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:mod:`!argparse` --- Parser for command-line options, arguments and sub-commands
================================================================================
.. module:: argparse
:synopsis: Command-line option and argument parsing library.
.. moduleauthor:: Steven Bethard <steven.bethard@gmail.com>
.. sectionauthor:: Steven Bethard <steven.bethard@gmail.com>
.. versionadded:: 3.2
**Source code:** :source:`Lib/argparse.py`
--------------
.. 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.
Quick Links for ArgumentParser
---------------------------------------
========================= =========================================================================================================== ==================================================================================
Name Description Values
========================= =========================================================================================================== ==================================================================================
prog_ The name of the program
usage_ The string describing the program usage
description_ A brief description of what the program does
epilog_ Additional description of the program after the argument help
parents_ A list of :class:`ArgumentParser` objects whose arguments should also be included
formatter_class_ A class for customizing the help output ``argparse.HelpFormatter``
prefix_chars_ The set of characters that prefix optional arguments Defaults to ``'-'``
fromfile_prefix_chars_ The set of characters that prefix files to read additional arguments from Defaults to ``None`` (meaning arguments will never be treated as file references)
argument_default_ The global default value for arguments
allow_abbrev_ Allows long options to be abbreviated if the abbreviation is unambiguous ``True`` or ``False`` (default: ``True``)
conflict_handler_ The strategy for resolving conflicting optionals
add_help_ Add a ``-h/--help`` option to the parser ``True`` or ``False`` (default: ``True``)
exit_on_error_ Determines whether or not to exit with error info when an error occurs ``True`` or ``False`` (default: ``True``)
========================= =========================================================================================================== ==================================================================================
Core Functionality
------------------
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)
Quick Links for add_argument()
------------------------------
============================ =========================================================== ==========================================================================================================================
Name Description Values
============================ =========================================================== ==========================================================================================================================
action_ Specify how an argument should be handled ``'store'``, ``'store_const'``, ``'store_true'``, ``'append'``, ``'append_const'``, ``'count'``, ``'help'``, ``'version'``
choices_ Limit values to a specific set of choices ``['foo', 'bar']``, ``range(1, 10)``, or :class:`~collections.abc.Container` instance
const_ Store a constant value
default_ Default value used when an argument is not provided Defaults to ``None``
dest_ Specify the attribute name used in the result namespace
help_ Help message for an argument
metavar_ Alternate display name for the argument as shown in help
nargs_ Number of times the argument can be used :class:`int`, ``'?'``, ``'*'``, or ``'+'``
required_ Indicate whether an argument is required or optional ``True`` or ``False``
:ref:`type <argparse-type>` Automatically convert an argument to the given type :class:`int`, :class:`float`, ``argparse.FileType('w')``, or callable function
============================ =========================================================== ==========================================================================================================================
Example
-------
The following code is a Python program that takes a list of integers and
produces either the sum or the max::
import argparse
parser = argparse.ArgumentParser(description='Process some integers.')
parser.add_argument('integers', metavar='N', type=int, nargs='+',
help='an integer for the accumulator')
parser.add_argument('--sum', dest='accumulate', action='store_const',
const=sum, default=max,
help='sum the integers (default: find the max)')
args = parser.parse_args()
print(args.accumulate(args.integers))
Assuming the above Python code is saved into a file called ``prog.py``, it can
be run at the command line and it provides useful help messages:
.. code-block:: shell-session
$ python prog.py -h
usage: prog.py [-h] [--sum] N [N ...]
Process some integers.
positional arguments:
N an integer for the accumulator
options:
-h, --help show this help message and exit
--sum sum the integers (default: find the max)
When run with the appropriate arguments, it prints either the sum or the max of
the command-line integers:
.. code-block:: shell-session
$ python prog.py 1 2 3 4
4
$ python prog.py 1 2 3 4 --sum
10
If invalid arguments are passed in, an error will be displayed:
.. code-block:: shell-session
$ python prog.py a b c
usage: prog.py [-h] [--sum] N [N ...]
prog.py: error: argument N: invalid int value: 'a'
The following sections walk you through this example.
Creating a parser
^^^^^^^^^^^^^^^^^
The first step in using the :mod:`argparse` is creating an
:class:`ArgumentParser` object::
>>> parser = argparse.ArgumentParser(description='Process some integers.')
The :class:`ArgumentParser` object will hold all the information necessary to
parse the command line into Python data types.
Adding arguments
^^^^^^^^^^^^^^^^
Filling an :class:`ArgumentParser` with information about program arguments is
done by making calls to the :meth:`~ArgumentParser.add_argument` method.
Generally, these calls tell the :class:`ArgumentParser` how to take the strings
on the command line and turn them into objects. This information is stored and
used when :meth:`~ArgumentParser.parse_args` is called. For example::
>>> parser.add_argument('integers', metavar='N', type=int, nargs='+',
... help='an integer for the accumulator')
>>> parser.add_argument('--sum', dest='accumulate', action='store_const',
... const=sum, default=max,
... help='sum the integers (default: find the max)')
Later, calling :meth:`~ArgumentParser.parse_args` will return an object with
two attributes, ``integers`` and ``accumulate``. The ``integers`` attribute
will be a list of one or more integers, and the ``accumulate`` attribute will be
either the :func:`sum` function, if ``--sum`` was specified at the command line,
or the :func:`max` function if it was not.
Parsing arguments
^^^^^^^^^^^^^^^^^
:class:`ArgumentParser` parses arguments through the
:meth:`~ArgumentParser.parse_args` method. This will inspect the command line,
convert each argument to the appropriate type and then invoke the appropriate action.
In most cases, this means a simple :class:`Namespace` object will be built up from
attributes parsed out of the command line::
>>> parser.parse_args(['--sum', '7', '-1', '42'])
Namespace(accumulate=<built-in function sum>, integers=[7, -1, 42])
In a script, :meth:`~ArgumentParser.parse_args` will typically be called with no
arguments, and the :class:`ArgumentParser` will automatically determine the
command-line arguments from :data:`sys.argv`.
ArgumentParser objects
----------------------
.. 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)
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 ArgumentParser exits with
error info when an error occurs. (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.
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 inerpreter was run:
* The :func:`base name <os.path.basename>` 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 :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. For example, consider a file
named ``myprogram.py`` with the following code::
import argparse
parser = argparse.ArgumentParser()
parser.add_argument('--foo', help='foo help')
args = parser.parse_args()
The help for this program will display ``myprogram.py`` as the program name
(regardless of where the program was invoked from) if it is run as a script:
.. code-block:: shell-session
$ python myprogram.py --help
usage: myprogram.py [-h] [--foo FOO]
options:
-h, --help show this help message and exit
--foo FOO foo help
$ cd ..
$ python subdir/myprogram.py --help
usage: myprogram.py [-h] [--foo FOO]
options:
-h, --help show this help message and exit
--foo FOO foo help
If it is executed via the :option:`-m` option, the help will display a corresponding command line:
.. code-block:: shell-session
$ /usr/bin/python3 -m subdir.myprogram --help
usage: python3 -m subdir.myprogram [-h] [--foo FOO]
options:
-h, --help show this help message and exit
--foo FOO foo help
To change this default behavior, another value can be supplied using the
``prog=`` argument to :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::
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('--foo', nargs='?', help='foo help')
>>> parser.add_argument('bar', nargs='+', help='bar help')
>>> parser.print_help()
usage: PROG [-h] [--foo [FOO]] bar [bar ...]
positional arguments:
bar bar help
options:
-h, --help show this help message and exit
--foo [FOO] foo help
The default message can be overridden with the ``usage=`` keyword argument::
>>> parser = argparse.ArgumentParser(prog='PROG', usage='%(prog)s [options]')
>>> parser.add_argument('--foo', nargs='?', help='foo help')
>>> parser.add_argument('bar', nargs='+', help='bar help')
>>> parser.print_help()
usage: PROG [options]
positional arguments:
bar bar help
options:
-h, --help show this help message and exit
--foo [FOO] foo help
The ``%(prog)s`` format specifier is available to fill in the program name in
your usage messages.
.. _description:
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::
>>> parser = argparse.ArgumentParser(description='A foo that bars')
>>> parser.print_help()
usage: argparse.py [-h]
A foo that bars
options:
-h, --help show this help message and exit
By default, the description will be line-wrapped so that it fits within the
given space. To change this behavior, see the formatter_class_ argument.
epilog
^^^^^^
Some programs like to display additional description of the program after the
description of the arguments. Such text can be specified using the ``epilog=``
argument to :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]
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
:class:`RawTextHelpFormatter` maintains whitespace for all sorts of help text,
including argument descriptions. However, multiple new lines are replaced with
one. If you wish to preserve multiple blank lines, add spaces between the
newlines.
: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 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 by default be one per line (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']``.
: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 :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, ArgumentParser objects add an option which simply displays
the parser's help message. For example, consider a file named
``myprogram.py`` containing the following code::
import argparse
parser = argparse.ArgumentParser()
parser.add_argument('--foo', help='foo help')
args = parser.parse_args()
If ``-h`` or ``--help`` is supplied at the command line, the ArgumentParser
help will be printed:
.. code-block:: shell-session
$ python myprogram.py --help
usage: myprogram.py [-h] [--foo FOO]
options:
-h, --help show this help message and exit
--foo FOO foo help
Occasionally, it may be useful to disable the addition of this help option.
This can be achieved by passing ``False`` as the ``add_help=`` argument to
: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 exit with error info.
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
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`_ - 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 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
.. _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. For example::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo')
>>> parser.parse_args('--foo 1'.split())
Namespace(foo='1')
* ``'store_const'`` - This stores the value specified by the const_ keyword
argument; note that the const_ keyword argument defaults to ``None``. The
``'store_const'`` action is most commonly used with optional arguments that
specify some sort of flag. For example::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', action='store_const', const=42)
>>> parser.parse_args(['--foo'])
Namespace(foo=42)
* ``'store_true'`` and ``'store_false'`` - These are special cases of
``'store_const'`` used for storing the values ``True`` and ``False``
respectively. In addition, they create default values of ``False`` and
``True`` respectively. For example::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', action='store_true')
>>> parser.add_argument('--bar', action='store_false')
>>> parser.add_argument('--baz', action='store_false')
>>> parser.parse_args('--foo --bar'.split())
Namespace(foo=True, bar=False, baz=True)
* ``'append'`` - This stores a list, and appends each argument value to the
list. It is useful to allow an option to be specified multiple times.
If the default value is non-empty, the default elements will be present
in the parsed value for the option, with any values from the
command line appended after those default values. Example usage::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', action='append')
>>> parser.parse_args('--foo 1 --foo 2'.split())
Namespace(foo=['1', '2'])
* ``'append_const'`` - This stores a list, and appends the value specified by
the const_ keyword argument to the list; note that the const_ keyword
argument defaults to ``None``. The ``'append_const'`` action is typically
useful when multiple arguments need to store constants to the same list. For
example::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--str', dest='types', action='append_const', const=str)
>>> parser.add_argument('--int', dest='types', action='append_const', const=int)
>>> parser.parse_args('--str --int'.split())
Namespace(types=[<class 'str'>, <class 'int'>])
* ``'count'`` - This counts the number of times a keyword argument occurs. For
example, this is useful for increasing verbosity levels::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--verbose', '-v', action='count', default=0)
>>> parser.parse_args(['-vvv'])
Namespace(verbose=3)
Note, the *default* will be ``None`` unless explicitly set to *0*.
* ``'help'`` - This prints a complete help message for all the options in the
current parser and then exits. By default a help action is automatically
added to the parser. See :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
* ``'extend'`` - This stores a list, and extends each argument value 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
You may also specify an arbitrary action by passing an Action subclass or
other object that implements the same interface. The ``BooleanOptionalAction``
is available in ``argparse`` and adds support for boolean actions such as
``--foo`` and ``--no-foo``::
>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', action=argparse.BooleanOptionalAction)
>>> parser.parse_args(['--no-foo'])
Namespace(foo=False)
.. versionadded:: 3.9
The recommended way to create a custom action is to extend :class:`Action`,
overriding the ``__call__`` method and optionally the ``__init__`` and
``format_usage`` methods.
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
^^^^^
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='?', type=argparse.FileType('r'),
... default=sys.stdin)
>>> parser.add_argument('outfile', nargs='?', type=argparse.FileType('w'),
... default=sys.stdout)
>>> parser.parse_args(['input.txt', 'output.txt'])
Namespace(infile=<_io.TextIOWrapper name='input.txt' encoding='UTF-8'>,
outfile=<_io.TextIOWrapper name='output.txt' encoding='UTF-8'>)
>>> parser.parse_args([])
Namespace(infile=<_io.TextIOWrapper name='<stdin>' encoding='UTF-8'>,
outfile=<_io.TextIOWrapper name='<stdout>' encoding='UTF-8'>)
.. 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 args present are gathered into a
list. Additionally, an error message will be generated if there wasn't at
least one command-line argument present. For example::
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('foo', nargs='+')
>>> parser.parse_args(['a', 'b'])
Namespace(foo=['a', 'b'])
>>> parser.parse_args([])
usage: PROG [-h] foo [foo ...]
PROG: error: the following arguments are required: foo
If the ``nargs`` keyword argument is not provided, the number of arguments consumed
is determined by the action_. Generally this means a single command-line argument
will be consumed and a single item (not a list) will be produced.
.. _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 of ``const`` will be assumed to
be ``None`` instead. 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 over write 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 any callable that accepts a single string.
If the function raises :exc:`ArgumentTypeError`, :exc:`TypeError`, or
:exc:`ValueError`, the exception is caught and a nicely formatted error
message is displayed. No other exception types are 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('source_file', type=open)
parser.add_argument('dest_file', type=argparse.FileType('w', encoding='latin-1'))
parser.add_argument('datapath', type=pathlib.Path)
User defined functions can be used as well:
.. 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.
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 *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')
Note that inclusion in the *choices* sequence is checked after any type_
conversions have been performed, so the type of the objects in the *choices*
sequence should match the type_ specified::
>>> parser = argparse.ArgumentParser(prog='doors.py')
>>> parser.add_argument('door', type=int, choices=range(1, 4))
>>> print(parser.parse_args(['3']))
Namespace(door=3)
>>> parser.parse_args(['4'])
usage: doors.py [-h] {1,2,3}
doors.py: error: argument door: invalid choice: 4 (choose from 1, 2, 3)
Any 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.
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::
>>> parser = argparse.ArgumentParser(prog='frobble')
>>> parser.add_argument('--foo', action='store_true',
... help='foo the bars before frobbling')
>>> parser.add_argument('bar', nargs='+',
... help='one of the bars to be frobbled')
>>> parser.parse_args(['-h'])
usage: frobble [-h] [--foo] bar [bar ...]
positional arguments:
bar one of the bars to be frobbled
options:
-h, --help show this help message and exit
--foo foo the bars before frobbling
The ``help`` strings can include various format specifiers to avoid repetition
of things like the program name or the argument default_. The available
specifiers include the program name, ``%(prog)s`` and most keyword arguments to
:meth:`~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, 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 long option string and stripping away the initial ``--``
string. If no long option strings were supplied, ``dest`` will be derived from
the first short option string by stripping the initial ``-`` character. Any
internal ``-`` characters will be converted to ``_`` characters to make sure
the string is a valid attribute name. The examples below illustrate this
behavior::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('-f', '--foo-bar', '--foo')
>>> parser.add_argument('-x', '-y')
>>> parser.parse_args('-f 1 -x 2'.split())
Namespace(foo_bar='1', x='2')
>>> parser.parse_args('--foo 1 -y 2'.split())
Namespace(foo_bar='1', x='2')
``dest`` allows a custom attribute name to be provided::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', dest='bar')
>>> parser.parse_args('--foo XXX'.split())
Namespace(bar='XXX')
.. _deprecated:
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
^^^^^^^^^^^^^^
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)
Action objects are used by an ArgumentParser to represent the information
needed to parse a single argument from one or more strings from the
command line. The Action class must accept the two positional arguments
plus any keyword arguments passed to :meth:`ArgumentParser.add_argument`
except for the ``action`` itself.
Instances of Action (or return value of any callable to the ``action``
parameter) should have attributes "dest", "option_strings", "default", "type",
"required", "help", etc. defined. The easiest way to ensure these attributes
are defined is to call ``Action.__init__``.
Action instances should be callable, so subclasses must override the
``__call__`` method, which should accept four parameters:
* ``parser`` - The ArgumentParser object which contains this action.
* ``namespace`` - The :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 ``__call__`` method may perform arbitrary actions, but will typically set
attributes on the ``namespace`` based on ``dest`` and ``values``.
Action subclasses can define a ``format_usage`` method that takes no argument
and return a string which will be used when printing the usage of the program.
If such method is not provided, a sensible default will be used.
The parse_args() method
-----------------------
.. 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 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'
Other utilities
---------------
Sub-commands
^^^^^^^^^^^^
.. method:: ArgumentParser.add_subparsers([title], [description], [prog], \
[parser_class], [action], \
[option_strings], [dest], [required], \
[help], [metavar])
Many programs split up their functionality into a number of sub-commands,
for example, the ``svn`` program can invoke sub-commands like ``svn
checkout``, ``svn update``, and ``svn commit``. Splitting up functionality
this way can be a particularly good idea when a program performs several
different functions which require different kinds of command-line arguments.
:class:`ArgumentParser` supports the creation of such sub-commands 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 sub-command help,
by default the name of the program and any positional arguments before the
subparser argument
* parser_class - class which will be used to create sub-parser instances, by
default the class of the current parser (e.g. ArgumentParser)
* action_ - the basic type of action to be taken when this argument is
encountered at the command line
* dest_ - name of the attribute under which sub-command name will be
stored; by default ``None`` and no value is stored
* required_ - Whether or not a subcommand must be provided, by default
``False`` (added in 3.7)
* help_ - help for sub-parser group in help output, by default ``None``
* metavar_ - string presenting available sub-commands in help; by default it
is ``None`` and presents sub-commands in form {cmd1, cmd2, ..}
Some example usage::
>>> # create the top-level parser
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('--foo', action='store_true', help='foo help')
>>> subparsers = parser.add_subparsers(help='sub-command help')
>>>
>>> # create the parser for the "a" command
>>> parser_a = subparsers.add_parser('a', help='a help')
>>> parser_a.add_argument('bar', type=int, help='bar help')
>>>
>>> # create the parser for the "b" command
>>> parser_b = subparsers.add_parser('b', help='b help')
>>> parser_b.add_argument('--baz', choices=('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:`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} sub-command 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:`add_subparsers` method also supports ``title`` and ``description``
keyword arguments. When either is present, the subparser's commands will
appear in their own group in the help output. For example::
>>> parser = argparse.ArgumentParser()
>>> subparsers = parser.add_subparsers(title='subcommands',
... description='valid subcommands',
... help='additional help')
>>> subparsers.add_parser('foo')
>>> subparsers.add_parser('bar')
>>> parser.parse_args(['-h'])
usage: [-h] {foo,bar} ...
options:
-h, --help show this help message and exit
subcommands:
valid subcommands
{foo,bar} additional help
Furthermore, :meth:`~_SubParsersAction.add_parser` supports an additional
*aliases* argument,
which allows multiple strings to refer to the same subparser. This example,
like ``svn``, aliases ``co`` as a shorthand for ``checkout``::
>>> parser = argparse.ArgumentParser()
>>> subparsers = parser.add_subparsers()
>>> checkout = subparsers.add_parser('checkout', aliases=['co'])
>>> checkout.add_argument('foo')
>>> parser.parse_args(['co', 'bar'])
Namespace(foo='bar')
:meth:`~_SubParsersAction.add_parser` supports also an additional
*deprecated* argument, which allows to deprecate the subparser.
>>> import argparse
>>> parser = argparse.ArgumentParser(prog='chicken.py')
>>> subparsers = parser.add_subparsers()
>>> run = subparsers.add_parser('run')
>>> fly = subparsers.add_parser('fly', deprecated=True)
>>> parser.parse_args(['fly']) # doctest: +SKIP
chicken.py: warning: command 'fly' is deprecated
Namespace()
.. versionadded:: 3.13
One particularly effective way of handling sub-commands is to combine the use
of the :meth:`add_subparsers` method with calls to :meth:`set_defaults` so
that each subparser knows which Python function it should execute. For
example::
>>> # sub-command functions
>>> def foo(args):
... print(args.x * args.y)
...
>>> def bar(args):
... print('((%s))' % args.z)
...
>>> # create the top-level parser
>>> parser = argparse.ArgumentParser()
>>> subparsers = parser.add_subparsers(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:`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:`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 argument.
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'>)
.. versionchanged:: 3.4
Added the *encodings* and *errors* parameters.
Argument groups
^^^^^^^^^^^^^^^
.. method:: ArgumentParser.add_argument_group(title=None, description=None)
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
Note that any arguments not in your user-defined groups will end up back
in the usual "positional arguments" and "optional arguments" sections.
.. versionchanged:: 3.11
Calling :meth:`add_argument_group` on an argument group is deprecated.
This feature was never supported and does not always work correctly.
The function exists on the API by accident through inheritance and
will be removed in the future.
Mutual exclusion
^^^^^^^^^^^^^^^^
.. 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
.. versionchanged:: 3.11
Calling :meth:`add_argument_group` or :meth:`add_mutually_exclusive_group`
on a mutually exclusive group is deprecated. These features were never
supported and do not always work correctly. The functions exist on the
API by accident through inheritance and will be removed in the future.
Parser defaults
^^^^^^^^^^^^^^^
.. 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 parser-level defaults always override argument-level defaults::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', default='bar')
>>> parser.set_defaults(foo='spam')
>>> parser.parse_args([])
Namespace(foo='spam')
Parser-level defaults can be particularly useful when working with multiple
parsers. See the :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 may only parse a few of the command-line arguments, passing
the remaining arguments on to another script or program. In these cases, the
:meth:`~ArgumentParser.parse_known_args` method can be useful. It works much like
:meth:`~ArgumentParser.parse_args` except that it does not produce an error when
extra arguments are present. Instead, it returns a two item tuple containing
the populated namespace and the list of remaining argument strings.
::
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', action='store_true')
>>> parser.add_argument('bar')
>>> parser.parse_known_args(['--foo', '--badger', 'BAR', 'spam'])
(Namespace(bar='BAR', foo=True), ['--badger', 'spam'])
.. warning::
: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 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
.. _upgrading-optparse-code:
Upgrading optparse code
-----------------------
Originally, the :mod:`argparse` module had attempted to maintain compatibility
with :mod:`optparse`. However, :mod:`optparse` was difficult to extend
transparently, particularly with the changes required to support the new
``nargs=`` specifiers and better usage messages. When most everything in
:mod:`optparse` had either been copy-pasted over or monkey-patched, it no
longer seemed practical to try to maintain the backwards compatibility.
The :mod:`argparse` module improves on the standard library :mod:`optparse`
module in a number of ways including:
* Handling positional arguments.
* Supporting sub-commands.
* Allowing alternative option prefixes like ``+`` and ``/``.
* Handling zero-or-more and one-or-more style arguments.
* Producing more informative usage messages.
* Providing a much simpler interface for custom ``type`` and ``action``.
A partial upgrade path from :mod:`optparse` to :mod:`argparse`:
* Replace all :meth:`optparse.OptionParser.add_option` calls with
:meth:`ArgumentParser.add_argument` calls.
* Replace ``(options, args) = parser.parse_args()`` with ``args =
parser.parse_args()`` and add additional :meth:`ArgumentParser.add_argument`
calls for the positional arguments. Keep in mind that what was previously
called ``options``, now in the :mod:`argparse` context is called ``args``.
* Replace :meth:`optparse.OptionParser.disable_interspersed_args`
by using :meth:`~ArgumentParser.parse_intermixed_args` instead of
:meth:`~ArgumentParser.parse_args`.
* Replace callback actions and the ``callback_*`` keyword arguments with
``type`` or ``action`` arguments.
* Replace string names for ``type`` keyword arguments with the corresponding
type objects (e.g. int, float, complex, etc).
* Replace :class:`optparse.Values` with :class:`Namespace` and
:exc:`optparse.OptionError` and :exc:`optparse.OptionValueError` with
:exc:`ArgumentError`.
* Replace strings with implicit arguments such as ``%default`` or ``%prog`` with
the standard Python syntax to use dictionaries to format strings, that is,
``%(default)s`` and ``%(prog)s``.
* Replace the OptionParser constructor ``version`` argument with a call to
``parser.add_argument('--version', action='version', version='<the version>')``.
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.
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