python_doc_md/Intermediate/library/argparse.md

# `!argparse`{.interpreted-text role="mod"} \-\-- Parser for command-line options, arguments and subcommands

::: {.module synopsis="Command-line option and argument parsing library."}
argparse
:::

::: versionadded
3.2
:::

**Source code:** `Lib/argparse.py`{.interpreted-text role="source"}

:::: note
::: title
Note
:::

While `!argparse`{.interpreted-text role="mod"} 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-parser`{.interpreted-text role="ref"} 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).
::::

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

::: sidebar
**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 <argparse-tutorial>`{.interpreted-text role="ref"}.
:::

The `!argparse`{.interpreted-text role="mod"} module makes it easy to write user-friendly command-line interfaces. The program defines what arguments it requires, and `!argparse`{.interpreted-text role="mod"} will figure out how to parse those out of `sys.argv`{.interpreted-text role="data"}. The `!argparse`{.interpreted-text role="mod"} module also automatically generates help and usage messages. The module will also issue errors when users give the program invalid arguments.

The `!argparse`{.interpreted-text role="mod"} module\'s support for command-line interfaces is built around an instance of `argparse.ArgumentParser`{.interpreted-text role="class"}. 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`{.interpreted-text role="meth"} 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`{.interpreted-text role="meth"} method runs the parser and places the extracted data in a `argparse.Namespace`{.interpreted-text role="class"} object:

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

:::: note
::: title
Note
:::

If you\'re looking for a guide about how to upgrade `optparse`{.interpreted-text role="mod"} code to `!argparse`{.interpreted-text role="mod"}, see `Upgrading Optparse Code <upgrading-optparse-code>`{.interpreted-text role="ref"}.
::::

## ArgumentParser objects

:::::::: {.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, .\\ .*, .suggest_on_error=True, .color=True)}
Create a new `ArgumentParser`{.interpreted-text role="class"} object. All parameters should be passed as keyword arguments. Each parameter has its own more detailed description below, but in short they are:

- [prog](#prog) - The name of the program (default: generated from the `__main__` module attributes and `sys.argv[0]`)
- [usage](#usage) - The string describing the program usage (default: generated from arguments added to parser)
- [description](#description) - Text to display before the argument help (by default, no text)
- [epilog](#epilog) - Text to display after the argument help (by default, no text)
- [parents](#parents) - A list of `ArgumentParser`{.interpreted-text role="class"} objects whose arguments should also be included
- [formatter_class](#formatter_class) - A class for customizing the help output
- [prefix_chars](#prefix_chars) - The set of characters that prefix optional arguments (default: \'-\')
- [fromfile_prefix_chars](#fromfile_prefix_chars) - The set of characters that prefix files from which additional arguments should be read (default: `None`)
- [argument_default](#argument_default) - The global default value for arguments (default: `None`)
- [conflict_handler](#conflict_handler) - The strategy for resolving conflicting optionals (usually unnecessary)
- [add_help](#add_help) - Add a `-h/--help` option to the parser (default: `True`)
- [allow_abbrev](#allow_abbrev) - Allows long options to be abbreviated if the abbreviation is unambiguous (default: `True`)
- [exit_on_error](#exit_on_error) - Determines whether or not `!ArgumentParser`{.interpreted-text role="class"} exits with error info when an error occurs. (default: `True`)
- [suggest_on_error](#suggest_on_error) - Enables suggestions for mistyped argument choices and subparser names (default: `True`)
- [color](#color) - Allow color output (default: `True`)

::: versionchanged
3.5 *allow_abbrev* parameter was added.
:::

::: versionchanged
3.8 In previous versions, *allow_abbrev* also disabled grouping of short flags such as `-vv` to mean `-v -v`.
:::

::: versionchanged
3.9 *exit_on_error* parameter was added.
:::

::: versionchanged
3.14 *suggest_on_error* and *color* parameters were added.
:::

::: versionchanged
3.15 *suggest_on_error* default changed to `True`.
:::
::::::::

The following sections describe how each of these are used.

### prog

By default, `ArgumentParser`{.interpreted-text role="class"} calculates the name of the program to display in help messages depending on the way the Python interpreter was run:

- The `base name <os.path.basename>`{.interpreted-text role="func"} 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`{.interpreted-text role="option"} 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`{.interpreted-text role="class"}:

    >>> 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]`, from the `__main__` module attributes 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

::: versionchanged
3.14 The default `prog` value now reflects how `__main__` was actually executed, rather than always being `os.path.basename(sys.argv[0])`.
:::

### usage

By default, `ArgumentParser`{.interpreted-text role="class"} 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.

When a custom usage message is specified for the main parser, you may also want to consider passing the `prog` argument to `~ArgumentParser.add_subparsers`{.interpreted-text role="meth"} or the `prog` and the `usage` arguments to `~_SubParsersAction.add_parser`{.interpreted-text role="meth"}, to ensure consistent command prefixes and usage information across subparsers.

### description

Most calls to the `ArgumentParser`{.interpreted-text role="class"} 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](#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`{.interpreted-text role="class"}:

    >>> 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](#description) argument, the `epilog=` text is by default line-wrapped, but this behavior can be adjusted with the [formatter_class](#formatter_class) argument to `ArgumentParser`{.interpreted-text role="class"}.

### 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`{.interpreted-text role="class"} can be used. The `parents=` argument takes a list of `ArgumentParser`{.interpreted-text role="class"} objects, collects all the positional and optional actions from them, and adds these actions to the `ArgumentParser`{.interpreted-text role="class"} 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`{.interpreted-text role="class"} will see two `-h/--help` options (one in the parent and one in the child) and raise an error.

:::: note
::: title
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`{.interpreted-text role="class"} objects allow the help formatting to be customized by specifying an alternate formatting class. Currently, there are four such classes:

::: {.RawDescriptionHelpFormatter .RawTextHelpFormatter .ArgumentDefaultsHelpFormatter .MetavarTypeHelpFormatter}
`RawDescriptionHelpFormatter`{.interpreted-text role="class"} and `RawTextHelpFormatter`{.interpreted-text role="class"} give more control over how textual descriptions are displayed. By default, `ArgumentParser`{.interpreted-text role="class"} objects line-wrap the [description](#description) and [epilog](#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`{.interpreted-text role="class"} as `formatter_class=` indicates that [description](#description) and [epilog](#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`{.interpreted-text role="class"} 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`{.interpreted-text role="class"} 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`{.interpreted-text role="class"} uses the name of the [type](#type) argument for each argument as the display name for its values (rather than using the [dest](#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`{.interpreted-text role="class"} 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`{.interpreted-text role="class"} 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 be one per line by default (but see also `~ArgumentParser.convert_arg_line_to_args`{.interpreted-text role="meth"}) 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']`.

:::: note
::: title
Note
:::

Empty lines are treated as empty strings (`''`), which are allowed as values but not as arguments. Empty lines that are read as arguments will result in an \"unrecognized arguments\" error.
::::

`ArgumentParser`{.interpreted-text role="class"} uses `filesystem encoding and error handler`{.interpreted-text role="term"} to read the file containing arguments.

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

::: versionchanged
3.12 `ArgumentParser`{.interpreted-text role="class"} changed encoding and errors to read arguments files from default (e.g. `locale.getpreferredencoding(False) <locale.getpreferredencoding>`{.interpreted-text role="func"} and `"strict"`) to the `filesystem encoding and error handler`{.interpreted-text role="term"}. 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 `~ArgumentParser.add_argument`{.interpreted-text role="meth"} or by calling the `~ArgumentParser.set_defaults`{.interpreted-text role="meth"} 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`{.interpreted-text role="class"}. For example, to globally suppress attribute creation on `~ArgumentParser.parse_args`{.interpreted-text role="meth"} 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 `~ArgumentParser.parse_args`{.interpreted-text role="meth"} method of an `ArgumentParser`{.interpreted-text role="class"}, it `recognizes abbreviations <prefix-matching>`{.interpreted-text role="ref"} 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

::: versionadded
3.5
:::

### conflict_handler

`ArgumentParser`{.interpreted-text role="class"} objects do not allow two actions with the same option string. By default, `ArgumentParser`{.interpreted-text role="class"} 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](#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`{.interpreted-text role="class"}:

    >>> 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`{.interpreted-text role="class"} 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`{.interpreted-text role="class"} objects add an option which simply displays the parser\'s help message. If `-h` or `--help` is supplied at the command line, the `!ArgumentParser`{.interpreted-text role="class"} 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`{.interpreted-text role="class"}:

    >>> 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 `~ArgumentParser.parse_args`{.interpreted-text role="meth"} method of an `ArgumentParser`{.interpreted-text role="class"}, it will print a *message* to `sys.stderr`{.interpreted-text role="data"} 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

::: versionadded
3.9
:::

### suggest_on_error

By default, when a user passes an invalid argument choice or subparser name, `ArgumentParser`{.interpreted-text role="class"} will exit with error info and provide suggestions for mistyped arguments. The error message will list the permissible argument choices (if specified) or subparser names, along with a \"maybe you meant\" suggestion if a close match is found. Note that this only applies for arguments when the choices specified are strings:

    >>> parser = argparse.ArgumentParser(description='Process some integers.',
                                         suggest_on_error=True)
    >>> parser.add_argument('--action', choices=['sum', 'max'])
    >>> parser.add_argument('integers', metavar='N', type=int, nargs='+',
    ...                     help='an integer for the accumulator')
    >>> parser.parse_args(['--action', 'sumn', 1, 2, 3])
    tester.py: error: argument --action: invalid choice: 'sumn', maybe you meant 'sum'? (choose from 'sum', 'max')

You can disable suggestions by setting `suggest_on_error` to `False`:

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

::: versionadded
3.14
:::

::: versionchanged
3.15 Changed default value of `suggest_on_error` from `False` to `True`.
:::

### color

By default, the help message is printed in color using [ANSI escape sequences](https://en.wikipedia.org/wiki/ANSI_escape_code). If you want plain text help messages, you can disable this `in your local
environment <using-on-controlling-color>`{.interpreted-text role="ref"}, or in the argument parser itself by setting `color` to `False`:

    >>> parser = argparse.ArgumentParser(description='Process some integers.',
    ...                                  color=False)
    >>> parser.add_argument('--action', choices=['sum', 'max'])
    >>> parser.add_argument('integers', metavar='N', type=int, nargs='+',
    ...                     help='an integer for the accumulator')
    >>> parser.parse_args(['--help'])

Note that when `color=True`, colored output depends on both environment variables and terminal capabilities. However, if `color=False`, colored output is always disabled, even if environment variables like `FORCE_COLOR` are set.

::: versionadded
3.14
:::

To highlight inline code in your description or epilog text, you can use backticks:

    >>> parser = argparse.ArgumentParser(
    ...     formatter_class=argparse.RawDescriptionHelpFormatter,
    ...     epilog='''Examples:
    ...   `python -m myapp --verbose`
    ...   `python -m myapp --config settings.json`
    ... ''')

When colors are enabled, the text inside backticks will be displayed in a distinct color to help examples stand out. When colors are disabled, backticks are preserved as-is, which is readable in plain text.

:::: note
::: title
Note
:::

Backtick markup only applies to description and epilog text. It does not apply to individual argument `help` strings.
::::

::: versionadded
3.15
:::

## The add_argument() method

::: 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](#name-or-flags) - Either a name or a list of option strings, e.g. `'foo'` or `'-f', '--foo'`.
- [action](#action) - The basic type of action to be taken when this argument is encountered at the command line.
- [nargs](#nargs) - The number of command-line arguments that should be consumed.
- [const](#const) - A constant value required by some [action](#action) and [nargs](#nargs) selections.
- [default](#default) - The value produced if the argument is absent from the command line and if it is absent from the namespace object.
- [type](#type) - The type to which the command-line argument should be converted.
- [choices](#choices) - A sequence of the allowable values for the argument.
- [required](#required) - Whether or not the command-line option may be omitted (optionals only).
- [help](#help) - A brief description of what the argument does.
- [metavar](#metavar) - A name for the argument in usage messages.
- [dest](#dest) - The name of the attribute to be added to the object returned by `parse_args`{.interpreted-text role="meth"}.
- [deprecated](#deprecated) - Whether or not use of the argument is deprecated.
:::

The following sections describe how each of these are used.

### name or flags {#name or flags}

The `~ArgumentParser.add_argument`{.interpreted-text role="meth"} 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 `~ArgumentParser.add_argument`{.interpreted-text role="meth"} 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 `~ArgumentParser.parse_args`{.interpreted-text role="meth"} 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

By default, `!argparse`{.interpreted-text role="mod"} automatically handles the internal naming and display names of arguments, simplifying the process without requiring additional configuration. As such, you do not need to specify the [dest](#dest) and [metavar](#metavar) parameters. The [dest](#dest) parameter defaults to the argument name with underscores `_` replacing hyphens `-` . The [metavar](#metavar) parameter defaults to the upper-cased name. For example:

    >>> parser = argparse.ArgumentParser(prog='PROG')
    >>> parser.add_argument('--foo-bar')
    >>> parser.parse_args(['--foo-bar', 'FOO-BAR'])
    Namespace(foo_bar='FOO-BAR')
    >>> parser.print_help()
    usage:  [-h] [--foo-bar FOO-BAR]

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

### action

`ArgumentParser`{.interpreted-text role="class"} 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 `~ArgumentParser.parse_args`{.interpreted-text role="meth"}. 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](#const) keyword argument; note that the [const](#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'` that respectively store the values `True` and `False` with default values of `False` and `True`:

      >>> 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 appends each argument value to a list. It is useful for allowing an option to be specified multiple times. If the default value is a non-empty list, the parsed value will start with the default list\'s elements and any values from the command line will be appended after those default values. Example usage:

      >>> parser = argparse.ArgumentParser()
      >>> parser.add_argument('--foo', action='append', default=['0'])
      >>> parser.parse_args('--foo 1 --foo 2'.split())
      Namespace(foo=['0', '1', '2'])

- `'append_const'` - This appends the value specified by the [const](#const) keyword argument to a list; note that the [const](#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 appends each item from a multi-value argument to a list. The `'extend'` action is typically used with the [nargs](#nargs) keyword argument value `'+'` or `'*'`. Note that when [nargs](#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'])

  ::: versionadded
  3.8
  :::

- `'count'` - This counts the number of times an 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`{.interpreted-text role="class"} for details of how the output is created.

- `'version'` - This expects a `version=` keyword argument in the `~ArgumentParser.add_argument`{.interpreted-text role="meth"} call, and prints version information and exits when invoked:

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

You may also specify an arbitrary action by passing an `Action`{.interpreted-text role="class"} subclass (e.g. `BooleanOptionalAction`{.interpreted-text role="class"}) or other object that implements the same interface. Only actions that consume command-line arguments (e.g. `'store'`, `'append'`, `'extend'`, or custom actions with non-zero `nargs`) can be used with positional arguments.

The recommended way to create a custom action is to extend `Action`{.interpreted-text role="class"}, overriding the `!__call__`{.interpreted-text role="meth"} method and optionally the `!__init__`{.interpreted-text role="meth"} and `!format_usage`{.interpreted-text role="meth"} methods. You can also register custom actions using the `~ArgumentParser.register`{.interpreted-text role="meth"} 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`{.interpreted-text role="class"}.

### nargs

`ArgumentParser`{.interpreted-text role="class"} 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`{.interpreted-text role="ref"}. 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.

::: index
single: ? (question mark); in argparse module
:::

- `'?'`. 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](#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](#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='?')
      >>> parser.add_argument('outfile', nargs='?')
      >>> parser.parse_args(['input.txt', 'output.txt'])
      Namespace(infile='input.txt', outfile='output.txt')
      >>> parser.parse_args(['input.txt'])
      Namespace(infile='input.txt', outfile=None)
      >>> parser.parse_args([])
      Namespace(infile=None, outfile=None)

::: index
single: \* (asterisk); in argparse module
:::

- `'*'`. 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'])

::: index
single: + (plus); in argparse module
:::

- `'+'`. Just like `'*'`, all command-line arguments 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](#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 `~ArgumentParser.add_argument`{.interpreted-text role="meth"} is used to hold constant values that are not read from the command line but are required for the various `ArgumentParser`{.interpreted-text role="class"} actions. The two most common uses of it are:

- When `~ArgumentParser.add_argument`{.interpreted-text role="meth"} 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 `~ArgumentParser.parse_args`{.interpreted-text role="meth"}. See the [action](#action) description for examples. If `const` is not provided to `~ArgumentParser.add_argument`{.interpreted-text role="meth"}, it will receive a default value of `None`.
- When `~ArgumentParser.add_argument`{.interpreted-text role="meth"} 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 from `const` will be used. See the [nargs](#nargs) description for examples.

::: versionchanged
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 `~ArgumentParser.add_argument`{.interpreted-text role="meth"}, 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](#type) conversion argument, if provided, before setting the attribute on the `Namespace`{.interpreted-text role="class"} 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](#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](#required) arguments, the `default` value is ignored. For example, this applies to positional arguments with [nargs](#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 {#argparse-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`{.interpreted-text role="class"} or `int`{.interpreted-text role="class"}. The `type` keyword for `~ArgumentParser.add_argument`{.interpreted-text role="meth"} allows any necessary type-checking and type conversions to be performed.

If the [type](#type) keyword is used with the [default](#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 `~ArgumentParser.register`{.interpreted-text role="meth"}) If the function raises `ArgumentTypeError`{.interpreted-text role="exc"}, `TypeError`{.interpreted-text role="exc"}, or `ValueError`{.interpreted-text role="exc"}, 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:

::: testcode
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(\'datapath\', type=pathlib.Path)
:::

User defined functions can be used as well:

::: doctest
\>\>\> 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`{.interpreted-text role="func"} 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 `~json.JSONDecodeError`{.interpreted-text role="exc"} would not be well formatted and a `FileNotFoundError`{.interpreted-text role="exc"} exception would not be handled at all.

Even `~argparse.FileType`{.interpreted-text role="class"} has its limitations for use with the `type` keyword. If one argument uses `~argparse.FileType`{.interpreted-text role="class"} 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`{.interpreted-text role="keyword"}-statement to manage the files.

For type checkers that simply check against a fixed set of values, consider using the [choices](#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 `~ArgumentParser.add_argument`{.interpreted-text role="meth"}. 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')

Any sequence can be passed as the *choices* value, so `list`{.interpreted-text role="class"} objects, `tuple`{.interpreted-text role="class"} objects, and custom sequences are all supported.

Use of `enum.Enum`{.interpreted-text role="class"} is not recommended because it is difficult to control its appearance in usage, help, and error messages.

Note that *choices* are checked after any [type](#type) conversions have been performed, so objects in *choices* should match the [type](#type) specified. This can make *choices* appear unfamiliar in usage, help, or error messages.

To keep *choices* user-friendly, consider a custom type wrapper that converts and formats values, or omit [type](#type) and handle conversion in your application code.

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](#metavar).

### required

In general, the `!argparse`{.interpreted-text role="mod"} 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 `~ArgumentParser.add_argument`{.interpreted-text role="meth"}:

    >>> 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`, `~ArgumentParser.parse_args`{.interpreted-text role="meth"} will report an error if that option is not present at the command line.

:::: note
::: title
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](#default). The available specifiers include the program name, `%(prog)s` and most keyword arguments to `~ArgumentParser.add_argument`{.interpreted-text role="meth"}, 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`{.interpreted-text role="mod"} 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`{.interpreted-text role="class"} generates help messages, it needs some way to refer to each expected argument. By default, `!ArgumentParser`{.interpreted-text role="class"} objects use the [dest](#dest) value as the \"name\" of each object. By default, for positional argument actions, the [dest](#dest) value is used directly, and for optional argument actions, the [dest](#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 `~ArgumentParser.parse_args`{.interpreted-text role="meth"} object is still determined by the [dest](#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`{.interpreted-text role="class"} actions add some value as an attribute of the object returned by `~ArgumentParser.parse_args`{.interpreted-text role="meth"}. The name of this attribute is determined by the `dest` keyword argument of `~ArgumentParser.add_argument`{.interpreted-text role="meth"}. For positional argument actions, `dest` is normally supplied as the first argument to `~ArgumentParser.add_argument`{.interpreted-text role="meth"}:

    >>> 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`{.interpreted-text role="class"} generates the value of `dest` by taking the first double-dash long option string and stripping away the initial `-` characters. If no double-dash long option strings were supplied, `dest` will be derived from the first single-dash long option string by stripping the initial `-` character. 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('-q', '-quz')
    >>> parser.add_argument('-x', '-y')
    >>> parser.parse_args('-f 1 -q 2 -x 3'.split())
    Namespace(foo_bar='1', quz='2', x='3')
    >>> parser.parse_args('--foo 1 -quz 2 -y 3'.split())
    Namespace(foo_bar='1', quz='2', 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')

::: versionchanged
3.15 Single-dash long option now takes precedence over short options.
:::

### 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 `~ArgumentParser.add_argument`{.interpreted-text role="meth"}, 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`{.interpreted-text role="data"} 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'])  # doctest: +SKIP
    snake.py: warning: option '--legs' is deprecated
    Namespace(legs=4)

::: versionadded
3.13
:::

### Action classes

`!Action`{.interpreted-text role="class"} 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 `~ArgumentParser.add_argument`{.interpreted-text role="meth"}.

::::: {.Action(option_strings, .dest, .nargs=None, .const=None, .default=None, .\\ .type=None, .choices=None, .required=False, .help=None, .\\ .metavar=None)}
`!Action`{.interpreted-text role="class"} objects are used by an `ArgumentParser`{.interpreted-text role="class"} to represent the information needed to parse a single argument from one or more strings from the command line. The `!Action`{.interpreted-text role="class"} class must accept the two positional arguments plus any keyword arguments passed to `ArgumentParser.add_argument`{.interpreted-text role="meth"} except for the `action` itself.

Instances of `!Action`{.interpreted-text role="class"} (or return value of any callable to the `action` parameter) should have attributes `!dest`{.interpreted-text role="attr"}, `!option_strings`{.interpreted-text role="attr"}, `!default`{.interpreted-text role="attr"}, `!type`{.interpreted-text role="attr"}, `!required`{.interpreted-text role="attr"}, `!help`{.interpreted-text role="attr"}, etc. defined. The easiest way to ensure these attributes are defined is to call `!Action.__init__`{.interpreted-text role="meth"}.

::: method
\_\_call\_\_(parser, namespace, values, option_string=None)

`!Action`{.interpreted-text role="class"} instances should be callable, so subclasses must override the `!__call__`{.interpreted-text role="meth"} method, which should accept four parameters:

- *parser* - The `ArgumentParser`{.interpreted-text role="class"} object which contains this action.
- *namespace* - The `Namespace`{.interpreted-text role="class"} object that will be returned by `~ArgumentParser.parse_args`{.interpreted-text role="meth"}. Most actions add an attribute to this object using `setattr`{.interpreted-text role="func"}.
- *values* - The associated command-line arguments, with any type conversions applied. Type conversions are specified with the [type](#type) keyword argument to `~ArgumentParser.add_argument`{.interpreted-text role="meth"}.
- *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__`{.interpreted-text role="meth"} method may perform arbitrary actions, but will typically set attributes on the `namespace` based on `dest` and `values`.
:::

::: method
format_usage()

`!Action`{.interpreted-text role="class"} subclasses can define a `!format_usage`{.interpreted-text role="meth"} 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.
:::
:::::

::::: BooleanOptionalAction
A subclass of `Action`{.interpreted-text role="class"} for handling boolean flags with positive and negative options. Adding a single argument such as `--foo` automatically creates both `--foo` and `--no-foo` options, storing `True` and `False` respectively:

    >>> import argparse
    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--foo', action=argparse.BooleanOptionalAction)
    >>> parser.parse_args(['--no-foo'])
    Namespace(foo=False)

Single-dash long options are also supported. For example, negative option `-nofoo` is automatically added for positive option `-foo`. But no additional options are added for short options such as `-f`.

::: versionadded
3.9
:::

::: versionchanged
3.15 Added support for single-dash options.

Added support for alternate [prefix_chars](#prefix_chars).
:::
:::::

## The parse_args() method

::: 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`{.interpreted-text role="meth"} determine exactly what objects are created and how they are assigned. See the documentation for `!add_argument`{.interpreted-text role="meth"} for details.

- [args](#args) - List of strings to parse. The default is taken from `sys.argv`{.interpreted-text role="data"}.
- [namespace](#namespace) - An object to take the attributes. The default is a new empty `Namespace`{.interpreted-text role="class"} object.
:::

### Option value syntax

The `~ArgumentParser.parse_args`{.interpreted-text role="meth"} 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, `~ArgumentParser.parse_args`{.interpreted-text role="meth"} 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 `~ArgumentParser.parse_args`{.interpreted-text role="meth"} 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 `~ArgumentParser.parse_args`{.interpreted-text role="meth"} 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 `~ArgumentParser.parse_args`{.interpreted-text role="meth"} 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 <specifying-ambiguous-arguments>`{.interpreted-text role="ref"} for more details.

### Argument abbreviations (prefix matching) {#prefix-matching}

The `~ArgumentParser.parse_args`{.interpreted-text role="meth"} method `by default <allow_abbrev>`{.interpreted-text role="ref"} 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`{.interpreted-text role="ref"} to `False`.

### Beyond `sys.argv` {#args}

Sometimes it may be useful to have an `ArgumentParser`{.interpreted-text role="class"} parse arguments other than those of `sys.argv`{.interpreted-text role="data"}. This can be accomplished by passing a list of strings to `~ArgumentParser.parse_args`{.interpreted-text role="meth"}. 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 {#namespace}

::: Namespace
Simple class used by default by `~ArgumentParser.parse_args`{.interpreted-text role="meth"} to create an object holding attributes and return it.

This class is deliberately simple, just an `object`{.interpreted-text role="class"} 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`{.interpreted-text role="func"}:

    >>> 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`{.interpreted-text role="class"} assign attributes to an already existing object, rather than a new `Namespace`{.interpreted-text role="class"} 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

### Subcommands

::::: method
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`{.interpreted-text role="class"} supports the creation of such subcommands with the `!add_subparsers`{.interpreted-text role="meth"} method. The `!add_subparsers`{.interpreted-text role="meth"} method is normally called with no arguments and returns a special action object. This object has a single method, `~_SubParsersAction.add_parser`{.interpreted-text role="meth"}, which takes a command name and any `!ArgumentParser`{.interpreted-text role="class"} constructor arguments, and returns an `!ArgumentParser`{.interpreted-text role="class"} 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 subcommand 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`{.interpreted-text role="class"})
- [action](#action) - the basic type of action to be taken when this argument is encountered at the command line
- [dest](#dest) - name of the attribute under which subcommand name will be stored; by default `None` and no value is stored
- [required](#required) - Whether or not a subcommand must be provided, by default `False` (added in 3.7)
- [help](#help) - help for sub-parser group in help output, by default `None`
- [metavar](#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 `~ArgumentParser.parse_args`{.interpreted-text role="meth"} 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 `~_SubParsersAction.add_parser`{.interpreted-text role="meth"} 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 `~ArgumentParser.add_subparsers`{.interpreted-text role="meth"} 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

One particularly effective way of handling subcommands is to combine the use of the `~ArgumentParser.add_subparsers`{.interpreted-text role="meth"} method with calls to `~ArgumentParser.set_defaults`{.interpreted-text role="meth"} 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 `~ArgumentParser.parse_args`{.interpreted-text role="meth"} 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 `~ArgumentParser.add_subparsers`{.interpreted-text role="meth"} 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')

::: versionchanged
3.7 New *required* keyword-only parameter.
:::

::: versionchanged
3.14 Subparser\'s *prog* is no longer affected by a custom usage message in the main parser.
:::
:::::

:::: method
[SubParsersAction.add_parser]{#subparsersaction.add_parser}(name, *, help=None, aliases=None, deprecated=False,*\*kwargs)

Create and return a new `ArgumentParser`{.interpreted-text role="class"} object for the subcommand *name*.

The *name* argument is the name of the sub-command.

The *help* argument provides a short description for this sub-command.

The *aliases* argument allows providing alternative names for this sub-command. For example:

    >>> 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')

The *deprecated* argument, if `True`, marks the sub-command as deprecated and will issue a warning when used. For example:

    >>> parser = argparse.ArgumentParser(prog='chicken.py')
    >>> subparsers = parser.add_subparsers()
    >>> fly = subparsers.add_parser('fly', deprecated=True)
    >>> args = parser.parse_args(['fly'])
    chicken.py: warning: command 'fly' is deprecated
    Namespace()

All other keyword arguments are passed directly to the `!ArgumentParser`{.interpreted-text role="class"} constructor.

::: versionadded
3.13 Added the *deprecated* parameter.
:::
::::

### FileType objects

::::::: {.FileType(mode='r', .bufsize=-1, .encoding=None, .errors=None)}
The `FileType`{.interpreted-text role="class"} factory creates objects that can be passed to the type argument of `ArgumentParser.add_argument`{.interpreted-text role="meth"}. Arguments that have `FileType`{.interpreted-text role="class"} objects as their type will open command-line arguments as files with the requested modes, buffer sizes, encodings and error handling (see the `open`{.interpreted-text role="func"} 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`{.interpreted-text role="data"} for readable `FileType`{.interpreted-text role="class"} objects and `sys.stdout`{.interpreted-text role="data"} for writable `FileType`{.interpreted-text role="class"} objects:

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

:::: note
::: title
Note
:::

If one argument uses *FileType* and then a subsequent argument fails, an error is reported but the file is not automatically closed. This can also clobber the output files. In this case, it would be better to wait until after the parser has run and then use the `with`{.interpreted-text role="keyword"}-statement to manage the files.
::::

::: versionchanged
3.4 Added the *encodings* and *errors* parameters.
:::

::: deprecated
3.14
:::
:::::::

### Argument groups

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

By default, `ArgumentParser`{.interpreted-text role="class"} 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`{.interpreted-text role="meth"} 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`{.interpreted-text role="meth"} method returns an argument group object which has an `~ArgumentParser.add_argument`{.interpreted-text role="meth"} method just like a regular `ArgumentParser`{.interpreted-text role="class"}. 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`{.interpreted-text role="meth"} 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](#argument_default) and [conflict_handler](#conflict_handler) allow for finer-grained control of the behavior of the argument group. These parameters have the same meaning as in the `ArgumentParser`{.interpreted-text role="class"} 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.

::: deprecated-removed
3.11 3.14 Calling `add_argument_group`{.interpreted-text role="meth"} on an argument group now raises an exception. This nesting was never supported, often failed to work correctly, and was unintentionally exposed through inheritance.
:::

::: deprecated
3.14 Passing [prefix_chars](#prefix_chars) to `add_argument_group`{.interpreted-text role="meth"} is now deprecated.
:::
:::::

### Mutual exclusion

:::: method
ArgumentParser.add_mutually_exclusive_group(required=False)

Create a mutually exclusive group. `!argparse`{.interpreted-text role="mod"} 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`{.interpreted-text role="meth"} 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 `~ArgumentParser.add_argument_group`{.interpreted-text role="meth"}. 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

::: deprecated-removed
3.11 3.14 Calling `add_argument_group`{.interpreted-text role="meth"} or `add_mutually_exclusive_group`{.interpreted-text role="meth"} on a mutually exclusive group now raises an exception. This nesting was never supported, often failed to work correctly, and was unintentionally exposed through inheritance.
:::
::::

### Parser defaults

::: method
ArgumentParser.set_defaults(\*\*kwargs)

Most of the time, the attributes of the object returned by `parse_args`{.interpreted-text role="meth"} will be fully determined by inspecting the command-line arguments and the argument actions. `set_defaults`{.interpreted-text role="meth"} 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 defaults can be set at both the parser level using `set_defaults`{.interpreted-text role="meth"} and at the argument level using `add_argument`{.interpreted-text role="meth"}. If both are called for the same argument, the last default set for an argument is used:

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

Parser-level defaults can be particularly useful when working with multiple parsers. See the `~ArgumentParser.add_subparsers`{.interpreted-text role="meth"} method for an example of this type.
:::

::: method
ArgumentParser.get_default(dest)

Get the default value for a namespace attribute, as set by either `~ArgumentParser.add_argument`{.interpreted-text role="meth"} or by `~ArgumentParser.set_defaults`{.interpreted-text role="meth"}:

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

### Printing help

In most typical applications, `~ArgumentParser.parse_args`{.interpreted-text role="meth"} will take care of formatting and printing any usage or error messages. However, several formatting methods are available:

::: method
ArgumentParser.print_usage(file=None)

Print a brief description of how the `ArgumentParser`{.interpreted-text role="class"} should be invoked on the command line. If *file* is `None`, `sys.stdout`{.interpreted-text role="data"} is assumed.
:::

::: method
ArgumentParser.print_help(file=None)

Print a help message, including the program usage and information about the arguments registered with the `ArgumentParser`{.interpreted-text role="class"}. If *file* is `None`, `sys.stdout`{.interpreted-text role="data"} is assumed.
:::

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

::: method
ArgumentParser.format_usage()

Return a string containing a brief description of how the `ArgumentParser`{.interpreted-text role="class"} should be invoked on the command line.
:::

::: method
ArgumentParser.format_help()

Return a string containing a help message, including the program usage and information about the arguments registered with the `ArgumentParser`{.interpreted-text role="class"}.
:::

### Partial parsing

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

Sometimes a script only needs to handle a specific set of command-line arguments, leaving any unrecognized arguments for another script or program. In these cases, the `~ArgumentParser.parse_known_args`{.interpreted-text role="meth"} method can be useful.

This method works similarly to `~ArgumentParser.parse_args`{.interpreted-text role="meth"}, but it does not raise an error for extra, unrecognized arguments. Instead, it parses the known arguments and returns a two item tuple that contains the populated namespace and the list of any unrecognized arguments.

    >>> 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
::: title
Warning
:::

`Prefix matching <prefix-matching>`{.interpreted-text role="ref"} rules apply to `~ArgumentParser.parse_known_args`{.interpreted-text role="meth"}. 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

::: method
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`{.interpreted-text role="class"} constructor) are read one argument per line. `convert_arg_line_to_args`{.interpreted-text role="meth"} 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

::: method
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`{.interpreted-text role="data"} 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)
:::

::: method
ArgumentParser.error(message)

This method prints a usage message, including the *message*, to `sys.stderr`{.interpreted-text role="data"} and terminates the program with a status code of 2.
:::

### Intermixed parsing

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

:::: method
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 `~ArgumentParser.parse_intermixed_args`{.interpreted-text role="meth"} and `~ArgumentParser.parse_known_intermixed_args`{.interpreted-text role="meth"} methods support this parsing style.

These parsers do not support all the `!argparse`{.interpreted-text role="mod"} 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 `~ArgumentParser.parse_known_args`{.interpreted-text role="meth"} and `~ArgumentParser.parse_intermixed_args`{.interpreted-text role="meth"}: 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])

`~ArgumentParser.parse_known_intermixed_args`{.interpreted-text role="meth"} returns a two item tuple containing the populated namespace and the list of remaining argument strings. `~ArgumentParser.parse_intermixed_args`{.interpreted-text role="meth"} raises an error if there are any remaining unparsed argument strings.

::: versionadded
3.7
:::
::::

### Registering custom types or actions

::: method
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`{.interpreted-text role="meth"} 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`{.interpreted-text role="meth"} 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
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
ArgumentTypeError

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

**Guides and Tutorials**

::: {.toctree maxdepth="1"}
../howto/argparse.rst ../howto/argparse-optparse.rst
:::