Doc/library/argparse.rst
!argparse --- Parser for command-line options, arguments and subcommands.. module:: argparse :synopsis: Command-line option and argument parsing library.
.. versionadded:: 3.2
Source code: :source:Lib/argparse.py
.. note::
While :mod:!argparse is the default recommended standard library module
for implementing basic command line applications, authors with more
exacting requirements for exactly how their command line applications
behave may find it doesn't provide the necessary level of control.
Refer to :ref:choosing-an-argument-parser 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
:ref:argparse tutorial <argparse-tutorial>.
The :mod:!argparse module makes it easy to write user-friendly command-line
interfaces. The program defines what arguments it requires, and :mod:!argparse
will figure out how to parse those out of :data:sys.argv. The :mod:!argparse
module also automatically generates help and usage messages. The module
will also issue errors when users give the program invalid arguments.
The :mod:!argparse module's support for command-line interfaces is built
around an instance of :class:argparse.ArgumentParser. It is a container for
argument specifications and has options that apply to the parser as whole::
parser = argparse.ArgumentParser( prog='ProgramName', description='What the program does', epilog='Text at the bottom of help')
The :meth:ArgumentParser.add_argument method attaches individual argument
specifications to the parser. It supports positional arguments, options that
accept values, and on/off flags::
parser.add_argument('filename') # positional argument parser.add_argument('-c', '--count') # option that takes a value parser.add_argument('-v', '--verbose', action='store_true') # on/off flag
The :meth:ArgumentParser.parse_args method runs the parser and places
the extracted data in a :class:argparse.Namespace object::
args = parser.parse_args() print(args.filename, args.count, args.verbose)
.. note::
If you're looking for a guide about how to upgrade :mod:optparse code
to :mod:!argparse, see :ref:Upgrading Optparse Code <upgrading-optparse-code>.
.. class:: 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 :class:ArgumentParser object. All parameters should be passed
as keyword arguments. Each parameter has its own more detailed description
below, but in short they are:
prog_ - The name of the program (default: generated from the __main__
module attributes and sys.argv[0])
usage_ - The string describing the program usage (default: generated from arguments added to parser)
description_ - Text to display before the argument help (by default, no text)
epilog_ - Text to display after the argument help (by default, no text)
parents_ - A list of :class:ArgumentParser objects whose arguments should
also be included
formatter_class_ - A class for customizing the help output
prefix_chars_ - The set of characters that prefix optional arguments (default: '-')
fromfile_prefix_chars_ - The set of characters that prefix files from
which additional arguments should be read (default: None)
argument_default_ - The global default value for arguments
(default: None)
conflict_handler_ - The strategy for resolving conflicting optionals (usually unnecessary)
add_help_ - Add a -h/--help option to the parser (default: True)
allow_abbrev_ - Allows long options to be abbreviated if the
abbreviation is unambiguous (default: True)
exit_on_error_ - Determines whether or not :class:!ArgumentParser exits with
error info when an error occurs. (default: True)
suggest_on_error_ - Enables suggestions for mistyped argument choices
and subparser names (default: True)
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:
prog ^^^^
By default, :class:ArgumentParser calculates the name of the program
to display in help messages depending on the way the Python interpreter was run:
base name <os.path.basename> of sys.argv[0] if a file was
passed as argument.sys.argv[0] if a directory or
a zipfile was passed as argument.-m followed by the
module or package name if the :option:-m option was used.This default is almost always desirable because it will make the help messages
match the string that was used to invoke the program on the command line.
However, to change this default behavior, another value can be supplied using
the prog= argument to :class:ArgumentParser::
parser = argparse.ArgumentParser(prog='myprogram') parser.print_help() usage: myprogram [-h]
options: -h, --help show this help message and exit
Note that the program name, whether determined from sys.argv[0],
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, :class:ArgumentParser calculates the usage message from the
arguments it contains. The default message can be overridden with the
usage= keyword argument::
parser = argparse.ArgumentParser(prog='PROG', usage='%(prog)s [options]') parser.add_argument('--foo', nargs='?', help='foo help') parser.add_argument('bar', nargs='+', help='bar help') parser.print_help() usage: PROG [options]
positional arguments: bar bar help
options: -h, --help show this help message and exit --foo [FOO] foo help
The %(prog)s format specifier is available to fill in the program name in
your usage messages.
When a custom usage message is specified for the main parser, you may also want to
consider passing the prog argument to :meth:~ArgumentParser.add_subparsers
or the prog and the usage arguments to
:meth:~_SubParsersAction.add_parser, to ensure consistent command prefixes and
usage information across subparsers.
.. _description:
description ^^^^^^^^^^^
Most calls to the :class:ArgumentParser constructor will use the
description= keyword argument. This argument gives a brief description of
what the program does and how it works. In help messages, the description is
displayed between the command-line usage string and the help messages for the
various arguments.
By default, the description will be line-wrapped so that it fits within the given space. To change this behavior, see the formatter_class_ argument.
epilog ^^^^^^
Some programs like to display additional description of the program after the
description of the arguments. Such text can be specified using the epilog=
argument to :class:ArgumentParser::
parser = argparse.ArgumentParser( ... description='A foo that bars', ... epilog="And that's how you'd foo a bar") parser.print_help() usage: argparse.py [-h]
A foo that bars
options: -h, --help show this help message and exit
And that's how you'd foo a bar
As with the description_ argument, the epilog= text is by default
line-wrapped, but this behavior can be adjusted with the formatter_class_
argument to :class:ArgumentParser.
parents ^^^^^^^
Sometimes, several parsers share a common set of arguments. Rather than
repeating the definitions of these arguments, a single parser with all the
shared arguments and passed to parents= argument to :class:ArgumentParser
can be used. The parents= argument takes a list of :class:ArgumentParser
objects, collects all the positional and optional actions from them, and adds
these actions to the :class:ArgumentParser object being constructed::
parent_parser = argparse.ArgumentParser(add_help=False) parent_parser.add_argument('--parent', type=int)
foo_parser = argparse.ArgumentParser(parents=[parent_parser]) foo_parser.add_argument('foo') foo_parser.parse_args(['--parent', '2', 'XXX']) Namespace(foo='XXX', parent=2)
bar_parser = argparse.ArgumentParser(parents=[parent_parser]) bar_parser.add_argument('--bar') bar_parser.parse_args(['--bar', 'YYY']) Namespace(bar='YYY', parent=None)
Note that most parent parsers will specify add_help=False. Otherwise, the
:class:ArgumentParser will see two -h/--help options (one in the parent
and one in the child) and raise an error.
.. note::
You must fully initialize the parsers before passing them via parents=.
If you change the parent parsers after the child parser, those changes will
not be reflected in the child.
.. _formatter_class:
formatter_class ^^^^^^^^^^^^^^^
:class:ArgumentParser objects allow the help formatting to be customized by
specifying an alternate formatting class. Currently, there are four such
classes:
.. class:: RawDescriptionHelpFormatter RawTextHelpFormatter ArgumentDefaultsHelpFormatter MetavarTypeHelpFormatter
:class:RawDescriptionHelpFormatter and :class:RawTextHelpFormatter give
more control over how textual descriptions are displayed.
By default, :class:ArgumentParser objects line-wrap the description_ and
epilog_ texts in command-line help messages::
parser = argparse.ArgumentParser( ... prog='PROG', ... description='''this description ... was indented weird ... but that is okay''', ... epilog=''' ... likewise for this epilog whose whitespace will ... be cleaned up and whose words will be wrapped ... across a couple lines''') parser.print_help() usage: PROG [-h]
this description was indented weird but that is okay
options: -h, --help show this help message and exit
likewise for this epilog whose whitespace will be cleaned up and whose words will be wrapped across a couple lines
Passing :class:RawDescriptionHelpFormatter as formatter_class=
indicates that description_ and epilog_ are already correctly formatted and
should not be line-wrapped::
parser = argparse.ArgumentParser( ... prog='PROG', ... formatter_class=argparse.RawDescriptionHelpFormatter, ... description=textwrap.dedent('''
... Please do not mess up this text! ... -------------------------------- ... I have indented it ... exactly the way ... I want it ... ''')) parser.print_help() usage: PROG [-h]
I have indented it
exactly the way
I want it
options: -h, --help show this help message and exit
:class:RawTextHelpFormatter maintains whitespace for all sorts of help text,
including argument descriptions. However, multiple newlines are replaced with
one. If you wish to preserve multiple blank lines, add spaces between the
newlines.
:class:ArgumentDefaultsHelpFormatter automatically adds information about
default values to each of the argument help messages::
parser = argparse.ArgumentParser( ... prog='PROG', ... formatter_class=argparse.ArgumentDefaultsHelpFormatter) parser.add_argument('--foo', type=int, default=42, help='FOO!') parser.add_argument('bar', nargs='*', default=[1, 2, 3], help='BAR!') parser.print_help() usage: PROG [-h] [--foo FOO] [bar ...]
positional arguments: bar BAR! (default: [1, 2, 3])
options: -h, --help show this help message and exit --foo FOO FOO! (default: 42)
:class:MetavarTypeHelpFormatter uses the name of the type_ argument for each
argument as the display name for its values (rather than using the dest_
as the regular formatter does)::
parser = argparse.ArgumentParser( ... prog='PROG', ... formatter_class=argparse.MetavarTypeHelpFormatter) parser.add_argument('--foo', type=int) parser.add_argument('bar', type=float) parser.print_help() usage: PROG [-h] [--foo int] float
positional arguments: float
options: -h, --help show this help message and exit --foo int
prefix_chars ^^^^^^^^^^^^
Most command-line options will use - as the prefix, e.g. -f/--foo.
Parsers that need to support different or additional prefix
characters, e.g. for options
like +f or /foo, may specify them using the prefix_chars= argument
to the :class:ArgumentParser constructor::
parser = argparse.ArgumentParser(prog='PROG', prefix_chars='-+') parser.add_argument('+f') parser.add_argument('++bar') parser.parse_args('+f X ++bar Y'.split()) Namespace(bar='Y', f='X')
The prefix_chars= argument defaults to '-'. Supplying a set of
characters that does not include - will cause -f/--foo options to be
disallowed.
fromfile_prefix_chars ^^^^^^^^^^^^^^^^^^^^^
Sometimes, when dealing with a particularly long argument list, it
may make sense to keep the list of arguments in a file rather than typing it out
at the command line. If the fromfile_prefix_chars= argument is given to the
:class:ArgumentParser constructor, then arguments that start with any of the
specified characters will be treated as files, and will be replaced by the
arguments they contain. For example::
with open('args.txt', 'w', encoding=sys.getfilesystemencoding()) as fp: ... fp.write('-f\nbar') ... parser = argparse.ArgumentParser(fromfile_prefix_chars='@') parser.add_argument('-f') parser.parse_args(['-f', 'foo', '@args.txt']) Namespace(f='bar')
Arguments read from a file must be one per line by default (but see also
:meth:~ArgumentParser.convert_arg_line_to_args) and are treated as if they
were in the same place as the original file referencing argument on the command
line. So in the example above, the expression ['-f', 'foo', '@args.txt']
is considered equivalent to the expression ['-f', 'foo', '-f', 'bar'].
.. 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.
:class:ArgumentParser uses :term:filesystem encoding and error handler
to read the file containing arguments.
The fromfile_prefix_chars= argument defaults to None, meaning that
arguments will never be treated as file references.
.. versionchanged:: 3.12
:class:ArgumentParser changed encoding and errors to read arguments files
from default (e.g. :func:locale.getpreferredencoding(False) <locale.getpreferredencoding>
and "strict") to the :term:filesystem encoding and error handler.
Arguments file should be encoded in UTF-8 instead of ANSI Codepage on Windows.
argument_default ^^^^^^^^^^^^^^^^
Generally, argument defaults are specified either by passing a default to
:meth:~ArgumentParser.add_argument or by calling the
:meth:~ArgumentParser.set_defaults methods with a specific set of name-value
pairs. Sometimes however, it may be useful to specify a single parser-wide
default for arguments. This can be accomplished by passing the
argument_default= keyword argument to :class:ArgumentParser. For example,
to globally suppress attribute creation on :meth:~ArgumentParser.parse_args
calls, we supply argument_default=SUPPRESS::
parser = argparse.ArgumentParser(argument_default=argparse.SUPPRESS) parser.add_argument('--foo') parser.add_argument('bar', nargs='?') parser.parse_args(['--foo', '1', 'BAR']) Namespace(bar='BAR', foo='1') parser.parse_args([]) Namespace()
.. _allow_abbrev:
allow_abbrev ^^^^^^^^^^^^
Normally, when you pass an argument list to the
:meth:~ArgumentParser.parse_args method of an :class:ArgumentParser,
it :ref:recognizes abbreviations <prefix-matching> 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 ^^^^^^^^^^^^^^^^
:class:ArgumentParser objects do not allow two actions with the same option
string. By default, :class:ArgumentParser objects raise an exception if an
attempt is made to create an argument with an option string that is already in
use::
parser = argparse.ArgumentParser(prog='PROG') parser.add_argument('-f', '--foo', help='old foo help') parser.add_argument('--foo', help='new foo help') Traceback (most recent call last): .. ArgumentError: argument --foo: conflicting option string(s): --foo
Sometimes (e.g. when using parents_) it may be useful to simply override any
older arguments with the same option string. To get this behavior, the value
'resolve' can be supplied to the conflict_handler= argument of
:class:ArgumentParser::
parser = argparse.ArgumentParser(prog='PROG', conflict_handler='resolve') parser.add_argument('-f', '--foo', help='old foo help') parser.add_argument('--foo', help='new foo help') parser.print_help() usage: PROG [-h] [-f FOO] [--foo FOO]
options: -h, --help show this help message and exit -f FOO old foo help --foo FOO new foo help
Note that :class:ArgumentParser objects only remove an action if all of its
option strings are overridden. So, in the example above, the old -f/--foo
action is retained as the -f action, because only the --foo option
string was overridden.
add_help ^^^^^^^^
By default, :class:ArgumentParser objects add an option which simply displays
the parser's help message. If -h or --help is supplied at the command
line, the :class:!ArgumentParser help will be printed.
Occasionally, it may be useful to disable the addition of this help option.
This can be achieved by passing False as the add_help= argument to
:class:ArgumentParser::
parser = argparse.ArgumentParser(prog='PROG', add_help=False) parser.add_argument('--foo', help='foo help') parser.print_help() usage: PROG [--foo FOO]
options: --foo FOO foo help
The help option is typically -h/--help. The exception to this is
if the prefix_chars= is specified and does not include -, in
which case -h and --help are not valid options. In
this case, the first character in prefix_chars is used to prefix
the help options::
parser = argparse.ArgumentParser(prog='PROG', prefix_chars='+/') parser.print_help() usage: PROG [+h]
options: +h, ++help show this help message and exit
exit_on_error ^^^^^^^^^^^^^
Normally, when you pass an invalid argument list to the :meth:~ArgumentParser.parse_args
method of an :class:ArgumentParser, it will print a message to :data:sys.stderr and exit with a status
code of 2.
If the user would like to catch errors manually, the feature can be enabled by setting
exit_on_error to False::
parser = argparse.ArgumentParser(exit_on_error=False) parser.add_argument('--integers', type=int) _StoreAction(option_strings=['--integers'], dest='integers', nargs=None, const=None, default=None, type=<class 'int'>, choices=None, help=None, metavar=None) try: ... parser.parse_args('--integers a'.split()) ... except argparse.ArgumentError: ... print('Catching an argumentError') ... Catching an argumentError
.. versionadded:: 3.9
suggest_on_error ^^^^^^^^^^^^^^^^
By default, when a user passes an invalid argument choice or subparser name,
:class:ArgumentParser 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(suggest_on_error=True) parser.add_argument('--action', choices=['debug', 'dryrun']) parser.parse_args(['--action', 'debugg']) usage: tester.py [-h] [--action {debug,dryrun}] tester.py: error: argument --action: invalid choice: 'debugg', maybe you meant 'debug'? (choose from debug, dryrun)
You can disable suggestions by setting suggest_on_error to 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 :ref:in your local environment <using-on-controlling-color>, 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::
Backtick markup only applies to description and epilog text. It does not
apply to individual argument help strings.
.. versionadded:: 3.15
.. method:: ArgumentParser.add_argument(name or flags..., *, [action], [nargs],
[const], [default], [type], [choices], [required],
[help], [metavar], [dest], [deprecated])
Define how a single command-line argument should be parsed. Each parameter has its own more detailed description below, but in short they are:
name or flags_ - Either a name or a list of option strings, e.g. 'foo'
or '-f', '--foo'.
action_ - The basic type of action to be taken when this argument is encountered at the command line.
nargs_ - The number of command-line arguments that should be consumed.
const_ - A constant value required by some action_ and nargs_ selections.
default_ - The value produced if the argument is absent from the command line and if it is absent from the namespace object.
type_ - The type to which the command-line argument should be converted.
choices_ - A sequence of the allowable values for the argument.
required_ - Whether or not the command-line option may be omitted (optionals only).
help_ - A brief description of what the argument does.
metavar_ - A name for the argument in usage messages.
dest_ - The name of the attribute to be added to the object returned by
:meth:parse_args.
deprecated_ - Whether or not use of the argument is deprecated.
The method returns an :class:Action object representing the argument.
The following sections describe how each of these are used.
.. _name or flags:
name or flags ^^^^^^^^^^^^^
The :meth:~ArgumentParser.add_argument method must know whether an optional
argument, like -f or --foo, or a positional argument, like a list of
filenames, is expected. The first arguments passed to
:meth:~ArgumentParser.add_argument must therefore be either a series of
flags, or a simple argument name.
For example, an optional argument could be created like::
parser.add_argument('-f', '--foo')
while a positional argument could be created like::
parser.add_argument('bar')
When :meth:~ArgumentParser.parse_args is called, optional arguments will be
identified by the - prefix, and the remaining arguments will be assumed to
be positional::
parser = argparse.ArgumentParser(prog='PROG') parser.add_argument('-f', '--foo') parser.add_argument('bar') parser.parse_args(['BAR']) Namespace(bar='BAR', foo=None) parser.parse_args(['BAR', '--foo', 'FOO']) Namespace(bar='BAR', foo='FOO') parser.parse_args(['--foo', 'FOO']) usage: PROG [-h] [-f FOO] bar PROG: error: the following arguments are required: bar
By default, :mod:!argparse 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_ and metavar_ parameters.
For optional arguments, the dest_ parameter defaults to the argument name, with
underscores _ replacing hyphens -. The 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:
action ^^^^^^
:class:ArgumentParser objects associate command-line arguments with actions. These
actions can do just about anything with the command-line arguments associated with
them, though most actions simply add an attribute to the object returned by
:meth:~ArgumentParser.parse_args. The action keyword argument specifies
how the command-line arguments should be handled. The supplied actions are:
'store' - This just stores the argument's value. This is the default
action.
'store_const' - This stores the value specified by the const_ keyword
argument; note that the const_ keyword argument defaults to None. The
'store_const' action is most commonly used with optional arguments that
specify some sort of flag. For example::
parser = argparse.ArgumentParser() parser.add_argument('--foo', action='store_const', const=42) parser.parse_args(['--foo']) Namespace(foo=42)
'store_true' and 'store_false' - These are special cases of
'store_const' 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_ keyword argument to a list; note that the const_ keyword
argument defaults to None. The 'append_const' action is typically
useful when multiple arguments need to store constants to the same list. For
example::
parser = argparse.ArgumentParser() parser.add_argument('--str', dest='types', action='append_const', const=str) parser.add_argument('--int', dest='types', action='append_const', const=int) parser.parse_args('--str --int'.split()) Namespace(types=[<class 'str'>, <class 'int'>])
'extend' - This appends each item from a multi-value
argument to a list.
The 'extend' action is typically used with the nargs_ keyword argument
value '+' or '*'.
Note that when nargs_ is None (the default) or '?', each
character of the argument string will be appended to the list.
Example usage::
parser = argparse.ArgumentParser() parser.add_argument("--foo", action="extend", nargs="+", type=str) parser.parse_args(["--foo", "f1", "--foo", "f2", "f3", "f4"]) Namespace(foo=['f1', 'f2', 'f3', 'f4'])
.. 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 :class:ArgumentParser for details of how the
output is created.
'version' - This expects a version= keyword argument in the
:meth:~ArgumentParser.add_argument call, and prints version information
and exits when invoked::
import argparse parser = argparse.ArgumentParser(prog='PROG') parser.add_argument('--version', action='version', version='%(prog)s 2.0') parser.parse_args(['--version']) PROG 2.0
You may also specify an arbitrary action by passing an :class:Action subclass
(e.g. :class:BooleanOptionalAction) 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 :class:Action,
overriding the :meth:!__call__ method and optionally the :meth:!__init__ and
:meth:!format_usage methods. You can also register custom actions using the
:meth:~ArgumentParser.register method and reference them by their registered name.
An example of a custom action::
class FooAction(argparse.Action): ... def init(self, option_strings, dest, nargs=None, **kwargs): ... if nargs is not None: ... raise ValueError("nargs not allowed") ... super().init(option_strings, dest, **kwargs) ... def call(self, parser, namespace, values, option_string=None): ... print('%r %r %r' % (namespace, values, option_string)) ... setattr(namespace, self.dest, values) ... parser = argparse.ArgumentParser() parser.add_argument('--foo', action=FooAction) parser.add_argument('bar', action=FooAction) args = parser.parse_args('1 --foo 2'.split()) Namespace(bar=None, foo=None) '1' None Namespace(bar='1', foo=None) '2' '--foo' args Namespace(bar='1', foo='2')
For more details, see :class:Action.
.. _nargs:
nargs ^^^^^
:class:ArgumentParser objects usually associate a single command-line argument with a
single action to be taken. The nargs keyword argument associates a
different number of command-line arguments with a single action.
See also :ref:specifying-ambiguous-arguments. The supported values are:
N (an integer). N arguments from the command line will be gathered
together into a list. For example::
parser = argparse.ArgumentParser() parser.add_argument('--foo', nargs=2) parser.add_argument('bar', nargs=1) parser.parse_args('c --foo a b'.split()) Namespace(bar=['c'], foo=['a', 'b'])
Note that nargs=1 produces a list of one item. This is different from
the default, in which the item is produced by itself.
.. 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_ will be produced. Note that for optional arguments, there is an
additional case - the option string is present but not followed by a
command-line argument. In this case the value from const_ will be produced. Some
examples to illustrate this::
parser = argparse.ArgumentParser() parser.add_argument('--foo', nargs='?', const='c', default='d') parser.add_argument('bar', nargs='?', default='d') parser.parse_args(['XX', '--foo', 'YY']) Namespace(bar='XX', foo='YY') parser.parse_args(['XX', '--foo']) Namespace(bar='XX', foo='c') parser.parse_args([]) Namespace(bar='d', foo='d')
One of the more common uses of nargs='?' is to allow optional input and
output files::
parser = argparse.ArgumentParser() parser.add_argument('infile', nargs='?') 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_. 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:
const ^^^^^
The const argument of :meth:~ArgumentParser.add_argument is used to hold
constant values that are not read from the command line but are required for
the various :class:ArgumentParser actions. The two most common uses of it are:
When :meth:~ArgumentParser.add_argument is called with
action='store_const' or action='append_const'. These actions add the
const value to one of the attributes of the object returned by
:meth:~ArgumentParser.parse_args. See the action_ description for examples.
If const is not provided to :meth:~ArgumentParser.add_argument, it will
receive a default value of None.
When :meth:~ArgumentParser.add_argument is called with option strings
(like -f or --foo) and nargs='?'. This creates an optional
argument that can be followed by zero or one command-line arguments.
When parsing the command line, if the option string is encountered with no
command-line argument following it, the value from const will be used.
See the nargs_ description for examples.
.. versionchanged:: 3.11
const=None by default, including when action='append_const' or
action='store_const'.
.. _default:
default ^^^^^^^
All optional arguments and some positional arguments may be omitted at the
command line. The default keyword argument of
:meth:~ArgumentParser.add_argument, whose value defaults to None,
specifies what value should be used if the command-line argument is not present.
For optional arguments, the default value is used when the option string
was not present at the command line::
parser = argparse.ArgumentParser() parser.add_argument('--foo', default=42) parser.parse_args(['--foo', '2']) Namespace(foo='2') parser.parse_args([]) Namespace(foo=42)
If the target namespace already has an attribute set, the action default will not overwrite it::
parser = argparse.ArgumentParser() parser.add_argument('--foo', default=42) parser.parse_args([], namespace=argparse.Namespace(foo=101)) Namespace(foo=101)
If the default value is a string, the parser parses the value as if it
were a command-line argument. In particular, the parser applies any type_
conversion argument, if provided, before setting the attribute on the
:class:Namespace return value. Otherwise, the parser uses the value as is::
parser = argparse.ArgumentParser() parser.add_argument('--length', default='10', type=int) parser.add_argument('--width', default=10.5, type=int) parser.parse_args() Namespace(length=10, width=10.5)
For positional arguments with nargs_ equal to ? or *, the default value
is used when no command-line argument was present::
parser = argparse.ArgumentParser() parser.add_argument('foo', nargs='?', default=42) parser.parse_args(['a']) Namespace(foo='a') parser.parse_args([]) Namespace(foo=42)
For required_ arguments, the default value is ignored. For example, this
applies to positional arguments with nargs_ values other than ? or *,
or optional arguments marked as required=True.
Providing default=argparse.SUPPRESS causes no attribute to be added if the
command-line argument was not present::
parser = argparse.ArgumentParser() parser.add_argument('--foo', default=argparse.SUPPRESS) parser.parse_args([]) Namespace() parser.parse_args(['--foo', '1']) Namespace(foo='1')
.. _argparse-type:
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 :class:float or :class:int. The
type keyword for :meth:~ArgumentParser.add_argument allows any
necessary type-checking and type conversions to be performed.
If the type_ keyword is used with the default_ keyword, the type converter is only applied if the default is a string.
The argument to type can be a callable that accepts a single string or
the name of a registered type (see :meth:~ArgumentParser.register)
If the function raises :exc:ArgumentTypeError, :exc:TypeError, or
:exc:ValueError, the exception is caught and a nicely formatted error
message is displayed. Other exception types are not handled.
Common built-in types and functions can be used as type converters:
.. 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 :func:bool function is not recommended as a type converter. All it does
is convert empty strings to False and non-empty strings to True.
This is usually not what is desired::
parser = argparse.ArgumentParser() _ = parser.add_argument('--verbose', type=bool) parser.parse_args(['--verbose', 'False']) Namespace(verbose=True)
See :class:BooleanOptionalAction or action='store_true' for common
alternatives.
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
:exc:~json.JSONDecodeError would not be well formatted and a
:exc:FileNotFoundError exception would not be handled at all.
Even :class:~argparse.FileType has its limitations for use with the type
keyword. If one argument uses :class:~argparse.FileType and then a
subsequent argument fails, an error is reported but the file is not
automatically closed. In this case, it would be better to wait until after
the parser has run and then use the :keyword:with-statement to manage the
files.
For type checkers that simply check against a fixed set of values, consider using the choices_ keyword instead.
.. _choices:
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 :meth:~ArgumentParser.add_argument. When the command line is
parsed, argument values will be checked, and an error message will be displayed
if the argument was not one of the acceptable values::
parser = argparse.ArgumentParser(prog='game.py') parser.add_argument('move', choices=['rock', 'paper', 'scissors']) parser.parse_args(['rock']) Namespace(move='rock') parser.parse_args(['fire']) usage: game.py [-h] {rock,paper,scissors} game.py: error: argument move: invalid choice: 'fire' (choose from 'rock', 'paper', 'scissors')
Any sequence can be passed as the choices value, so :class:list objects,
:class:tuple objects, and custom sequences are all supported.
Use of :class:enum.Enum 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_ conversions have been performed, so objects in choices should match the 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_ 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_.
.. _required:
required ^^^^^^^^
In general, the :mod:!argparse module assumes that flags like -f and --bar
indicate optional arguments, which can always be omitted at the command line.
To make an option required, True can be specified for the required=
keyword argument to :meth:~ArgumentParser.add_argument::
parser = argparse.ArgumentParser() parser.add_argument('--foo', required=True) parser.parse_args(['--foo', 'BAR']) Namespace(foo='BAR') parser.parse_args([]) usage: [-h] --foo FOO : error: the following arguments are required: --foo
As the example shows, if an option is marked as required,
:meth:~ArgumentParser.parse_args will report an error if that option is not
present at the command line.
.. note::
Required options are generally considered bad form because users expect
*options* to be *optional*, and thus they should be avoided when possible.
.. _help:
help ^^^^
The help value is a string containing a brief description of the argument.
When a user requests help (usually by using -h or --help at the
command line), these help descriptions will be displayed with each
argument.
The help strings can include various format specifiers to avoid repetition
of things like the program name or the argument default_. The available
specifiers include the program name, %(prog)s and most keyword arguments to
:meth:~ArgumentParser.add_argument, e.g. %(default)s, %(type)s, etc.::
parser = argparse.ArgumentParser(prog='frobble') parser.add_argument('bar', nargs='?', type=int, default=42, ... help='the bar to %(prog)s (default: %(default)s)') parser.print_help() usage: frobble [-h] [bar]
positional arguments: bar the bar to frobble (default: 42)
options: -h, --help show this help message and exit
As the help string supports %-formatting, if you want a literal % to appear
in the help string, you must escape it as %%.
:mod:!argparse supports silencing the help entry for certain options, by
setting the help value to argparse.SUPPRESS::
parser = argparse.ArgumentParser(prog='frobble') parser.add_argument('--foo', help=argparse.SUPPRESS) parser.print_help() usage: frobble [-h]
options: -h, --help show this help message and exit
.. _metavar:
metavar ^^^^^^^
When :class:ArgumentParser generates help messages, it needs some way to refer
to each expected argument. By default, :class:!ArgumentParser objects use the dest_
value as the "name" of each object. By default, for positional argument
actions, the dest_ value is used directly, and for optional argument actions,
the dest_ value is uppercased. So, a single positional argument with
dest='bar' will be referred to as bar. A single
optional argument --foo that should be followed by a single command-line argument
will be referred to as FOO. An example::
parser = argparse.ArgumentParser() parser.add_argument('--foo') parser.add_argument('bar') parser.parse_args('X --foo Y'.split()) Namespace(bar='X', foo='Y') parser.print_help() usage: [-h] [--foo FOO] bar
positional arguments: bar
options: -h, --help show this help message and exit --foo FOO
An alternative name can be specified with metavar::
parser = argparse.ArgumentParser() parser.add_argument('--foo', metavar='YYY') parser.add_argument('bar', metavar='XXX') parser.parse_args('X --foo Y'.split()) Namespace(bar='X', foo='Y') parser.print_help() usage: [-h] [--foo YYY] XXX
positional arguments: XXX
options: -h, --help show this help message and exit --foo YYY
Note that metavar only changes the displayed name - the name of the
attribute on the :meth:~ArgumentParser.parse_args object is still determined
by the dest_ value.
Different values of nargs may cause the metavar to be used multiple times.
Providing a tuple to metavar specifies a different display for each of the
arguments::
parser = argparse.ArgumentParser(prog='PROG') parser.add_argument('-x', nargs=2) parser.add_argument('--foo', nargs=2, metavar=('bar', 'baz')) parser.print_help() usage: PROG [-h] [-x X X] [--foo bar baz]
options: -h, --help show this help message and exit -x X X --foo bar baz
.. _dest:
dest ^^^^
Most :class:ArgumentParser actions add some value as an attribute of the
object returned by :meth:~ArgumentParser.parse_args. The name of this
attribute is determined by the dest keyword argument of
:meth:~ArgumentParser.add_argument. For positional argument actions,
dest is normally supplied as the first argument to
:meth:~ArgumentParser.add_argument::
parser = argparse.ArgumentParser() parser.add_argument('bar') parser.parse_args(['XXX']) Namespace(bar='XXX')
For optional argument actions, the value of dest is normally inferred from
the option strings. :class:ArgumentParser 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:
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
:meth:~ArgumentParser.add_argument, which defaults to False,
specifies if the argument is deprecated and will be removed
in the future.
For arguments, if deprecated is True, then a warning will be
printed to :data:sys.stderr when the argument is used::
import argparse parser = argparse.ArgumentParser(prog='snake.py') parser.add_argument('--legs', default=0, type=int, deprecated=True) parser.parse_args([]) Namespace(legs=0) parser.parse_args(['--legs', '4']) # doctest: +SKIP snake.py: warning: option '--legs' is deprecated Namespace(legs=4)
.. versionadded:: 3.13
Action classes ^^^^^^^^^^^^^^
:class:!Action classes implement the Action API, a callable which returns a callable
which processes arguments from the command-line. Any object which follows
this API may be passed as the action parameter to
:meth:~ArgumentParser.add_argument.
.. class:: Action(option_strings, dest, nargs=None, const=None, default=None,
type=None, choices=None, required=False, help=None,
metavar=None)
:class:!Action objects are used by an :class:ArgumentParser to represent the information
needed to parse a single argument from one or more strings from the
command line. The :class:!Action class must accept the two positional arguments
plus any keyword arguments passed to :meth:ArgumentParser.add_argument
except for the action itself.
Instances of :class:!Action (or return value of any callable to the
action parameter) should have attributes :attr:!dest,
:attr:!option_strings, :attr:!default, :attr:!type, :attr:!required,
:attr:!help, etc. defined. The easiest way to ensure these attributes
are defined is to call :meth:!Action.__init__.
.. method:: call(parser, namespace, values, option_string=None)
:class:`!Action` instances should be callable, so subclasses must override the
:meth:`!__call__` method, which should accept four parameters:
* *parser* - The :class:`ArgumentParser` object which contains this action.
* *namespace* - The :class:`Namespace` object that will be returned by
:meth:`~ArgumentParser.parse_args`. Most actions add an attribute to this
object using :func:`setattr`.
* *values* - The associated command-line arguments, with any type conversions
applied. Type conversions are specified with the type_ keyword argument to
:meth:`~ArgumentParser.add_argument`.
* *option_string* - The option string that was used to invoke this action.
The ``option_string`` argument is optional, and will be absent if the action
is associated with a positional argument.
The :meth:`!__call__` method may perform arbitrary actions, but will typically set
attributes on the ``namespace`` based on ``dest`` and ``values``.
.. method:: format_usage()
:class:`!Action` subclasses can define a :meth:`!format_usage` method that takes no argument
and return a string which will be used when printing the usage of the program.
If such method is not provided, a sensible default will be used.
.. class:: BooleanOptionalAction
A subclass of :class:Action 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_.
.. 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 :meth:add_argument determine exactly what objects are
created and how they are assigned. See the documentation for
:meth:!add_argument for details.
args_ - List of strings to parse. The default is taken from
:data:sys.argv.
namespace_ - An object to take the attributes. The default is a new empty
:class:Namespace object.
Option value syntax ^^^^^^^^^^^^^^^^^^^
The :meth:~ArgumentParser.parse_args method supports several ways of
specifying the value of an option (if it takes one). In the simplest case, the
option and its value are passed as two separate arguments::
parser = argparse.ArgumentParser(prog='PROG') parser.add_argument('-x') parser.add_argument('--foo') parser.parse_args(['-x', 'X']) Namespace(foo=None, x='X') parser.parse_args(['--foo', 'FOO']) Namespace(foo='FOO', x=None)
For long options (options with names longer than a single character), the option
and value can also be passed as a single command-line argument, using = to
separate them::
parser.parse_args(['--foo=FOO']) Namespace(foo='FOO', x=None)
For short options (options only one character long), the option and its value can be concatenated::
parser.parse_args(['-xX']) Namespace(foo=None, x='X')
Several short options can be joined together, using only a single - prefix,
as long as only the last option (or none of them) requires a value::
parser = argparse.ArgumentParser(prog='PROG') parser.add_argument('-x', action='store_true') parser.add_argument('-y', action='store_true') parser.add_argument('-z') parser.parse_args(['-xyzZ']) Namespace(x=True, y=True, z='Z')
Invalid arguments ^^^^^^^^^^^^^^^^^
While parsing the command line, :meth:~ArgumentParser.parse_args checks for a
variety of errors, including ambiguous options, invalid types, invalid options,
wrong number of positional arguments, etc. When it encounters such an error,
it exits and prints the error along with a usage message::
parser = argparse.ArgumentParser(prog='PROG') parser.add_argument('--foo', type=int) parser.add_argument('bar', nargs='?')
invalid type
parser.parse_args(['--foo', 'spam']) usage: PROG [-h] [--foo FOO] [bar] PROG: error: argument --foo: invalid int value: 'spam'
invalid option
parser.parse_args(['--bar']) usage: PROG [-h] [--foo FOO] [bar] PROG: error: no such option: --bar
wrong number of arguments
parser.parse_args(['spam', 'badger']) usage: PROG [-h] [--foo FOO] [bar] PROG: error: extra arguments found: badger
Arguments containing -
^^^^^^^^^^^^^^^^^^^^^^^^^^
The :meth:~ArgumentParser.parse_args method attempts to give errors whenever
the user has clearly made a mistake, but some situations are inherently
ambiguous. For example, the command-line argument -1 could either be an
attempt to specify an option or an attempt to provide a positional argument.
The :meth:~ArgumentParser.parse_args method is cautious here: positional
arguments may only begin with - if they look like negative numbers and
there are no options in the parser that look like negative numbers::
parser = argparse.ArgumentParser(prog='PROG') parser.add_argument('-x') parser.add_argument('foo', nargs='?')
no negative number options, so -1 is a positional argument
parser.parse_args(['-x', '-1']) Namespace(foo=None, x='-1')
no negative number options, so -1 and -5 are positional arguments
parser.parse_args(['-x', '-1', '-5']) Namespace(foo='-5', x='-1')
parser = argparse.ArgumentParser(prog='PROG') parser.add_argument('-1', dest='one') parser.add_argument('foo', nargs='?')
negative number options present, so -1 is an option
parser.parse_args(['-1', 'X']) Namespace(foo=None, one='X')
negative number options present, so -2 is an option
parser.parse_args(['-2']) usage: PROG [-h] [-1 ONE] [foo] PROG: error: no such option: -2
negative number options present, so both -1s are options
parser.parse_args(['-1', '-1']) usage: PROG [-h] [-1 ONE] [foo] PROG: error: argument -1: expected one argument
If you have positional arguments that must begin with - and don't look
like negative numbers, you can insert the pseudo-argument '--' which tells
:meth:~ArgumentParser.parse_args that everything after that is a positional
argument::
parser.parse_args(['--', '-f']) Namespace(foo='-f', one=None)
See also :ref:the argparse howto on ambiguous arguments <specifying-ambiguous-arguments>
for more details.
.. _prefix-matching:
Argument abbreviations (prefix matching) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The :meth:~ArgumentParser.parse_args method :ref:by default <allow_abbrev>
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 :ref:allow_abbrev to False.
.. _args:
Beyond sys.argv
^^^^^^^^^^^^^^^^^^^
Sometimes it may be useful to have an :class:ArgumentParser parse arguments other than those
of :data:sys.argv. This can be accomplished by passing a list of strings to
:meth:~ArgumentParser.parse_args. This is useful for testing at the
interactive prompt::
parser = argparse.ArgumentParser() parser.add_argument( ... 'integers', metavar='int', type=int, choices=range(10), ... nargs='+', help='an integer in the range 0..9') parser.add_argument( ... '--sum', dest='accumulate', action='store_const', const=sum, ... default=max, help='sum the integers (default: find the max)') parser.parse_args(['1', '2', '3', '4']) Namespace(accumulate=<built-in function max>, integers=[1, 2, 3, 4]) parser.parse_args(['1', '2', '3', '4', '--sum']) Namespace(accumulate=<built-in function sum>, integers=[1, 2, 3, 4])
.. _namespace:
The Namespace object ^^^^^^^^^^^^^^^^^^^^
.. class:: Namespace
Simple class used by default by :meth:~ArgumentParser.parse_args to create
an object holding attributes and return it.
This class is deliberately simple, just an :class:object subclass with a
readable string representation. If you prefer to have dict-like view of the
attributes, you can use the standard Python idiom, :func:vars::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo')
>>> args = parser.parse_args(['--foo', 'BAR'])
>>> vars(args)
{'foo': 'BAR'}
It may also be useful to have an :class:ArgumentParser assign attributes to an
already existing object, rather than a new :class:Namespace object. This can
be achieved by specifying the namespace= keyword argument::
>>> class C:
... pass
...
>>> c = C()
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo')
>>> parser.parse_args(args=['--foo', 'BAR'], namespace=c)
>>> c.foo
'BAR'
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.
:class:ArgumentParser supports the creation of such subcommands with the
:meth:!add_subparsers method. The :meth:!add_subparsers method is normally
called with no arguments and returns a special action object. This object
has a single method, :meth:~_SubParsersAction.add_parser, which takes a
command name and any :class:!ArgumentParser constructor arguments, and
returns an :class:!ArgumentParser object that can be modified as usual.
Description of parameters:
title - title for the sub-parser group in help output; by default "subcommands" if description is provided, otherwise uses title for positional arguments
description - description for the sub-parser group in help output, by
default None
prog - usage information that will be displayed with 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. :class:ArgumentParser)
action_ - the basic type of action to be taken when this argument is encountered at the command line
dest_ - name of the attribute under which subcommand name will be
stored; by default None and no value is stored
required_ - Whether or not a subcommand must be provided, by default
False (added in 3.7)
help_ - help for sub-parser group in help output, by default None
metavar_ - string presenting available subcommands in help; by default it
is None and presents subcommands in form {cmd1, cmd2, ..}
Some example usage::
>>> # create the top-level parser
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('--foo', action='store_true', help='foo help')
>>> subparsers = parser.add_subparsers(help='subcommand help')
>>>
>>> # create the parser for the "a" command
>>> parser_a = subparsers.add_parser('a', help='a help')
>>> parser_a.add_argument('bar', type=int, help='bar help')
>>>
>>> # create the parser for the "b" command
>>> parser_b = subparsers.add_parser('b', help='b help')
>>> parser_b.add_argument('--baz', choices=('X', 'Y', 'Z'), help='baz help')
>>>
>>> # parse some argument lists
>>> parser.parse_args(['a', '12'])
Namespace(bar=12, foo=False)
>>> parser.parse_args(['--foo', 'b', '--baz', 'Z'])
Namespace(baz='Z', foo=True)
Note that the object returned by :meth:~ArgumentParser.parse_args will only contain
attributes for the main parser and the subparser that was selected by the
command line (and not any other subparsers). So in the example above, when
the a command is specified, only the foo and bar attributes are
present, and when the b command is specified, only the foo and
baz attributes are present.
Similarly, when a help message is requested from a subparser, only the help
for that particular parser will be printed. The help message will not
include parent parser or sibling parser messages. (A help message for each
subparser command, however, can be given by supplying the help= argument
to :meth:~_SubParsersAction.add_parser as above.)
::
>>> parser.parse_args(['--help'])
usage: PROG [-h] [--foo] {a,b} ...
positional arguments:
{a,b} subcommand help
a a help
b b help
options:
-h, --help show this help message and exit
--foo foo help
>>> parser.parse_args(['a', '--help'])
usage: PROG a [-h] bar
positional arguments:
bar bar help
options:
-h, --help show this help message and exit
>>> parser.parse_args(['b', '--help'])
usage: PROG b [-h] [--baz {X,Y,Z}]
options:
-h, --help show this help message and exit
--baz {X,Y,Z} baz help
The :meth:~ArgumentParser.add_subparsers method also supports title and description
keyword arguments. When either is present, the subparser's commands will
appear in their own group in the help output. For example::
>>> parser = argparse.ArgumentParser()
>>> subparsers = parser.add_subparsers(title='subcommands',
... description='valid subcommands',
... help='additional help')
>>> subparsers.add_parser('foo')
>>> subparsers.add_parser('bar')
>>> parser.parse_args(['-h'])
usage: [-h] {foo,bar} ...
options:
-h, --help show this help message and exit
subcommands:
valid subcommands
{foo,bar} additional help
One particularly effective way of handling subcommands is to combine the use
of the :meth:~ArgumentParser.add_subparsers method with calls to :meth:~ArgumentParser.set_defaults so
that each subparser knows which Python function it should execute. For
example::
>>> # subcommand functions
>>> def foo(args):
... print(args.x * args.y)
...
>>> def bar(args):
... print('((%s))' % args.z)
...
>>> # create the top-level parser
>>> parser = argparse.ArgumentParser()
>>> subparsers = parser.add_subparsers(required=True)
>>>
>>> # create the parser for the "foo" command
>>> parser_foo = subparsers.add_parser('foo')
>>> parser_foo.add_argument('-x', type=int, default=1)
>>> parser_foo.add_argument('y', type=float)
>>> parser_foo.set_defaults(func=foo)
>>>
>>> # create the parser for the "bar" command
>>> parser_bar = subparsers.add_parser('bar')
>>> parser_bar.add_argument('z')
>>> parser_bar.set_defaults(func=bar)
>>>
>>> # parse the args and call whatever function was selected
>>> args = parser.parse_args('foo 1 -x 2'.split())
>>> args.func(args)
2.0
>>>
>>> # parse the args and call whatever function was selected
>>> args = parser.parse_args('bar XYZYX'.split())
>>> args.func(args)
((XYZYX))
This way, you can let :meth:~ArgumentParser.parse_args do the job of calling the
appropriate function after argument parsing is complete. Associating
functions with actions like this is typically the easiest way to handle the
different actions for each of your subparsers. However, if it is necessary
to check the name of the subparser that was invoked, the dest keyword
argument to the :meth:~ArgumentParser.add_subparsers call will work::
>>> parser = argparse.ArgumentParser()
>>> subparsers = parser.add_subparsers(dest='subparser_name')
>>> subparser1 = subparsers.add_parser('1')
>>> subparser1.add_argument('-x')
>>> subparser2 = subparsers.add_parser('2')
>>> subparser2.add_argument('y')
>>> parser.parse_args(['2', 'frobble'])
Namespace(subparser_name='2', y='frobble')
.. 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(name, *, help=None, aliases=None,
deprecated=False, **kwargs)
Create and return a new :class:ArgumentParser 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
:class:!ArgumentParser constructor.
.. versionadded:: 3.13 Added the deprecated parameter.
FileType objects ^^^^^^^^^^^^^^^^
.. class:: FileType(mode='r', bufsize=-1, encoding=None, errors=None)
The :class:FileType factory creates objects that can be passed to the type
argument of :meth:ArgumentParser.add_argument. Arguments that have
:class:FileType objects as their type will open command-line arguments as
files with the requested modes, buffer sizes, encodings and error handling
(see the :func:open function for more details)::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--raw', type=argparse.FileType('wb', 0))
>>> parser.add_argument('out', type=argparse.FileType('w', encoding='UTF-8'))
>>> parser.parse_args(['--raw', 'raw.dat', 'file.txt'])
Namespace(out=<_io.TextIOWrapper name='file.txt' mode='w' encoding='UTF-8'>, raw=<_io.FileIO name='raw.dat' mode='wb'>)
FileType objects understand the pseudo-argument '-' and automatically
convert this into :data:sys.stdin for readable :class:FileType objects and
:data:sys.stdout for writable :class:FileType objects::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('infile', type=argparse.FileType('r'))
>>> parser.parse_args(['-'])
Namespace(infile=<_io.TextIOWrapper name='<stdin>' encoding='UTF-8'>)
.. 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 :keyword:`with`-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, :class:ArgumentParser groups command-line arguments into
"positional arguments" and "options" when displaying help
messages. When there is a better conceptual grouping of arguments than this
default one, appropriate groups can be created using the
:meth:!add_argument_group method::
>>> parser = argparse.ArgumentParser(prog='PROG', add_help=False)
>>> group = parser.add_argument_group('group')
>>> group.add_argument('--foo', help='foo help')
>>> group.add_argument('bar', help='bar help')
>>> parser.print_help()
usage: PROG [--foo FOO] bar
group:
bar bar help
--foo FOO foo help
The :meth:add_argument_group method returns an argument group object which
has an :meth:~ArgumentParser.add_argument method just like a regular
:class:ArgumentParser. When an argument is added to the group, the parser
treats it just like a normal argument, but displays the argument in a
separate group for help messages. The :meth:!add_argument_group method
accepts title and description arguments which can be used to
customize this display::
>>> parser = argparse.ArgumentParser(prog='PROG', add_help=False)
>>> group1 = parser.add_argument_group('group1', 'group1 description')
>>> group1.add_argument('foo', help='foo help')
>>> group2 = parser.add_argument_group('group2', 'group2 description')
>>> group2.add_argument('--bar', help='bar help')
>>> parser.print_help()
usage: PROG [--bar BAR] foo
group1:
group1 description
foo foo help
group2:
group2 description
--bar BAR bar help
The optional, keyword-only parameters argument_default_ and conflict_handler_
allow for finer-grained control of the behavior of the argument group. These
parameters have the same meaning as in the :class:ArgumentParser constructor,
but apply specifically to the argument group rather than the entire parser.
Note that any arguments not in your user-defined groups will end up back in the usual "positional arguments" and "optional arguments" sections.
.. deprecated-removed:: 3.11 3.14
Calling :meth:add_argument_group 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_ to :meth:add_argument_group
is now deprecated.
Mutual exclusion ^^^^^^^^^^^^^^^^
.. method:: ArgumentParser.add_mutually_exclusive_group(required=False)
Create a mutually exclusive group. :mod:!argparse will make sure that only
one of the arguments in the mutually exclusive group was present on the
command line::
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> group = parser.add_mutually_exclusive_group()
>>> group.add_argument('--foo', action='store_true')
>>> group.add_argument('--bar', action='store_false')
>>> parser.parse_args(['--foo'])
Namespace(bar=True, foo=True)
>>> parser.parse_args(['--bar'])
Namespace(bar=False, foo=False)
>>> parser.parse_args(['--foo', '--bar'])
usage: PROG [-h] [--foo | --bar]
PROG: error: argument --bar: not allowed with argument --foo
The :meth:add_mutually_exclusive_group method also accepts a required
argument, to indicate that at least one of the mutually exclusive arguments
is required::
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> group = parser.add_mutually_exclusive_group(required=True)
>>> group.add_argument('--foo', action='store_true')
>>> group.add_argument('--bar', action='store_false')
>>> parser.parse_args([])
usage: PROG [-h] (--foo | --bar)
PROG: error: one of the arguments --foo --bar is required
Note that currently mutually exclusive argument groups do not support the
title and description arguments of
:meth:~ArgumentParser.add_argument_group. However, a mutually exclusive
group can be added to an argument group that has a title and description.
For example::
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> group = parser.add_argument_group('Group title', 'Group description')
>>> exclusive_group = group.add_mutually_exclusive_group(required=True)
>>> exclusive_group.add_argument('--foo', help='foo help')
>>> exclusive_group.add_argument('--bar', help='bar help')
>>> parser.print_help()
usage: PROG [-h] (--foo FOO | --bar BAR)
options:
-h, --help show this help message and exit
Group title:
Group description
--foo FOO foo help
--bar BAR bar help
.. deprecated-removed:: 3.11 3.14
Calling :meth:add_argument_group or :meth:add_mutually_exclusive_group
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 :meth:parse_args
will be fully determined by inspecting the command-line arguments and the argument
actions. :meth:set_defaults allows some additional
attributes that are determined without any inspection of the command line to
be added::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('foo', type=int)
>>> parser.set_defaults(bar=42, baz='badger')
>>> parser.parse_args(['736'])
Namespace(bar=42, baz='badger', foo=736)
Note that defaults can be set at both the parser level using :meth:set_defaults
and at the argument level using :meth:add_argument. 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 :meth:~ArgumentParser.add_subparsers method for an
example of this type.
.. method:: ArgumentParser.get_default(dest)
Get the default value for a namespace attribute, as set by either
:meth:~ArgumentParser.add_argument or by
:meth:~ArgumentParser.set_defaults::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', default='badger')
>>> parser.get_default('foo')
'badger'
Printing help ^^^^^^^^^^^^^
In most typical applications, :meth:~ArgumentParser.parse_args 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 :class:ArgumentParser should be
invoked on the command line. If file is None, :data:sys.stdout is
assumed.
.. method:: ArgumentParser.print_help(file=None)
Print a help message, including the program usage and information about the
arguments registered with the :class:ArgumentParser. If file is
None, :data:sys.stdout 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
:class:ArgumentParser 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 :class:ArgumentParser.
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 :meth:~ArgumentParser.parse_known_args method can be
useful.
This method works similarly to :meth:~ArgumentParser.parse_args, 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::
:ref:Prefix matching <prefix-matching> rules apply to
:meth:~ArgumentParser.parse_known_args. The parser may consume an option even if it's just
a prefix of one of its known options, instead of leaving it in the remaining
arguments list.
Customizing file parsing ^^^^^^^^^^^^^^^^^^^^^^^^
.. 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 :class:ArgumentParser constructor) are read one
argument per line. :meth:convert_arg_line_to_args can be overridden for
fancier reading.
This method takes a single argument arg_line which is a string read from the argument file. It returns a list of arguments parsed from this string. The method is called once per line read from the argument file, in order.
A useful override of this method is one that treats each space-separated word as an argument. The following example demonstrates how to do this::
class MyArgumentParser(argparse.ArgumentParser):
def convert_arg_line_to_args(self, arg_line):
return arg_line.split()
Exiting methods ^^^^^^^^^^^^^^^
.. 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 :data:sys.stderr before that.
The user can override this method to handle these steps differently::
class ErrorCatchingArgumentParser(argparse.ArgumentParser):
def exit(self, status=0, message=None):
if status:
raise Exception(f'Exiting because of an error: {message}')
exit(status)
.. method:: ArgumentParser.error(message)
This method prints a usage message, including the message, to
:data:sys.stderr 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 :meth:~ArgumentParser.parse_intermixed_args
and :meth:~ArgumentParser.parse_known_intermixed_args methods
support this parsing style.
These parsers do not support all the :mod:!argparse features, and will raise
exceptions if unsupported features are used. In particular, subparsers,
and mutually exclusive groups that include both
optionals and positionals are not supported.
The following example shows the difference between
:meth:~ArgumentParser.parse_known_args and
:meth:~ArgumentParser.parse_intermixed_args: the former returns ['2', '3'] as unparsed arguments, while the latter collects all the positionals
into rest. ::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo')
>>> parser.add_argument('cmd')
>>> parser.add_argument('rest', nargs='*', type=int)
>>> parser.parse_known_args('doit 1 --foo bar 2 3'.split())
(Namespace(cmd='doit', foo='bar', rest=[1]), ['2', '3'])
>>> parser.parse_intermixed_args('doit 1 --foo bar 2 3'.split())
Namespace(cmd='doit', foo='bar', rest=[1, 2, 3])
:meth:~ArgumentParser.parse_known_intermixed_args returns a two item tuple
containing the populated namespace and the list of remaining argument strings.
:meth:~ArgumentParser.parse_intermixed_args 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, :meth:!register can be used to
register custom actions or types with a parser and allow you to reference the
type by their registered name instead of their callable name.
The :meth:!register method accepts three arguments - a registry_name,
specifying the internal registry where the object will be stored (e.g.,
action, type), value, which is the key under which the object will
be registered, and object, the callable to be registered.
The following example shows how to register a custom type with a parser::
>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.register('type', 'hexadecimal integer', lambda s: int(s, 16))
>>> parser.add_argument('--foo', type='hexadecimal integer')
_StoreAction(option_strings=['--foo'], dest='foo', nargs=None, const=None, default=None, type='hexadecimal integer', choices=None, required=False, help=None, metavar=None, deprecated=False)
>>> parser.parse_args(['--foo', '0xFA'])
Namespace(foo=250)
>>> parser.parse_args(['--foo', '1.2'])
usage: PROG [-h] [--foo FOO]
PROG: error: argument --foo: invalid 'hexadecimal integer' value: '1.2'
.. 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.
.. rubric:: Guides and Tutorials
.. toctree:: :maxdepth: 1
../howto/argparse.rst ../howto/argparse-optparse.rst