Python 3.6.5 Documentation >  "argparse" — Parser for command-line options, arguments and sub-commands

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

New in version 3.2.

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

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

Tutorial
^^^^^^^^

This page contains the API reference information. For a more gentle
introduction to Python command-line parsing, have a look at the
argparse tutorial.

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:

$ python 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:

$ python prog.py 1 2 3 4
4

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

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

$ python 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 argument 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 argparse.ArgumentParser(prog=None, usage=None, description=None, epilog=None, parents=[], formatter_class=argparse.HelpFormatter, prefix_chars='-', fromfile_prefix_chars=None, argument_default=None, conflict_handler='error', add_help=True, allow_abbrev=True)

Create a new "ArgumentParser" object. All parameters should be
passed as keyword arguments. Each parameter has its own more
detailed description below, but in short they are:

* prog - The name of the program (default: "sys.argv[0]")

* usage - The string describing the program usage (default:
generated from arguments added to parser)

* description - Text to display before the argument help
(default: none)

* epilog - Text to display after the argument help (default:
none)

* parents - A list of "ArgumentParser" objects whose arguments
should also be included

* formatter_class - A class for customizing the help output

* 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")

* argument_default - The global default value for arguments
(default: "None")

* conflict_handler - The strategy for resolving conflicting
optionals (usually unnecessary)

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

* allow_abbrev - Allows long options to be abbreviated if the
abbreviation is unambiguous. (default: "True")

Changed in version 3.5: *allow_abbrev* parameter was added.

The following sections describe how each of these are used.

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 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.

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".

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 four
such classes:

class argparse.RawDescriptionHelpFormatter
class argparse.RawTextHelpFormatter
class argparse.ArgumentDefaultsHelpFormatter
class argparse.MetavarTypeHelpFormatter

"RawDescriptionHelpFormatter" and "RawTextHelpFormatter" give more
control over how textual descriptions are displayed. 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. However, multiple new lines are
replaced with one. If you wish to preserve multiple blank lines, add
spaces between the newlines.

"ArgumentDefaultsHelpFormatter" automatically adds information about
default values to each of the argument help messages:

>>> 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)

"MetavarTypeHelpFormatter" uses the name of the type argument for each
argument as the display name for its values (rather than using the
dest as the regular formatter does):

>>> parser = argparse.ArgumentParser(
... prog='PROG',
... formatter_class=argparse.MetavarTypeHelpFormatter)
>>> parser.add_argument('--foo', type=int)
>>> parser.add_argument('bar', type=float)
>>> parser.print_help()
usage: PROG [-h] [--foo int] float

positional arguments:
float

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

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()

allow_abbrev
------------

Normally, when you pass an argument list to the "parse_args()" method
of an "ArgumentParser", it recognizes abbreviations of long options.

This feature can be disabled by setting "allow_abbrev" to "False":

>>> parser = argparse.ArgumentParser(prog='PROG', allow_abbrev=False)
>>> parser.add_argument('--foobar', action='store_true')
>>> parser.add_argument('--foonley', action='store_false')
>>> parser.parse_args(['--foon'])
usage: PROG [-h] [--foobar] [--foonley]
PROG: error: unrecognized arguments: --foon

New in version 3.5.

conflict_handler
----------------

"ArgumentParser" objects do not allow two actions with the same option
string. By default, "ArgumentParser" objects raise 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.

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

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 supplied 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. 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'])
Namespace(foo=42)

* "'store_true'" and "'store_false'" - These are special cases of
"'store_const'" used for storing the values "True" and "False"
respectively. In addition, they create default values of "False"
and "True" respectively. For example:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', action='store_true')
>>> parser.add_argument('--bar', action='store_false')
>>> parser.add_argument('--baz', action='store_false')
>>> parser.parse_args('--foo --bar'.split())
Namespace(foo=True, bar=False, baz=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=[<class 'str'>, <class 'int'>])

* "'count'" - This counts the number of times a keyword argument
occurs. For example, this is useful for increasing verbosity levels:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--verbose', '-v', action='count')
>>> parser.parse_args(['-vvv'])
Namespace(verbose=3)

* "'help'" - This prints a complete help message for all the options
in the current parser and then exits. By default a help action is
automatically added to the parser. See "ArgumentParser" for details
of how the output is created.

* "'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 may also specify an arbitrary action by passing an Action subclass
or other object that implements the same interface. The recommended
way to do this is to extend "Action", overriding the "__call__" method
and optionally the "__init__" method.

An example of a custom action:

>>> class FooAction(argparse.Action):
... def __init__(self, option_strings, dest, nargs=None, **kwargs):
... if nargs is not None:
... raise ValueError("nargs not allowed")
... super(FooAction, self).__init__(option_strings, dest, **kwargs)
... 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')

For more details, see "Action".

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 argument will be consumed from the command line if
possible, and produced as a single item. If no command-line
argument is present, the value from 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
argument. 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'])
Namespace(bar='XX', foo='YY')
>>> parser.parse_args(['XX', '--foo'])
Namespace(bar='XX', foo='c')
>>> parser.parse_args([])
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 argument present. For
example:

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

* "argparse.REMAINDER". All the remaining command-line arguments
are gathered into a list. This is commonly useful for command line
utilities that dispatch to other command line utilities:

>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('--foo')
>>> parser.add_argument('command')
>>> parser.add_argument('args', nargs=argparse.REMAINDER)
>>> print(parser.parse_args('--foo B cmd --arg1 XX ZZ'.split()))
Namespace(args=['--arg1', 'XX', 'ZZ'], command='cmd', foo='B')

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 argument 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 argument following it, the value of "const" will be
assumed instead. See the nargs description for examples.

With the "'store_const'" and "'append_const'" actions, the "const"
keyword argument must be given. For other actions, it 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 argument 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'])
Namespace(foo='2')
>>> parser.parse_args([])
Namespace(foo=42)

If the "default" value is a string, the parser parses the value as if
it were a command-line argument. In particular, the parser applies
any type conversion argument, if provided, before setting the
attribute on the "Namespace" return value. Otherwise, the parser uses
the value as is:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--length', default='10', type=int)
>>> parser.add_argument('--width', default=10.5, type=int)
>>> parser.parse_args()
Namespace(length=10, width=10.5)

For positional arguments with nargs equal to "?" or "*", the "default"
value is used when no command-line argument was present:

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

See the section on the default keyword argument for information on
when the "type" argument is applied to default arguments.

To ease the use of various types of files, the argparse module
provides the factory FileType which takes the "mode=", "bufsize=",
"encoding=" and "errors=" 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'])
Namespace(foo=9)
>>> parser.parse_args(['7'])
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'])
Namespace(foo=7)
>>> parser.parse_args(['11'])
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, argument values will be checked, and an error message will
be displayed if the argument was not one of the acceptable values:

>>> parser = argparse.ArgumentParser(prog='game.py')
>>> parser.add_argument('move', choices=['rock', 'paper', 'scissors'])
>>> parser.parse_args(['rock'])
Namespace(move='rock')
>>> parser.parse_args(['fire'])
usage: game.py [-h] {rock,paper,scissors}
game.py: error: argument move: invalid choice: 'fire' (choose from 'rock',
'paper', 'scissors')

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='doors.py')
>>> parser.add_argument('door', type=int, choices=range(1, 4))
>>> print(parser.parse_args(['3']))
Namespace(door=3)
>>> parser.parse_args(['4'])
usage: doors.py [-h] {1,2,3}
doors.py: error: argument door: invalid choice: 4 (choose from 1, 2, 3)

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'])
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

As the help string supports %-formatting, if you want a literal "%" to
appear in the help string, you must escape it as "%%".

"argparse" supports silencing the help entry for certain options, by
setting the "help" value to "argparse.SUPPRESS":

>>> parser = argparse.ArgumentParser(prog='frobble')
>>> parser.add_argument('--foo', help=argparse.SUPPRESS)
>>> parser.print_help()
usage: frobble [-h]

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

metavar
-------

When "ArgumentParser" generates help messages, it needs 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 be referred to as "bar". A
single optional argument "--foo" that should be followed by a single
command-line argument 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'])
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')

Action classes
--------------

Action classes implement the Action API, a callable which returns a
callable which processes arguments from the command-line. Any object
which follows this API may be passed as the "action" parameter to
"add_argument()".

class argparse.Action(option_strings, dest, nargs=None, const=None, default=None, type=None, choices=None, required=False, help=None, metavar=None)

Action objects are used by an ArgumentParser to represent the
information needed to parse a single argument from one or more strings
from the command line. The Action class must accept the two positional
arguments plus any keyword arguments passed to
"ArgumentParser.add_argument()" except for the "action" itself.

Instances of Action (or return value of any callable to the "action"
parameter) should have attributes “dest”, “option_strings”, “default”,
“type”, “required”, “help”, etc. defined. The easiest way to ensure
these attributes are defined is to call "Action.__init__".

Action instances should be callable, so subclasses must override the
"__call__" method, which 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 using
"setattr()".

* "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.

The "__call__" method may perform arbitrary actions, but will
typically set attributes on the "namespace" based on "dest" and
"values".

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.

* args - List of strings to parse. The default is taken from
"sys.argv".

* namespace - An object to take the attributes. The default is a
new empty "Namespace" object.

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'])
Namespace(foo=None, x='X')
>>> parser.parse_args(['--foo', 'FOO'])
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'])
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'])
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'])
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 argument "-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 (prefix matching)
----------------------------------------

The "parse_args()" method by default allows long options to be
abbreviated to a prefix, if the abbreviation is unambiguous (the
prefix matches a unique option):

>>> 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. This feature can be disabled by setting allow_abbrev to
"False".

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'])
Namespace(accumulate=<built-in function sum>, integers=[1, 2, 3, 4])

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

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([title][, description][, prog][, parser_class][, action][, option_string][, dest][, help][, metavar])

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 a 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.

Description of parameters:

* title - title for the sub-parser group in help output; by
default “subcommands” if description is provided, otherwise uses
title for positional arguments

* description - description for the sub-parser group in help
output, by default "None"

* prog - usage information that will be displayed with sub-
command help, by default the name of the program and any
positional arguments before the subparser argument

* parser_class - class which will be used to create sub-parser
instances, by default the class of the current parser (e.g.
ArgumentParser)

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

* dest - name of the attribute under which sub-command name will
be stored; by default "None" and no value is stored

* help - help for sub-parser group in help output, by default
"None"

* metavar - string presenting available sub-commands in help; by
default it is "None" and presents sub-commands in form {cmd1,
cmd2, ..}

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 argument 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 argparse.FileType(mode='r', bufsize=-1, encoding=None, errors=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, buffer sizes,
encodings and error handling (see the "open()" function for more
details):

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--raw', type=argparse.FileType('wb', 0))
>>> parser.add_argument('out', type=argparse.FileType('w', encoding='UTF-8'))
>>> parser.parse_args(['--raw', 'raw.dat', 'file.txt'])
Namespace(out=<_io.TextIOWrapper name='file.txt' mode='w' encoding='UTF-8'>, raw=<_io.FileIO name='raw.dat' mode='wb'>)

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'>)

New in version 3.4: The *encodings* and *errors* keyword arguments.

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 in your user-defined groups will end up
back in the usual “positional arguments” and “optional arguments”
sections.

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

ArgumentParser.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'])

Warning: Prefix matching rules apply to "parse_known_args()". The
parser may consume an option even if it’s just a prefix of one of
its known options, instead of leaving it in the remaining arguments
list.

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 overridden 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. The following example demonstrates
how to do this:

class MyArgumentParser(argparse.ArgumentParser):
def convert_arg_line_to_args(self, arg_line):
return arg_line.split()

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.

The "argparse" module improves on the standard library "optparse"
module in a number of ways including:

* Handling positional arguments.

* Supporting sub-commands.

* Allowing alternative option prefixes like "+" and "/".

* Handling zero-or-more and one-or-more style arguments.

* Producing more informative usage messages.

* Providing a much simpler interface for custom "type" and "action".

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.
Keep in mind that what was previously called "options", now in the
"argparse" context is called "args".

* Replace "optparse.OptionParser.disable_interspersed_args()" by
setting "nargs" of a positional argument to argparse.REMAINDER, or
use "parse_known_args()" to collect unparsed argument strings in a
separate list.

* 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>')".