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

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

Example

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

import argparse

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

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

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

$ prog.py -h
usage: prog.py [-h] [--sum] N [N ...]

Process some integers.

positional arguments:
 N           an integer for the accumulator

optional arguments:
 -h, --help  show this help message and exit
 --sum       sum the integers (default: find the max)

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

$ prog.py 1 2 3 4
4

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

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

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

The following sections walk you through this example.

Creating a parser

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

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

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

Adding arguments

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

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

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

Parsing arguments

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

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

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

ArgumentParser objects

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

• description - Text to display before the argument help.
• epilog - Text to display after the argument help.
• add_help - Add a -h/--help option to the parser. (default: True)
• argument_default - Set the global default value for arguments. (default: None)
• parents - A list of :class:ArgumentParser objects whose arguments should also be included.
• prefix_chars - The set of characters that prefix optional arguments. (default: '-')
• fromfile_prefix_chars - The set of characters that prefix files from which additional arguments should be read. (default: None)
• formatter_class - A class for customizing the help output.
• conflict_handler - Usually unnecessary, defines strategy for resolving conflicting optionals.
• prog - Usually unnecessary, the name of the program (default: sys.argv[0])
• usage - Usually unnecessary, the string describing the program usage (default: generated)

The following sections describe how each of these are used.

description

Most calls to the ArgumentParser constructor will use the description= keyword argument. This argument gives a brief description of what the program does and how it works. In help messages, the description is displayed between the command-line usage string and the help messages for the various arguments:

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

A foo that bars

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

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

epilog

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

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

A foo that bars

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

And that's how you'd foo a bar

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

add_help

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

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

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

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

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

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

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

optional arguments:
 --foo FOO  foo help

prefix_chars

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

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

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

fromfile_prefix_chars

Sometimes, e.g. for particularly long argument lists, it may make sense to keep the list of arguments in a file rather than typing it out at the command line. If the fromfile_prefix_chars= argument is given to the ArgumentParser constructor, then arguments that start with any of the specified characters will be treated as files, and will be replaced by the arguments they contain. For example:

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

Arguments read from a file must by default be one per line (but see also :meth:`convert_arg_line_to_args`) and are treated as if they were in the same place as the original file referencing argument on the command line. So in the example above, the expression ['-f', 'foo', '@args.txt'] is considered equivalent to the expression ['-f', 'foo', '-f', 'bar'].

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

argument_default

Generally, argument defaults are specified either by passing a default to :meth:`add_argument` or by calling the :meth:`set_defaults` methods with a specific set of name-value pairs. Sometimes however, it may be useful to specify a single parser-wide default for arguments. This can be accomplished by passing the argument_default= keyword argument to ArgumentParser. For example, to globally suppress attribute creation on :meth:`parse_args` calls, we supply argument_default=SUPPRESS:

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

parents

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

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

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

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

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

formatter_class

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

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

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

this description was indented weird but that is okay

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

likewise for this epilog whose whitespace will be cleaned up and whose words
will be wrapped across a couple lines

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

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

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

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

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

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

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

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

optional arguments:
 -h, --help  show this help message and exit
 --foo FOO   FOO! (default: 42)

conflict_handler

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

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

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

>>> parser = argparse.ArgumentParser(prog='PROG', conflict_handler='resolve')
>>> parser.add_argument('-f', '--foo', help='old foo help')
>>> parser.add_argument('--foo', help='new foo help')
>>> parser.print_help()
usage: PROG [-h] [-f FOO] [--foo FOO]

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

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

prog

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

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

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

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

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

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

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

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

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

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

>>> parser = argparse.ArgumentParser(prog='myprogram')
>>> parser.add_argument('--foo', help='foo of the %(prog)s program')
>>> parser.print_help()
usage: myprogram [-h] [--foo FOO]

optional arguments:
 -h, --help  show this help message and exit
 --foo FOO   foo of the myprogram program

usage

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

>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('--foo', nargs='?', help='foo help')
>>> parser.add_argument('bar', nargs='+', help='bar help')
>>> parser.print_help()
usage: PROG [-h] [--foo [FOO]] bar [bar ...]

positional arguments:
 bar          bar help

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

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

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

positional arguments:
 bar          bar help

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

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

The add_argument() method

name or flags

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

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

while a positional argument could be created like:

>>> parser.add_argument('bar')

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

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

action

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

• 'store' - This just stores the argument's value. This is the default

  action. For example:

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

• 'store_const' - This stores the value specified by the const keyword

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

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

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

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

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

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

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

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

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

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

You can also specify an arbitrary action by passing an object that implements the Action API. The easiest way to do this is to extend argparse.Action, supplying an appropriate __call__ method. The __call__ method accepts four parameters:

• parser - The ArgumentParser object which contains this action.
• namespace - The namespace object that will be returned by :meth:`parse_args`. Most actions add an attribute to this object.
• values - The associated command-line args, with any type-conversions applied. (Type-conversions are specified with the type keyword argument to :meth:`add_argument`.
• option_string - The option string that was used to invoke this action. The option_string argument is optional, and will be absent if the action is associated with a positional argument.

So for example:

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

nargs

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

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

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', nargs=2)
   >>> parser.add_argument('bar', nargs=1)
   >>> parser.parse_args('c --foo a b'.split())
   Namespace(bar=['c'], foo=['a', 'b'])
  
  Note that ``nargs=1`` produces a list of one item.  This is different from
  the default, in which the item is produced by itself.
  

• '?'. One arg will be consumed from the command-line if possible, and produced as a single item. If no command-line arg is present, the value from default will be produced. Note that for optional arguments, there is an additional case - the option string is present but not followed by a command-line arg. In this case the value from const will be produced. Some examples to illustrate this:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', nargs='?', const='c', default='d')
   >>> parser.add_argument('bar', nargs='?', default='d')
   >>> parser.parse_args('XX --foo YY'.split())
   Namespace(bar='XX', foo='YY')
   >>> parser.parse_args('XX --foo'.split())
   Namespace(bar='XX', foo='c')
   >>> parser.parse_args(''.split())
   Namespace(bar='d', foo='d')
  
  One of the more common uses of ``nargs='?'`` is to allow optional input and
  output files::
  
   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('infile', nargs='?', type=argparse.FileType('r'), default=sys.stdin)
   >>> parser.add_argument('outfile', nargs='?', type=argparse.FileType('w'), default=sys.stdout)
   >>> parser.parse_args(['input.txt', 'output.txt'])
   Namespace(infile=<open file 'input.txt', mode 'r' at 0x...>, outfile=<open file 'output.txt', mode 'w' at 0x...>)
   >>> parser.parse_args([])
   Namespace(infile=<open file '<stdin>', mode 'r' at 0x...>, outfile=<open file '<stdout>', mode 'w' at 0x...>)
  

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

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

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

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

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

const

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

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

The const keyword argument defaults to None.

default

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

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

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

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

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

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

type

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

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

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

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

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

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

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

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

See the choices section for more details.

choices

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

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

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

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

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

required

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

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

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

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

help

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

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

positional arguments:
 bar     one of the bars to be frobbled

optional arguments:
 -h, --help  show this help message and exit
 --foo   foo the bars before frobbling

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

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

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

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

metavar

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

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

positional arguments:
 bar

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

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

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

positional arguments:
 XXX

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

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

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

>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-x', nargs=2)
>>> parser.add_argument('--foo', nargs=2, metavar=('bar', 'baz'))
>>> parser.print_help()
usage: PROG [-h] [-x X X] [--foo bar baz]

optional arguments:
 -h, --help     show this help message and exit
 -x X X
 --foo bar baz

dest

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

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

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

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

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

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

The parse_args() method

Option value syntax

The :meth:`parse_args` method supports several ways of specifying the value of an option (if it takes one). In the simplest case, the option and its value are passed as two separate arguments:

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

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

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

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

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

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

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

Invalid arguments

While parsing the command-line, parse_args checks for a variety of errors, including ambiguous options, invalid types, invalid options, wrong number of positional arguments, etc. When it encounters such an error, it exits and prints the error along with a usage message:

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

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

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

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

Arguments containing "-"

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

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

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

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

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

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

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

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

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

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

Argument abbreviations

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

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

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

Beyond sys.argv

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

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

Custom namespaces

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

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

Other utilities

Sub-commands

FileType objects

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

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

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

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

Argument groups

Mutual exclusion

Parser defaults

Printing help

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

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

Partial parsing

Sometimes a script may only parse a few of the command line arguments, passing the remaining arguments on to another script or program. In these cases, the :meth:`parse_known_args` method can be useful. It works much like :meth:`parse_args` except that it does not produce an error when extra arguments are present. Instead, it returns a two item tuple containing the populated namespace and the list of remaining argument strings.

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

Customizing file parsing

Upgrading optparse code

Originally, the argparse module had attempted to maintain compatibility with

optparse. However, optparse was difficult to extend transparently, particularly with the changes required to support the new nargs= specifiers and better usage messges. When most everything in optparse had either been copy-pasted over or monkey-patched, it no longer seemed practical to try to maintain the backwards compatibility.

A partial upgrade path from optparse to argparse:

• Replace all add_option() calls with :meth:`add_argument` calls.
• Replace options, args = parser.parse_args() with args = parser.parse_args() and add additional :meth:`add_argument` calls for the positional arguments.
• Replace callback actions and the callback_* keyword arguments with type or action arguments.
• Replace string names for type keyword arguments with the corresponding type objects (e.g. int, float, complex, etc).
• Replace Values with Namespace and OptionError/OptionValueError with ArgumentError.
• Replace strings with implicit arguments such as %default or %prog with the standard python syntax to use dictionaries to format strings, that is, %(default)s and %(prog)s.