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])


The Namespace object
--------------------

class class argparse.Namespace

   Simple class used by default by ``parse_args()`` to create an
   object holding attributes and return it.

This class is deliberately simple, just an ``object`` subclass with a
readable string representation. If you prefer to have dict-like view
of the attributes, you can use the standard Python idiom, ``vars()``:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo')
   >>> args = parser.parse_args(['--foo', 'BAR'])
   >>> vars(args)
   {'foo': 'BAR'}

It may also be useful to have an ``ArgumentParser`` assign attributes
to an already existing object, rather than a new ``Namespace`` object.
This can be achieved by specifying the ``namespace=`` keyword
argument:

   >>> class C:
   ...     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
------------

ArgumentParser.add_subparsers()

   Many 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``, and ``svn commit``.
   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``
   supports the creation of such sub-commands with the
   ``add_subparsers()`` method.  The ``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 ``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.
   (A help message for each subparser command, however, can be given
   by supplying 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 ``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

   Furthermore, ``add_parser`` supports an additional ``aliases``
   argument, which allows multiple strings to refer to the same
   subparser. This example, like ``svn``, aliases ``co`` as a
   shorthand for ``checkout``:

      >>> parser = argparse.ArgumentParser()
      >>> subparsers = parser.add_subparsers()
      >>> checkout = subparsers.add_parser('checkout', aliases=['co'])
      >>> checkout.add_argument('foo')
      >>> parser.parse_args(['co', 'bar'])
      Namespace(foo='bar')

   One particularly effective way of handling sub-commands is to
   combine the use of the ``add_subparsers()`` method with calls to
   ``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 ``parse_args()`` 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 ``dest`` keyword argument to the
   ``add_subparsers()`` call will work:

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

   The ``FileType`` factory creates objects that can be passed to the
   type argument of ``ArgumentParser.add_argument()``.  Arguments that
   have ``FileType`` objects as their type will open command-line
   arguments 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=<_io.BufferedWriter name='out'>)

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

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('infile', type=argparse.FileType('r'))
   >>> parser.parse_args(['-'])
   Namespace(infile=<_io.TextIOWrapper name='<stdin>' encoding='UTF-8'>)


Argument groups
---------------

ArgumentParser.add_argument_group(title=None, description=None)

   By default, ``ArgumentParser`` groups 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 ``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 ``add_argument_group()`` method returns an argument group
   object which has an ``add_argument()`` method just like a regular
   ``ArgumentParser``.  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
   ``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 your user defined groups will end up
   back in the usual "positional arguments" and "optional arguments"
   sections.


Mutual exclusion
----------------

argparse.add_mutually_exclusive_group(required=False)

   Create a mutually exclusive group. ``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 ``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
   ``add_argument_group()``.


Parser defaults
---------------

ArgumentParser.set_defaults(**kwargs)

   Most of the time, the attributes of the object returned by
   ``parse_args()`` will be fully determined by inspecting the
   command-line arguments and the argument actions.
   ``set_defaults()`` allows some additional attributes that are
   determined without any inspection of the command line to be added:

      >>> 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:

      >>> 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 working with
   multiple parsers.  See the ``add_subparsers()`` method for an
   example of this type.

ArgumentParser.get_default(dest)

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

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


Printing help
-------------

In most typical applications, ``parse_args()`` will take care of
formatting and printing any usage or error messages.  However, several
formatting methods are available:

ArgumentParser.print_usage(file=None)

   Print a brief description of how the ``ArgumentParser`` should be
   invoked on the command line.  If *file* is ``None``, ``sys.stdout``
   is assumed.

ArgumentParser.print_help(file=None)

   Print a help message, including the program usage and information
   about the arguments registered with the ``ArgumentParser``.  If
   *file* is ``None``, ``sys.stdout`` is assumed.

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

ArgumentParser.format_usage()

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

ArgumentParser.format_help()

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


Partial parsing
---------------

ArgumentParser.parse_known_args(args=None, namespace=None)

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 ``parse_known_args()`` method can be useful.  It
works much like ``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
------------------------

ArgumentParser.convert_arg_line_to_args(arg_line)

   Arguments that are read from a file (see the
   *fromfile_prefix_chars* keyword argument to the ``ArgumentParser``
   constructor) are read one argument per line.
   ``convert_arg_line_to_args()`` 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:

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


Exiting methods
---------------

ArgumentParser.exit(status=0, message=None)

   This method terminates the program, exiting with the specified
   *status* and, if given, it prints a *message* before that.

ArgumentParser.error(message)

   This method prints a usage message including the *message* to the
   standard error and terminates the program with a status code of 2.


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 messages.  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 ``optparse.OptionParser.add_option()`` calls with
  ``ArgumentParser.add_argument()`` calls.

* Replace ``options, args = parser.parse_args()`` with ``args =
  parser.parse_args()`` and add additional
  ``ArgumentParser.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 ``optparse.Values`` with ``Namespace`` and
  ``optparse.OptionError`` and ``optparse.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``.

* Replace the OptionParser constructor ``version`` argument with a
  call to ``parser.add_argument('--version', action='version',
  version='<the version>')``
