Argparse — Parser for command-line options, arguments and subcommands18.03.25

argparse — Parser for command-line options, arguments and subcommands

Added in version 3.2.

Source code: Lib/argparse.py

Note

While argparse is the default recommended standard library module for implementing basic command line applications, authors with more exacting requirements for exactly how their command line applications behave may find it doesn’t provide the necessary level of control. Refer to Choosing an argument parsing library for alternatives to consider when argparse doesn’t support behaviors that the application requires (such as entirely disabling support for interspersed options and positional arguments, or accepting option parameter values that start with - even when they correspond to another defined option).

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. The module will also issue errors when users give the program invalid arguments.

The argparse module’s support for command-line interfaces is built around an instance of argparse.ArgumentParser. It is a container for argument specifications and has options that apply to the parser as whole:

parser = argparse.ArgumentParser(
                    prog='ProgramName',
                    description='What the program does',
                    epilog='Text at the bottom of help')

The ArgumentParser.add_argument() method attaches individual argument specifications to the parser. It supports positional arguments, options that accept values, and on/off flags:

parser.add_argument('filename')           # positional argument
parser.add_argument('-c', '--count')      # option that takes a value
parser.add_argument('-v', '--verbose',
                    action='store_true')  # on/off flag

The ArgumentParser.parse_args() method runs the parser and places the extracted data in a argparse.Namespace object:

args = parser.parse_args()
print(args.filename, args.count, args.verbose)

Note

If you’re looking for a guide about how to upgrade optparse code to argparse, see Upgrading Optparse Code.
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, exit_on_error=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: os.path.basename(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 (by default, no text)

epilog - Text to display after the argument help (by default, no text)

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)

exit_on_error - Determines whether or not ArgumentParser exits with error info when an error occurs. (default: True)

Changed in version 3.5: allow_abbrev parameter was added.

Changed in version 3.8: In previous versions, allow_abbrev also disabled grouping of short flags such as -vv to mean -v -v.

Changed in version 3.9: exit_on_error parameter was added.

The following sections describe how each of these are used.
prog

By default, ArgumentParser calculates the name of the program to display in help messages depending on the way the Python interpreter was run:

The base name of sys.argv[0] if a file was passed as argument.

The Python interpreter name followed by sys.argv[0] if a directory or a zipfile was passed as argument.

The Python interpreter name followed by -m followed by the module or package name if the -m option was used.

This default is almost always desirable because it will make the help messages match the string that was used to invoke the program on the command line. However, 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]

options:
 -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]

options:
 -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. 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

options:
 -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.

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

options:
 -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

options:
 -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

options:
 -h, --help  show this help message and exit

RawTextHelpFormatter maintains whitespace for all sorts of help text, including argument descriptions. However, multiple newlines 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 ...]

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

options:
 -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

options:
  -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, when dealing with a particularly long argument list, 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', encoding=sys.getfilesystemencoding()) 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'].

ArgumentParser uses filesystem encoding and error handler to read the file containing arguments.

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

Changed in version 3.12: ArgumentParser changed encoding and errors to read arguments files from default (e.g. locale.getpreferredencoding(False) and "strict") to the filesystem encoding and error handler. Arguments file should be encoded in UTF-8 instead of ANSI Codepage on Windows.
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

Added 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]

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

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]

options:
 --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]

options:
  +h, ++help  show this help message and exit

exit_on_error

Normally, when you pass an invalid argument list to the parse_args() method of an ArgumentParser, it will print a message to sys.stderr and exit with a status code of 2.

If the user would like to catch errors manually, the feature can be enabled by setting exit_on_error to False:

parser = argparse.ArgumentParser(exit_on_error=False)

parser.add_argument('--integers', type=int)
_StoreAction(option_strings=['--integers'], dest='integers', nargs=None, const=None, default=None, type=<class 'int'>, choices=None, help=None, metavar=None)

try:

parser.parse_args('--integers a'.split())

except argparse.ArgumentError:

print('Catching an argumentError')

Catching an argumentError

Added in version 3.9.
The add_argument() method

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

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 and if it is absent from the namespace object.

type - The type to which the command-line argument should be converted.

choices - A sequence 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().

deprecated - Whether or not use of the argument is deprecated.

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: the following arguments are required: bar

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.

'store_const' - This stores the value specified by the const keyword argument; note that the const keyword argument defaults to None. The 'store_const' action is most commonly used with optional arguments that specify some sort of flag. For example:

parser = argparse.ArgumentParser()

parser.add_argument('--foo', action='store_const', const=42)

parser.parse_args(['--foo'])
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:

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. It is useful to allow an option to be specified multiple times. If the default value is non-empty, the default elements will be present in the parsed value for the option, with any values from the command line appended after those default values. 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'>])

'extend' - This stores a list and appends each item from the multi-value argument list to it. The 'extend' action is typically used with the nargs keyword argument value '+' or '*'. Note that when nargs is None (the default) or '?', each character of the argument string will be appended to the list. Example usage:

parser = argparse.ArgumentParser()

parser.add_argument("--foo", action="extend", nargs="+", type=str)

parser.parse_args(["--foo", "f1", "--foo", "f2", "f3", "f4"])
Namespace(foo=['f1', 'f2', 'f3', 'f4'])

Added in version 3.8.

'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', default=0)

parser.parse_args(['-vvv'])
Namespace(verbose=3)

Note, the default will be None unless explicitly set to 0.

'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

Only actions that consume command-line arguments (e.g. 'store', 'append' or 'extend') can be used with positional arguments.

class argparse.BooleanOptionalAction

You may also specify an arbitrary action by passing an Action subclass or other object that implements the same interface. The BooleanOptionalAction is available in argparse and adds support for boolean actions such as --foo and --no-foo:

import argparse

parser = argparse.ArgumentParser()

parser.add_argument('--foo', action=argparse.BooleanOptionalAction)

parser.parse_args(['--no-foo'])
    Namespace(foo=False)

Added in version 3.9.

The recommended way to create a custom action is to extend Action, overriding the __call__() method and optionally the __init__() and format_usage() methods. You can also register custom actions using the register() method and reference them by their registered name.

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().__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. See also Specifying ambiguous arguments. 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: the following arguments are required: foo

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. Actions that do not consume command-line arguments (e.g. 'store_const') set nargs=0.
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. If const is not provided to add_argument(), it will receive a default value of None.

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 to be None instead. See the nargs description for examples.

Changed in version 3.11: const=None by default, including when action='append_const' or action='store_const'.
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 target namespace already has an attribute set, the action default will not overwrite it:

parser = argparse.ArgumentParser()

parser.add_argument('--foo', default=42)

parser.parse_args([], namespace=argparse.Namespace(foo=101))
Namespace(foo=101)

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)

For required arguments, the default value is ignored. For example, this applies to positional arguments with nargs values other than ? or *, or optional arguments marked as required=True.

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, the parser reads command-line arguments in as simple strings. However, quite often the command-line string should instead be interpreted as another type, such as a float or int. The type keyword for add_argument() allows any necessary type-checking and type conversions to be performed.

If the type keyword is used with the default keyword, the type converter is only applied if the default is a string.

The argument to type can be a callable that accepts a single string or the name of a registered type (see register()) If the function raises ArgumentTypeError, TypeError, or ValueError, the exception is caught and a nicely formatted error message is displayed. Other exception types are not handled.

Common built-in types and functions can be used as type converters:

import argparse
import pathlib

parser = argparse.ArgumentParser()
parser.add_argument('count', type=int)
parser.add_argument('distance', type=float)
parser.add_argument('street', type=ascii)
parser.add_argument('code_point', type=ord)
parser.add_argument('dest_file', type=argparse.FileType('w', encoding='latin-1'))
parser.add_argument('datapath', type=pathlib.Path)

User defined functions can be used as well:

def hyphenated(string):

return '-'.join([word[:4] for word in string.casefold().split()])

parser = argparse.ArgumentParser()

_ = parser.add_argument('short_title', type=hyphenated)

parser.parse_args(['"The Tale of Two Cities"'])
Namespace(short_title='"the-tale-of-two-citi')

The bool() function is not recommended as a type converter. All it does is convert empty strings to False and non-empty strings to True. This is usually not what is desired.

In general, the type keyword is a convenience that should only be used for simple conversions that can only raise one of the three supported exceptions. Anything with more interesting error-handling or resource management should be done downstream after the arguments are parsed.

For example, JSON or YAML conversions have complex error cases that require better reporting than can be given by the type keyword. A JSONDecodeError would not be well formatted and a FileNotFoundError exception would not be handled at all.

Even FileType has its limitations for use with the type keyword. If one argument uses FileType and then a subsequent argument fails, an error is reported but the file is not automatically closed. In this case, it would be better to wait until after the parser has run and then use the with-statement to manage the files.

For type checkers that simply check against a fixed set of values, consider using the choices keyword instead.
choices

Some command-line arguments should be selected from a restricted set of values. These can be handled by passing a sequence 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 sequence is checked after any type conversions have been performed, so the type of the objects in the choices sequence should match the type specified.

Any sequence can be passed as the choices value, so list objects, tuple objects, and custom sequences are all supported.

Use of enum.Enum is not recommended because it is difficult to control its appearance in usage, help, and error messages.

Formatted choices override the default metavar which is normally derived from dest. This is usually what you want because the user never sees the dest parameter. If this display isn’t desirable (perhaps because there are many choices), just specify an explicit metavar.
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: [-h] --foo FOO
: error: the following arguments are required: --foo

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.

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)

options:
 -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]

options:
  -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

options:
 -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

options:
 -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]

options:
 -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')

deprecated

During a project’s lifetime, some arguments may need to be removed from the command line. Before removing them, you should inform your users that the arguments are deprecated and will be removed. The deprecated keyword argument of add_argument(), which defaults to False, specifies if the argument is deprecated and will be removed in the future. For arguments, if deprecated is True, then a warning will be printed to sys.stderr when the argument is used:

import argparse

parser = argparse.ArgumentParser(prog='snake.py')

parser.add_argument('--legs', default=0, type=int, deprecated=True)

parser.parse_args([])
Namespace(legs=0)

parser.parse_args(['--legs', '4'])
snake.py: warning: option '--legs' is deprecated
Namespace(legs=4)

Added in version 3.13.
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__().

__call__(parser, namespace, values, option_string=None)

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.

format_usage()

Action subclasses can define a format_usage() method that takes no argument and return a string which will be used when printing the usage of the program. If such method is not provided, a sensible default will be used.

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)

See also the argparse howto on ambiguous arguments for more details.
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][, dest][, required][, help][, metavar])

Many programs split up their functionality into a number of subcommands, for example, the svn program can invoke subcommands 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 subcommands 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

required - Whether or not a subcommand must be provided, by default False (added in 3.7)

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

metavar - string presenting available subcommands in help; by default it is None and presents subcommands 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='subcommand 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=('X', 'Y', 'Z'), 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}   subcommand help
    a     a help
    b     b help

options:
  -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

options:
  -h, --help  show this help message and exit

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

options:
  -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} ...

options:
  -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')

add_parser() supports also an additional deprecated argument, which allows to deprecate the subparser.

import argparse

parser = argparse.ArgumentParser(prog='chicken.py')

subparsers = parser.add_subparsers()

run = subparsers.add_parser('run')

fly = subparsers.add_parser('fly', deprecated=True)

parser.parse_args(['fly'])
chicken.py: warning: command 'fly' is deprecated
Namespace()

Added in version 3.13.

One particularly effective way of handling subcommands 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:

# subcommand 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(required=True)

# 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')

Changed in version 3.7: New required keyword-only parameter.

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'>)

Changed in version 3.4: Added the encodings and errors parameters.

Argument groups

ArgumentParser.add_argument_group(title=None, description=None, *[, argument_default][, conflict_handler])

By default, ArgumentParser groups command-line arguments into “positional arguments” and “options” 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

The optional, keyword-only parameters argument_default and conflict_handler allow for finer-grained control of the behavior of the argument group. These parameters have the same meaning as in the ArgumentParser constructor, but apply specifically to the argument group rather than the entire parser.

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

Changed in version 3.11: Calling add_argument_group() on an argument group is deprecated. This feature was never supported and does not always work correctly. The function exists on the API by accident through inheritance and will be removed in the future.

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(). However, a mutually exclusive group can be added to an argument group that has a title and description. For example:

parser = argparse.ArgumentParser(prog='PROG')

group = parser.add_argument_group('Group title', 'Group description')

exclusive_group = group.add_mutually_exclusive_group(required=True)

exclusive_group.add_argument('--foo', help='foo help')

exclusive_group.add_argument('--bar', help='bar help')

parser.print_help()
    usage: PROG [-h] (--foo FOO | --bar BAR)

options:
      -h, --help  show this help message and exit

Group title:
      Group description

--foo FOO   foo help
      --bar BAR   bar help

Changed in version 3.11: Calling add_argument_group() or add_mutually_exclusive_group() on a mutually exclusive group is deprecated. These features were never supported and do not always work correctly. The functions exist on the API by accident through inheritance and will be removed in the future.

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 to sys.stderr before that. The user can override this method to handle these steps differently:

class ErrorCatchingArgumentParser(argparse.ArgumentParser):
        def exit(self, status=0, message=None):
            if status:
                raise Exception(f'Exiting because of an error: {message}')
            exit(status)

ArgumentParser.error(message)

This method prints a usage message, including the message, to sys.stderr and terminates the program with a status code of 2.

Intermixed parsing

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

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

A number of Unix commands allow the user to intermix optional arguments with positional arguments. The parse_intermixed_args() and parse_known_intermixed_args() methods support this parsing style.

These parsers do not support all the argparse features, and will raise exceptions if unsupported features are used. In particular, subparsers, and mutually exclusive groups that include both optionals and positionals are not supported.

The following example shows the difference between parse_known_args() and parse_intermixed_args(): the former returns ['2', '3'] as unparsed arguments, while the latter collects all the positionals into rest.

parser = argparse.ArgumentParser()

parser.add_argument('--foo')

parser.add_argument('cmd')

parser.add_argument('rest', nargs='*', type=int)

parser.parse_known_args('doit 1 --foo bar 2 3'.split())
(Namespace(cmd='doit', foo='bar', rest=[1]), ['2', '3'])

parser.parse_intermixed_args('doit 1 --foo bar 2 3'.split())
    Namespace(cmd='doit', foo='bar', rest=[1, 2, 3])

parse_known_intermixed_args() returns a two item tuple containing the populated namespace and the list of remaining argument strings. parse_intermixed_args() raises an error if there are any remaining unparsed argument strings.

Added in version 3.7.

Registering custom types or actions

ArgumentParser.register(registry_name, value, object)

Sometimes it’s desirable to use a custom string in error messages to provide more user-friendly output. In these cases, register() can be used to register custom actions or types with a parser and allow you to reference the type by their registered name instead of their callable name.

The register() method accepts three arguments - a registry_name, specifying the internal registry where the object will be stored (e.g., action, type), value, which is the key under which the object will be registered, and object, the callable to be registered.

The following example shows how to register a custom type with a parser:

import argparse

parser = argparse.ArgumentParser()

parser.register('type', 'hexadecimal integer', lambda s: int(s, 16))

parser.add_argument('--foo', type='hexadecimal integer')
_StoreAction(option_strings=['--foo'], dest='foo', nargs=None, const=None, default=None, type='hexadecimal integer', choices=None, required=False, help=None, metavar=None, deprecated=False)

parser.parse_args(['--foo', '0xFA'])
Namespace(foo=250)

parser.parse_args(['--foo', '1.2'])
    usage: PROG [-h] [--foo FOO]
    PROG: error: argument --foo: invalid 'hexadecimal integer' value: '1.2'

Exceptions

exception argparse.ArgumentError

An error from creating or using an argument (optional or positional).

The string value of this exception is the message, augmented with information about the argument that caused it.

exception argparse.ArgumentTypeError

Raised when something goes wrong converting a command line string to a type.

Guides and Tutorials

Argparse Tutorial
    Migrating optparse code to argparse

https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser.add_argument

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Argparse — Parser for command-line options, arguments and subcommands18.03.25

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