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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 and issues errors when users give the program invalid arguments. Example ------- The following code is a Python program that takes a list of integers and produces either the sum or the max:: import argparse parser = argparse.ArgumentParser(description='Process some integers.') parser.add_argument('integers', metavar='N', type=int, nargs='+', help='an integer for the accumulator') parser.add_argument('--sum', dest='accumulate', action='store_const', const=sum, default=max, help='sum the integers (default: find the max)') args = parser.parse_args() print(args.accumulate(args.integers)) Assuming the Python code above is saved into a file called prog.py, it can be run at the command line and provides useful help messages:: $ prog.py -h usage: prog.py [-h] [--sum] N [N ...] Process some integers. positional arguments: N an integer for the accumulator optional arguments: -h, --help show this help message and exit --sum sum the integers (default: find the max) When run with the appropriate arguments, it prints either the sum or the max of the command-line integers:: $ prog.py 1 2 3 4 4 $ prog.py 1 2 3 4 --sum 10 If invalid arguments are passed in, it will issue an error:: $ prog.py a b c usage: prog.py [-h] [--sum] N [N ...] prog.py: error: argument N: invalid int value: 'a' The following sections walk you through this example. Creating a parser ^^^^^^^^^^^^^^^^^ 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 ints, 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=, 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, [](#l133) epilog=None, parents=[], [](#l134) formatter_class=argparse.HelpFormatter, [](#l135) prefix_chars='-', fromfile_prefix_chars=None, [](#l136) argument_default=None, conflict_handler='error', [](#l137) add_help=True) Create a new :class:ArgumentParser object. Each parameter has its own more detailed description below, but in short they are:

• description_ - Text to display before the argument help.
• epilog_ - Text to display after the argument help.
• add_help_ - Add a -h/--help option to the parser. (default: True)
• argument_default_ - Set the global default value for arguments. (default: None)
• parents_ - A list of :class:ArgumentParser objects whose arguments should also be included.
• prefix_chars_ - The set of characters that prefix optional arguments. (default: '-')
• fromfile_prefix_chars_ - The set of characters that prefix files from which additional arguments should be read. (default: None)
• formatter_class_ - A class for customizing the help output.
• conflict_handler_ - Usually unnecessary, defines strategy for resolving conflicting optionals.
• prog_ - The name of the program (default: sys.argv[0])
• usage_ - The string describing the program usage (default: generated) The following sections describe how each of these are used. 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 optional arguments: -h, --help show this help message and exit By default, the description will be line-wrapped so that it fits within the given space. To change this behavior, see the formatter_class_ argument. epilog ^^^^^^ Some programs like to display additional description of the program after the description of the arguments. Such text can be specified using the epilog= argument to :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 optional arguments: -h, --help show this help message and exit And that's how you'd foo a bar As with the description_ argument, the epilog= text is by default line-wrapped, but this behavior can be adjusted with the formatter_class_ argument to :class:ArgumentParser. 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:: $ python myprogram.py --help usage: myprogram.py [-h] [--foo FOO] optional arguments: -h, --help show this help message and exit --foo FOO foo help Occasionally, it may be useful to disable the addition of this help option. This can be achieved by passing False as the add_help= argument to :class:ArgumentParser:: parser = argparse.ArgumentParser(prog='PROG', add_help=False) parser.add_argument('--foo', help='foo help') parser.print_help() usage: PROG [--foo FOO] optional arguments: --foo FOO foo help 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] optional arguments: +h, ++help show this help message and exit 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, for example when dealing with a particularly long argument lists, it may make sense to keep the list of arguments in a file rather than typing it out at the command line. If the fromfile_prefix_chars= argument is given to the :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') 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']. The fromfile_prefix_chars= argument defaults to None, meaning that arguments will never be treated as file references. argument_default ^^^^^^^^^^^^^^^^ Generally, argument defaults are specified either by passing a default to :meth:~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() 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 ^^^^^^^^^^^^^^^ :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 optional arguments: -h, --help show this help message and exit likewise for this epilog whose whitespace will be cleaned up and whose words will be wrapped across a couple lines 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('''[](#l414) ... Please do not mess up this text ... -------------------------------- ... I have indented it ... exactly the way ... I want it ... ''')) parser.print_help() usage: PROG [-h] Please do not mess up this text -------------------------------- I have indented it exactly the way I want it optional arguments: -h, --help show this help message and exit :class:RawTextHelpFormatter maintains whitespace for all sorts of help text, including argument descriptions. :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 [bar ...]] positional arguments: bar BAR! (default: [1, 2, 3]) optional arguments: -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 optional arguments: -h, --help show this help message and exit --foo int conflict_handler ^^^^^^^^^^^^^^^^ :class:ArgumentParser objects do not allow two actions with the same option string. By default, :class:ArgumentParser objects raises 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] optional arguments: -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. prog ^^^^ By default, :class:ArgumentParser objects uses sys.argv[0] to determine how to display the name of the program in help messages. This default is almost always desirable because it will make the help messages match how the program was invoked 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):: $ python myprogram.py --help usage: myprogram.py [-h] [--foo FOO] optional arguments: -h, --help show this help message and exit --foo FOO foo help $ cd .. $ python subdir\myprogram.py --help usage: myprogram.py [-h] [--foo FOO] optional arguments: -h, --help show this help message and exit --foo FOO foo help To change this default behavior, another value can be supplied using the prog= argument to :class:ArgumentParser:: parser = argparse.ArgumentParser(prog='myprogram') parser.print_help() usage: myprogram [-h] optional arguments: -h, --help show this help message and exit Note that the program name, whether determined from sys.argv[0] or from the prog= argument, is available to help messages using the %(prog)s format specifier. :: parser = argparse.ArgumentParser(prog='myprogram') parser.add_argument('--foo', help='foo of the %(prog)s program') parser.print_help() usage: myprogram [-h] [--foo FOO] optional arguments: -h, --help show this help message and exit --foo FOO foo of the myprogram program usage ^^^^^ By default, :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 optional arguments: -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 optional arguments: -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. The add_argument() method ------------------------- .. method:: ArgumentParser.add_argument(name or flags..., [action], [nargs], [](#l609) [const], [default], [type], [choices], [required], [](#l610) [help], [metavar], [dest]) 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.
• type_ - The type to which the command-line argument should be converted.
• choices_ - A container 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. The following sections describe how each of these are used. 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: too few arguments 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 supported actions are:

• 'store' - This just stores the argument's value. This is the default action. For example::

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

• 'store_const' - This stores the value specified by the const_ keyword argument. (Note that the const_ keyword argument defaults to the rather unhelpful 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'.split()) Namespace(foo=42)

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

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

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

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

• 'append_const' - This stores a list, and appends the value specified by the const_ keyword argument to the list. (Note that the const_ keyword argument defaults to None.) 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') parser.parse_args('-vvv'.split()) Namespace(verbose=3)

• '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 can also specify an arbitrary action by passing an object that implements the Action API. The easiest way to do this is to extend :class:argparse.Action, supplying an appropriate __call__ method. The __call__ method 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.
• 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. An example of a custom action::

  class FooAction(argparse.Action): ... def call(self, parser, namespace, values, option_string=None): ... print('%r %r %r' % (namespace, values, option_string)) ... setattr(namespace, self.dest, values) ... parser = argparse.ArgumentParser() parser.add_argument('--foo', action=FooAction) parser.add_argument('bar', action=FooAction) args = parser.parse_args('1 --foo 2'.split()) Namespace(bar=None, foo=None) '1' None Namespace(bar='1', foo=None) '2' '--foo' args Namespace(bar='1', foo='2') nargs ^^^^^ ArgumentParser objects usually associate a single command-line argument with a single action to be taken. The nargs keyword argument associates a different number of command-line arguments with a single action. The supported values are:

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

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

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

  parser = argparse.ArgumentParser() parser.add_argument('--foo', nargs='?', const='c', default='d') parser.add_argument('bar', nargs='?', default='d') parser.parse_args('XX --foo YY'.split()) Namespace(bar='XX', foo='YY') parser.parse_args('XX --foo'.split()) Namespace(bar='XX', foo='c') parser.parse_args(''.split()) Namespace(bar='d', foo='d') One of the more common uses of nargs='?' is to allow optional input and output files:: parser = argparse.ArgumentParser() parser.add_argument('infile', nargs='?', type=argparse.FileType('r'), ... default=sys.stdin) parser.add_argument('outfile', nargs='?', type=argparse.FileType('w'), ... default=sys.stdout) parser.parse_args(['input.txt', 'output.txt']) Namespace(infile=<_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='' encoding='UTF-8'>, outfile=<_io.TextIOWrapper name='' encoding='UTF-8'>)

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

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

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

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

• argparse.REMAINDER. All the remaining command-line arguments are gathered into a list. This is commonly useful for command line utilities that dispatch to other command line utilities::

  parser = argparse.ArgumentParser(prog='PROG') parser.add_argument('--foo') parser.add_argument('command') parser.add_argument('args', nargs=argparse.REMAINDER) print(parser.parse_args('--foo B cmd --arg1 XX ZZ'.split())) Namespace(args=['--arg1', 'XX', 'ZZ'], command='cmd', foo='B') 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 ^^^^^ 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.
• 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 instead. See the nargs_ description for examples. The const keyword argument defaults to None. default ^^^^^^^ All optional arguments and some positional arguments may be omitted at the command line. The default keyword argument of :meth:~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'.split()) Namespace(foo='2') parser.parse_args(''.split()) Namespace(foo=42) 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'.split()) Namespace(foo='a') parser.parse_args(''.split()) Namespace(foo=42) Providing default=argparse.SUPPRESS causes no attribute to be added if the command-line argument was not present.:: parser = argparse.ArgumentParser() parser.add_argument('--foo', default=argparse.SUPPRESS) parser.parse_args([]) Namespace() parser.parse_args(['--foo', '1']) Namespace(foo='1') type ^^^^ By default, :class:ArgumentParser objects read command-line arguments in as simple strings. However, quite often the command-line string should instead be interpreted as another type, like a :class:float or :class:int. The type keyword argument of :meth:~ArgumentParser.add_argument allows any necessary type-checking and type conversions to be performed. Common built-in types and functions can be used directly as the value of the type argument:: parser = argparse.ArgumentParser() parser.add_argument('foo', type=int) parser.add_argument('bar', type=open) parser.parse_args('2 temp.txt'.split()) Namespace(bar=<io.TextIOWrapper name='temp.txt' encoding='UTF-8'>, foo=2) To ease the use of various types of files, the argparse module provides the factory FileType which takes the mode= and bufsize= arguments of the :func:open function. For example, FileType('w') can be used to create a writable file:: parser = argparse.ArgumentParser() parser.add_argument('bar', type=argparse.FileType('w')) parser.parse_args(['out.txt']) Namespace(bar=<_io.TextIOWrapper name='out.txt' encoding='UTF-8'>) type= can take any callable that takes a single string argument and returns the converted value:: def perfect_square(string): ... value = int(string) ... sqrt = math.sqrt(value) ... if sqrt != int(sqrt): ... msg = "%r is not a perfect square" % string ... raise argparse.ArgumentTypeError(msg) ... return value ... parser = argparse.ArgumentParser(prog='PROG') parser.add_argument('foo', type=perfect_square) parser.parse_args('9'.split()) Namespace(foo=9) parser.parse_args('7'.split()) usage: PROG [-h] foo PROG: error: argument foo: '7' is not a perfect square The choices keyword argument may be more convenient for type checkers that simply check against a range of values:: parser = argparse.ArgumentParser(prog='PROG') parser.add_argument('foo', type=int, choices=range(5, 10)) parser.parse_args('7'.split()) Namespace(foo=7) parser.parse_args('11'.split()) usage: PROG [-h] {5,6,7,8,9} PROG: error: argument foo: invalid choice: 11 (choose from 5, 6, 7, 8, 9) See the choices_ section for more details. choices ^^^^^^^ Some command-line arguments should be selected from a restricted set of values. These can be handled by passing a container 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='PROG') parser.add_argument('foo', choices='abc') parser.parse_args('c'.split()) Namespace(foo='c') parser.parse_args('X'.split()) usage: PROG [-h] {a,b,c} PROG: error: argument foo: invalid choice: 'X' (choose from 'a', 'b', 'c') Note that inclusion in the choices container is checked after any type_ conversions have been performed, so the type of the objects in the choices container should match the type_ specified:: parser = argparse.ArgumentParser(prog='PROG') parser.add_argument('foo', type=complex, choices=[1, 1j]) parser.parse_args('1j'.split()) Namespace(foo=1j) parser.parse_args('-- -4'.split()) usage: PROG [-h] {1,1j} PROG: error: argument foo: invalid choice: (-4+0j) (choose from 1, 1j) Any object that supports the in operator can be passed as the choices value, so :class:dict objects, :class:set objects, custom containers, etc. are all supported. 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: argparse.py [-h] [--foo FOO] argparse.py: error: option --foo is required 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 ^^^^ 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'.split()) usage: frobble [-h] [--foo] bar [bar ...] positional arguments: bar one of the bars to be frobbled optional arguments: -h, --help show this help message and exit --foo foo the bars before frobbling The help strings can include various format specifiers to avoid repetition of things like the program name or the argument default_. The available specifiers include the program name, %(prog)s and most keyword arguments to :meth:~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) optional arguments: -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] optional arguments: -h, --help show this help message and exit metavar ^^^^^^^ When :class:ArgumentParser generates help messages, it need some way to refer to each expected argument. By default, ArgumentParser objects use the dest_ value as the "name" of each object. By default, for positional argument actions, the dest_ value is used directly, and for optional argument actions, the dest_ value is uppercased. So, 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 optional arguments: -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 optional arguments: -h, --help show this help message and exit --foo YYY Note that metavar only changes the displayed name - the name of the attribute on the :meth:~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] optional arguments: -h, --help show this help message and exit -x X X --foo bar baz 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'.split()) 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') 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. By default, the argument strings are taken from :data:sys.argv, and a new empty :class:Namespace object is created for the attributes. 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'.split()) Namespace(foo=None, x='X') parser.parse_args('--foo FOO'.split()) Namespace(foo='FOO', x=None) For long options (options with names longer than a single character), the option and value can also be passed as a single command-line argument, using = to separate them:: parser.parse_args('--foo=FOO'.split()) Namespace(foo='FOO', x=None) For short options (options only one character long), the option and its value can be concatenated:: parser.parse_args('-xX'.split()) 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'.split()) 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) Argument abbreviations ^^^^^^^^^^^^^^^^^^^^^^ The :meth:~ArgumentParser.parse_args method allows long options to be abbreviated if the abbreviation is unambiguous:: parser = argparse.ArgumentParser(prog='PROG') parser.add_argument('-bacon') parser.add_argument('-badger') parser.parse_args('-bac MMM'.split()) Namespace(bacon='MMM', badger=None) parser.parse_args('-bad WOOD'.split()) Namespace(bacon=None, badger='WOOD') parser.parse_args('-ba BA'.split()) usage: PROG [-h] [-bacon BACON] [-badger BADGER] PROG: error: ambiguous option: -ba could match -badger, -bacon An error is produced for arguments that could produce more than one options. 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=, integers=[1, 2, 3, 4]) parser.parse_args('1 2 3 4 --sum'.split()) Namespace(accumulate=, integers=[1, 2, 3, 4]) 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() Many programs split up their functionality into a number of sub-commands, for example, the svn program can invoke sub-commands like svn[](#l1442) 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 an special action object. This object has a single method, :meth:~ArgumentParser.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. 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='XYZ', 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: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 optional arguments: -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 optional arguments: -h, --help show this help message and exit >>> parser.parse_args(['b', '--help']) usage: PROG b [-h] [--baz {X,Y,Z}] optional arguments: -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} ... optional arguments: -h, --help show this help message and exit subcommands: valid subcommands {foo,bar} additional help Furthermore, add_parser supports an additional aliases argument, which allows multiple strings to refer to the same subparser. This example, like svn, aliases co as a shorthand for checkout:: >>> parser = argparse.ArgumentParser() >>> subparsers = parser.add_subparsers() >>> checkout = subparsers.add_parser('checkout', aliases=['co']) >>> checkout.add_argument('foo') >>> parser.parse_args(['co', 'bar']) Namespace(foo='bar') 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() >>> >>> # 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') FileType objects ^^^^^^^^^^^^^^^^ .. class:: FileType(mode='r', bufsize=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 and buffer sizes:: >>> parser = argparse.ArgumentParser() >>> parser.add_argument('--output', type=argparse.FileType('wb', 0)) >>> parser.parse_args(['--output', 'out']) Namespace(output=<_io.BufferedWriter name='out'>) FileType objects understand the pseudo-argument '-' and automatically convert this into sys.stdin for readable :class:FileType objects and sys.stdout for writable :class:FileType objects:: >>> parser = argparse.ArgumentParser() >>> parser.add_argument('infile', type=argparse.FileType('r')) >>> parser.parse_args(['-']) Namespace(infile=<_io.TextIOWrapper name='' encoding='UTF-8'>) Argument groups ^^^^^^^^^^^^^^^ .. method:: ArgumentParser.add_argument_group(title=None, description=None) By default, :class:ArgumentParser groups command-line arguments into "positional arguments" and "optional arguments" 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. Mutual exclusion ^^^^^^^^^^^^^^^^ .. method:: 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. 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']) 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 overriden 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:: def convert_arg_line_to_args(self, arg_line): for arg in arg_line.split(): if not arg.strip(): continue yield arg 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 before that. .. method:: ArgumentParser.error(message) This method prints a usage message including the message to the standard error and terminates the program with a status code of 2. .. _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. 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 =[](#l1867) 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 :mod:argparse context is called 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>')