Added in version 3.2.
Source code:
Note
While argparse is the default recommended standard library module for implementing basic command line applications, authors with more exacting requirements for exactly how their command line applications behave may find it doesn’t provide the necessary level of control. Refer to
Choosing an argument parsing library
for alternatives to consider when argparse doesn’t support behaviors that the application requires (such as entirely disabling support for interspersed options and positional arguments, or accepting option parameter values that start with - even when they correspond to another defined option).
———
The argparse module makes it easy to write user-friendly command-line interfaces. The program defines what arguments it requires, and argparse will figure out how to parse those out of
. The argparse module also automatically generates help and usage messages. The module will also issue errors when users give the program invalid arguments.
The argparse module’s support for command-line interfaces is built around an instance of
. 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
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 argumentparser.add_argument('-c','--count')# option that takes a valueparser.add_argument('-v','--verbose',action='store_true')# on/off flagThe
method runs the parser and places the extracted data in a
object:
args=parser.parse_args()print(args.filename,args.count,args.verbose)ArgumentParser objects
classargparse.ArgumentParser(prog=None, usage=None, description=None, epilog=None, parents=[], formatter_class=argparse.HelpFormatter, prefix_chars='-', fromfile_prefix_chars=None, argument_default=None, conflict_handler='error', add_help=True, allow_abbrev=True, exit_on_error=True, *, suggest_on_error=False, color=True)
Create a new ArgumentParser object. All parameters should be passed as keyword arguments. Each parameter has its own more detailed description below, but in short they are:
- The name of the program (default: generated from the __main__ module attributes and sys.argv[0])
- The string describing the program usage (default: generated from arguments added to parser)
- Text to display before the argument help (by default, no text)
- Text to display after the argument help (by default, no text)
- A list of ArgumentParser objects whose arguments should also be included
- A class for customizing the help output
- The set of characters that prefix optional arguments (default: ‘-‘)
- The set of characters that prefix files from which additional arguments should be read (default: None)
- The global default value for arguments (default: None)
- The strategy for resolving conflicting optionals (usually unnecessary)
- Add a -h/--help option to the parser (default: True)
- Allows long options to be abbreviated if the abbreviation is unambiguous (default: True)
- Determines whether or not ArgumentParser exits with error info when an error occurs. (default: True)
- Enables suggestions for mistyped argument choices and subparser names (default: False)
- Allow color output (default: True)
Changed in version 3.5: allow_abbrev parameter was added.
Changed in version 3.8: In previous versions, allow_abbrev also disabled grouping of short flags such as -vv to mean -v-v.
Changed in version 3.9: exit_on_error parameter was added.
Changed in version 3.14: suggest_on_error and color parameters were added.
The following sections describe how each of these are used.
prog
By default,
calculates the name of the program to display in help messages depending on the way the Python interpreter was run:
The
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 was used.
This default is almost always desirable because it will make the help messages match the string that was used to invoke the program on the command line. However, to change this default behavior, another value can be supplied using the prog= argument to
:
>>> parser=argparse.ArgumentParser(prog='myprogram')>>> parser.print_help()usage: myprogram [-h]options: -h, --help show this help message and exitNote 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 programChanged in version 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,
calculates the usage message from the arguments it contains. The default message can be overridden with the usage= keyword argument:
>>> parser=argparse.ArgumentParser(prog='PROG',usage='%(prog)s [options]')>>> parser.add_argument('--foo',nargs='?',help='foo help')>>> parser.add_argument('bar',nargs='+',help='bar help')>>> parser.print_help()usage: PROG [options]positional arguments: bar bar helpoptions: -h, --help show this help message and exit --foo [FOO] foo helpThe %(prog)s format specifier is available to fill in the program name in your usage messages.
When a custom usage message is specified for the main parser, you may also want to consider passing the prog argument to
or the prog and the usage arguments to add_parser(), to ensure consistent command prefixes and usage information across subparsers.
description
Most calls to the
constructor will use the description= keyword argument. This argument gives a brief description of what the program does and how it works. In help messages, the description is displayed between the command-line usage string and the help messages for the various arguments.
By default, the description will be line-wrapped so that it fits within the given space. To change this behavior, see the
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
:
>>> 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 barsoptions: -h, --help show this help message and exitAnd that's how you'd foo a barAs with the
argument, the epilog= text is by default line-wrapped, but this behavior can be adjusted with the
argument to
.
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
can be used. The parents= argument takes a list of ArgumentParser objects, collects all the positional and optional actions from them, and adds these actions to the ArgumentParser object being constructed:
>>> parent_parser=argparse.ArgumentParser(add_help=False)>>> parent_parser.add_argument('--parent',type=int)>>> foo_parser=argparse.ArgumentParser(parents=[parent_parser])>>> foo_parser.add_argument('foo')>>> foo_parser.parse_args(['--parent','2','XXX'])Namespace(foo='XXX', parent=2)>>> bar_parser=argparse.ArgumentParser(parents=[parent_parser])>>> bar_parser.add_argument('--bar')>>> bar_parser.parse_args(['--bar','YYY'])Namespace(bar='YYY', parent=None)Note that most parent parsers will specify add_help=False. Otherwise, the
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
objects allow the help formatting to be customized by specifying an alternate formatting class. Currently, there are four such classes:
classargparse.RawDescriptionHelpFormatter
classargparse.RawTextHelpFormatter
classargparse.ArgumentDefaultsHelpFormatter
classargparse.MetavarTypeHelpFormatter
and
give more control over how textual descriptions are displayed. By default,
objects line-wrap the
and
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 okayoptions: -h, --help show this help message and exitlikewise for this epilog whose whitespace will be cleaned up and whose wordswill be wrapped across a couple linesPassing
as formatter_class= indicates that
and
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 itoptions: -h, --help show this help message and exit
maintains whitespace for all sorts of help text, including argument descriptions. However, multiple newlines are replaced with one. If you wish to preserve multiple blank lines, add spaces between the newlines.
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)
uses the name of the
argument for each argument as the display name for its values (rather than using the
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] floatpositional arguments: floatoptions: -h, --help show this help message and exit --foo intprefix_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
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
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:
>>> withopen('args.txt','w',encoding=sys.getfilesystemencoding())asfp:... fp.write('-f\nbar')...>>> parser=argparse.ArgumentParser(fromfile_prefix_chars='@')>>> parser.add_argument('-f')>>> parser.parse_args(['-f','foo','@args.txt'])Namespace(f='bar')Arguments read from a file must be one per line by default (but see also
) and are treated as if they were in the same place as the original file referencing argument on the command line. So in the example above, the expression ['-f','foo','@args.txt'] is considered equivalent to the expression ['-f','foo','-f','bar'].
Note
Each line is treated as a single argument, so an empty line is read as an empty string ('').
uses
filesystem encoding and error handler
to read the file containing arguments.
The fromfile_prefix_chars= argument defaults to None, meaning that arguments will never be treated as file references.
argument_default
Generally, argument defaults are specified either by passing a default to
or by calling the
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
. For example, to globally suppress attribute creation on
calls, we supply argument_default=SUPPRESS:
>>> parser=argparse.ArgumentParser(argument_default=argparse.SUPPRESS)>>> parser.add_argument('--foo')>>> parser.add_argument('bar',nargs='?')>>> parser.parse_args(['--foo','1','BAR'])Namespace(bar='BAR', foo='1')>>> parser.parse_args([])Namespace()allow_abbrev
Normally, when you pass an argument list to the
method of an
, it
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: --foonAdded in version 3.5.
conflict_handler
objects do not allow two actions with the same option string. By default, ArgumentParser objects raise an exception if an attempt is made to create an argument with an option string that is already in use:
>>> parser=argparse.ArgumentParser(prog='PROG')>>> parser.add_argument('-f','--foo',help='old foo help')>>> parser.add_argument('--foo',help='new foo help')Traceback (most recent call last): ..ArgumentError: argument --foo: conflicting option string(s): --fooSometimes (e.g. when using
) 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
:
>>> 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 helpNote that
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,
objects add an option which simply displays the parser’s help message. If -h or --help is supplied at the command line, the ArgumentParser help will be printed.
Occasionally, it may be useful to disable the addition of this help option. This can be achieved by passing False as the add_help= argument to
:
>>> 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 helpThe 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 exitexit_on_error
Normally, when you pass an invalid argument list to the
method of an
, it will print a message to
and exit with a status code of 2.
If the user would like to catch errors manually, the feature can be enabled by setting exit_on_error to False:
>>> parser=argparse.ArgumentParser(exit_on_error=False)>>> parser.add_argument('--integers',type=int)_StoreAction(option_strings=['--integers'], dest='integers', nargs=None, const=None, default=None, type=<class 'int'>, choices=None, help=None, metavar=None)>>> try:... parser.parse_args('--integers a'.split())... exceptargparse.ArgumentError:... print('Catching an argumentError')...Catching an argumentErrorAdded in version 3.9.
suggest_on_error
By default, when a user passes an invalid argument choice or subparser name,
will exit with error info and list the permissible argument choices (if specified) or subparser names as part of the error message.
If the user would like to enable suggestions for mistyped argument choices and subparser names, the feature can be enabled by setting suggest_on_error to True. Note that this only applies for arguments when the choices specified are strings:
>>> parser=argparse.ArgumentParser(suggest_on_error=True)>>> parser.add_argument('--action',choices=['debug','dryrun'])>>> parser.parse_args(['--action','debugg'])usage: tester.py [-h] [--action {debug,dryrun}]tester.py: error: argument --action: invalid choice: 'debugg', maybe you meant 'debug'? (choose from debug, dryrun)If you’re writing code that needs to be compatible with older Python versions and want to opportunistically use suggest_on_error when it’s available, you can set it as an attribute after initializing the parser instead of using the keyword argument:
>>> parser=argparse.ArgumentParser(description='Process some integers.')>>> parser.suggest_on_error=TrueAdded in version 3.14.
color
By default, the help message is printed in color using
. If you want plain text help messages, you can disable this
, or in the argument parser itself by setting color to False:
>>> parser=argparse.ArgumentParser(description='Process some integers.',... color=False)>>> parser.add_argument('--action',choices=['sum','max'])>>> parser.add_argument('integers',metavar='N',type=int,nargs='+',... help='an integer for the accumulator')>>> parser.parse_args(['--help'])Note that when color=True, colored output depends on both environment variables and terminal capabilities. However, if color=False, colored output is always disabled, even if environment variables like FORCE_COLOR are set.
Note
Error messages will include color codes when redirecting stderr to a file. To avoid this, set the
or
environment variable (for example, NO_COLOR=1pythonscript.py2>errors.txt).
Added in version 3.14.
The add_argument() method
ArgumentParser.add_argument(nameorflags..., *[, 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:
- Either a name or a list of option strings, e.g. 'foo' or '-f','--foo'.
- The basic type of action to be taken when this argument is encountered at the command line.
- The number of command-line arguments that should be consumed.
- A constant value required by some
and
selections.
- The value produced if the argument is absent from the command line and if it is absent from the namespace object.
- The type to which the command-line argument should be converted.
- A sequence of the allowable values for the argument.
- Whether or not the command-line option may be omitted (optionals only).
- A brief description of what the argument does.
- A name for the argument in usage messages.
- The name of the attribute to be added to the object returned by
.
- Whether or not use of the argument is deprecated.
The method returns an
object representing the argument.
The following sections describe how each of these are used.
name or flags
The
method must know whether an optional argument, like -f or --foo, or a positional argument, like a list of filenames, is expected. The first arguments passed to add_argument() must therefore be either a series of flags, or a simple argument name.
For example, an optional argument could be created like:
>>> parser.add_argument('-f','--foo')while a positional argument could be created like:
>>> parser.add_argument('bar')When
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] barPROG: error: the following arguments are required: barBy default, argparse automatically handles the internal naming and display names of arguments, simplifying the process without requiring additional configuration. As such, you do not need to specify the
and
parameters. For optional arguments, the
parameter defaults to the argument name, with underscores _ replacing hyphens -. The
parameter defaults to the upper-cased name. For example:
>>> parser=argparse.ArgumentParser(prog='PROG')>>> parser.add_argument('--foo-bar')>>> parser.parse_args(['--foo-bar','FOO-BAR'])Namespace(foo_bar='FOO-BAR')>>> parser.print_help()usage: [-h] [--foo-bar FOO-BAR]optional arguments: -h, --help show this help message and exit --foo-bar FOO-BARaction
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
. The action keyword argument specifies how the command-line arguments should be handled. The supplied actions are:
'store' - This just stores the argument’s value. This is the default action.
'store_const' - This stores the value specified by the
keyword argument; note that the
keyword argument defaults to None. The 'store_const' action is most commonly used with optional arguments that specify some sort of flag. For example:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--foo',action='store_const',const=42)>>> parser.parse_args(['--foo'])Namespace(foo=42)
'store_true' and 'store_false' - These are special cases of 'store_const' that respectively store the values True and False with default values of False and True:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--foo',action='store_true')>>> parser.add_argument('--bar',action='store_false')>>> parser.add_argument('--baz',action='store_false')>>> parser.parse_args('--foo --bar'.split())Namespace(foo=True, bar=False, baz=True)
'append' - This appends each argument value to a list. It is useful for allowing an option to be specified multiple times. If the default value is a non-empty list, the parsed value will start with the default list’s elements and any values from the command line will be appended after those default values. Example usage:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--foo',action='append',default=['0'])>>> parser.parse_args('--foo 1 --foo 2'.split())Namespace(foo=['0', '1', '2'])
'append_const' - This appends the value specified by the
keyword argument to a list; note that the
keyword argument defaults to None. The 'append_const' action is typically useful when multiple arguments need to store constants to the same list. For example:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--str',dest='types',action='append_const',const=str)>>> parser.add_argument('--int',dest='types',action='append_const',const=int)>>> parser.parse_args('--str --int'.split())Namespace(types=[<class 'str'>, <class 'int'>])
'extend' - This appends each item from a multi-value argument to a list. The 'extend' action is typically used with the
keyword argument value '+' or '*'. Note that when
is None (the default) or '?', each character of the argument string will be appended to the list. Example usage:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument("--foo",action="extend",nargs="+",type=str)>>> parser.parse_args(["--foo","f1","--foo","f2","f3","f4"])Namespace(foo=['f1', 'f2', 'f3', 'f4'])Added in version 3.8.
'count' - This counts the number of times an argument occurs. For example, this is useful for increasing verbosity levels:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--verbose','-v',action='count',default=0)>>> parser.parse_args(['-vvv'])Namespace(verbose=3)Unless explicitly set, the default will be None. If the default value is a non-zero number, the count starts from that number rather than from zero.
'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
for details of how the output is created.
'version' - This expects a version= keyword argument in the
call, and prints version information and exits when invoked:
>>> importargparse>>> parser=argparse.ArgumentParser(prog='PROG')>>> parser.add_argument('--version',action='version',version='%(prog)s 2.0')>>> parser.parse_args(['--version'])PROG 2.0
You may also specify an arbitrary action by passing an
subclass (e.g.
) or other object that implements the same interface. Only actions that consume command-line arguments (e.g. 'store', 'append', 'extend', or custom actions with non-zero nargs) can be used with positional arguments.
The recommended way to create a custom action is to extend
, overriding the __call__() method and optionally the __init__() and format_usage() methods. You can also register custom actions using the
method and reference them by their registered name.
An example of a custom action:
>>> classFooAction(argparse.Action):... def__init__(self,option_strings,dest,nargs=None,**kwargs):... ifnargsisnotNone:... raiseValueError("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' NoneNamespace(bar='1', foo=None) '2' '--foo'>>> argsNamespace(bar='1', foo='2')For more details, see
.
nargs
objects usually associate a single command-line argument with a single action to be taken. The nargs keyword argument associates a different number of command-line arguments with a single action. See also
Specifying ambiguous arguments
. The supported values are:
N (an integer). N arguments from the command line will be gathered together into a list. For example:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--foo',nargs=2)>>> parser.add_argument('bar',nargs=1)>>> parser.parse_args('c --foo a b'.split())Namespace(bar=['c'], foo=['a', 'b'])Note that nargs=1 produces a list of one item. This is different from the default, in which the item is produced by itself.
'?'. One argument will be consumed from the command line if possible, and produced as a single item. If no command-line argument is present, the value from
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
will be produced. Some examples to illustrate this:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--foo',nargs='?',const='c',default='d')>>> parser.add_argument('bar',nargs='?',default='d')>>> parser.parse_args(['XX','--foo','YY'])Namespace(bar='XX', foo='YY')>>> parser.parse_args(['XX','--foo'])Namespace(bar='XX', foo='c')>>> parser.parse_args([])Namespace(bar='d', foo='d')One of the more common uses of nargs='?' is to allow optional input and output files:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('infile',nargs='?')>>> parser.add_argument('outfile',nargs='?')>>> parser.parse_args(['input.txt','output.txt'])Namespace(infile='input.txt', outfile='output.txt')>>> parser.parse_args(['input.txt'])Namespace(infile='input.txt', outfile=None)>>> parser.parse_args([])Namespace(infile=None, outfile=None)
'*'. 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 arguments present are gathered into a list. Additionally, an error message will be generated if there wasn’t at least one command-line argument present. For example:
>>> parser=argparse.ArgumentParser(prog='PROG')>>> parser.add_argument('foo',nargs='+')>>> parser.parse_args(['a','b'])Namespace(foo=['a', 'b'])>>> parser.parse_args([])usage: PROG [-h] foo [foo ...]PROG: error: the following arguments are required: foo
If the nargs keyword argument is not provided, the number of arguments consumed is determined by the
. Generally this means a single command-line argument will be consumed and a single item (not a list) will be produced. Actions that do not consume command-line arguments (e.g. 'store_const') set nargs=0.
const
The const argument of
is used to hold constant values that are not read from the command line but are required for the various
actions. The two most common uses of it are:
When
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
. See the
description for examples. If const is not provided to add_argument(), it will receive a default value of None.
When
is called with option strings (like -f or --foo) and nargs='?'. This creates an optional argument that can be followed by zero or one command-line arguments. When parsing the command line, if the option string is encountered with no command-line argument following it, the value from const will be used. See the
description for examples.
Changed in version 3.11: const=None by default, including when action='append_const' or action='store_const'.
default
All optional arguments and some positional arguments may be omitted at the command line. The default keyword argument of
, whose value defaults to None, specifies what value should be used if the command-line argument is not present. For optional arguments, the default value is used when the option string was not present at the command line:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--foo',default=42)>>> parser.parse_args(['--foo','2'])Namespace(foo='2')>>> parser.parse_args([])Namespace(foo=42)If the target namespace already has an attribute set, the action default will not overwrite it:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--foo',default=42)>>> parser.parse_args([],namespace=argparse.Namespace(foo=101))Namespace(foo=101)If the default value is a string, the parser parses the value as if it were a command-line argument. In particular, the parser applies any
conversion argument, if provided, before setting the attribute on the
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
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)Because nargs='*' gathers any supplied values into a list, an absent positional argument yields an empty list ([]). Only a non-Nonedefault overrides this (so default=None still gives []).
For
arguments, the default value is ignored. For example, this applies to positional arguments with
values other than ? or *, or optional arguments marked as required=True.
Providing default=argparse.SUPPRESS causes no attribute to be added if the command-line argument was not present:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--foo',default=argparse.SUPPRESS)>>> parser.parse_args([])Namespace()>>> parser.parse_args(['--foo','1'])Namespace(foo='1')type
By default, the parser reads command-line arguments in as simple strings. However, quite often the command-line string should instead be interpreted as another type, such as a
or
. The type keyword for
allows any necessary type-checking and type conversions to be performed.
If the
keyword is used with the
keyword, the type converter is only applied if the default is a string.
The argument to type can be a callable that accepts a single string or the name of a registered type (see
) If the function raises
,
, or
, the exception is caught and a nicely formatted error message is displayed. Other exception types are not handled.
Common built-in types and functions can be used as type converters:
importargparseimportpathlibparser=argparse.ArgumentParser()parser.add_argument('count',type=int)parser.add_argument('distance',type=float)parser.add_argument('street',type=ascii)parser.add_argument('code_point',type=ord)parser.add_argument('datapath',type=pathlib.Path)User defined functions can be used as well:
>>> defhyphenated(string):... return'-'.join([word[:4]forwordinstring.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
function is not recommended as a type converter. All it does is convert empty strings to False and non-empty strings to True. This is usually not what is desired:
>>> parser=argparse.ArgumentParser()>>> _=parser.add_argument('--verbose',type=bool)>>> parser.parse_args(['--verbose','False'])Namespace(verbose=True)See
or action='store_true' for common alternatives.
In general, the type keyword is a convenience that should only be used for simple conversions that can only raise one of the three supported exceptions. Anything with more interesting error-handling or resource management should be done downstream after the arguments are parsed.
For example, JSON or YAML conversions have complex error cases that require better reporting than can be given by the type keyword. A
would not be well formatted and a
exception would not be handled at all.
Even
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
-statement to manage the files.
For type checkers that simply check against a fixed set of values, consider using the
keyword instead.
choices
Some command-line arguments should be selected from a restricted set of values. These can be handled by passing a sequence object as the choices keyword argument to
. When the command line is parsed, argument values will be checked, and an error message will be displayed if the argument was not one of the acceptable values:
>>> parser=argparse.ArgumentParser(prog='game.py')>>> parser.add_argument('move',choices=['rock','paper','scissors'])>>> parser.parse_args(['rock'])Namespace(move='rock')>>> parser.parse_args(['fire'])usage: game.py [-h] {rock,paper,scissors}game.py: error: argument move: invalid choice: 'fire' (choose from 'rock','paper', 'scissors')Any sequence can be passed as the choices value, so
objects,
objects, and custom sequences are all supported.
Use of
is not recommended because it is difficult to control its appearance in usage, help, and error messages.
Note that choices are checked after any
conversions have been performed, so objects in choices should match the
specified. This can make choices appear unfamiliar in usage, help, or error messages.
To keep choices user-friendly, consider a custom type wrapper that converts and formats values, or omit
and handle conversion in your application code.
Formatted choices override the default metavar which is normally derived from dest. This is usually what you want because the user never sees the dest parameter. If this display isn’t desirable (perhaps because there are many choices), just specify an explicit
.
required
In general, the argparse module assumes that flags like -f and --bar indicate optional arguments, which can always be omitted at the command line. To make an option required, True can be specified for the required= keyword argument to
:
>>> 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: --fooAs the example shows, if an option is marked as required,
will report an error if that option is not present at the command line.
Note
Required options are generally considered bad form because users expect options to be optional, and thus they should be avoided when possible.
help
The help value is a string containing a brief description of the argument. When a user requests help (usually by using -h or --help at the command line), these help descriptions will be displayed with each argument.
The help strings can include various format specifiers to avoid repetition of things like the program name or the argument
. The available specifiers include the program name, %(prog)s and most keyword arguments to
, 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 exitAs the help string supports %-formatting, if you want a literal % to appear in the help string, you must escape it as %%.
argparse supports silencing the help entry for certain options, by setting the help value to argparse.SUPPRESS:
>>> parser=argparse.ArgumentParser(prog='frobble')>>> parser.add_argument('--foo',help=argparse.SUPPRESS)>>> parser.print_help()usage: frobble [-h]options: -h, --help show this help message and exitmetavar
When
generates help messages, it needs some way to refer to each expected argument. By default, ArgumentParser objects use the
value as the “name” of each object. By default, for positional argument actions, the
value is used directly, and for optional argument actions, the
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] barpositional arguments: baroptions: -h, --help show this help message and exit --foo FOOAn 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] XXXpositional arguments: XXXoptions: -h, --help show this help message and exit --foo YYYNote that metavar only changes the displayed name - the name of the attribute on the
object is still determined by the
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 bazdest
Most
actions add some value as an attribute of the object returned by
. The name of this attribute is determined by the dest keyword argument of
. For positional argument actions, dest is normally supplied as the first argument to add_argument():
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('bar')>>> parser.parse_args(['XXX'])Namespace(bar='XXX')For optional argument actions, the value of dest is normally inferred from the option strings.
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')Multiple arguments may share the same dest. By default, the value from the last such argument given on the command line wins. Use action='append' to collect values from all of them into a list instead. For conflicting option strings rather than dest names, see
.
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
, 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
when the argument is used:
>>> importargparse>>> parser=argparse.ArgumentParser(prog='snake.py')>>> parser.add_argument('--legs',default=0,type=int,deprecated=True)>>> parser.parse_args([])Namespace(legs=0)>>> parser.parse_args(['--legs','4'])snake.py: warning: option '--legs' is deprecatedNamespace(legs=4)Added in version 3.13.
Action classes
Action classes implement the Action API, a callable which returns a callable which processes arguments from the command-line. Any object which follows this API may be passed as the action parameter to
.
classargparse.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
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
except for the action itself.
Instances of Action (or return value of any callable to the action parameter) should have attributes dest, option_strings, default, type, required, help, etc. defined. The easiest way to ensure these attributes are defined is to call Action.__init__().
__call__(parser, namespace, values, option_string=None)
Action instances should be callable, so subclasses must override the __call__() method, which should accept four parameters:
parser - The
object which contains this action.
namespace - The
object that will be returned by
. Most actions add an attribute to this object using
.
values - The associated command-line arguments, with any type conversions applied. Type conversions are specified with the
keyword argument to
.
option_string - The option string that was used to invoke this action. The option_string argument is optional, and will be absent if the action is associated with a positional argument.
The __call__() method may perform arbitrary actions, but will typically set attributes on the namespace based on dest and values.
format_usage()
Action subclasses can define a format_usage() method that takes no argument and return a string which will be used when printing the usage of the program. If such method is not provided, a sensible default will be used.
classargparse.BooleanOptionalAction
A subclass of
for handling boolean flags with positive and negative options. Adding a single argument such as --foo automatically creates both --foo and --no-foo options, storing True and False respectively:
>>> importargparse>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--foo',action=argparse.BooleanOptionalAction)>>> parser.parse_args(['--no-foo'])Namespace(foo=False)Added in version 3.9.
The parse_args() method
ArgumentParser.parse_args(args=None, namespace=None)
Convert argument strings to objects and assign them as attributes of the namespace. Return the populated namespace.
Previous calls to
determine exactly what objects are created and how they are assigned. See the documentation for add_argument() for details.
- List of strings to parse. The default is taken from
.
- An object to take the attributes. The default is a new empty
object.
Option value syntax
The
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,
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: unrecognized arguments: --bar>>> # wrong number of arguments>>> parser.parse_args(['spam','badger'])usage: PROG [-h] [--foo FOO] [bar]PROG: error: unrecognized arguments: badgerArguments containing -
The
method attempts to give errors whenever the user has clearly made a mistake, but some situations are inherently ambiguous. For example, the command-line argument -1 could either be an attempt to specify an option or an attempt to provide a positional argument. The parse_args() method is cautious here: positional arguments may only begin with - if they look like negative numbers and there are no options in the parser that look like negative numbers:
>>> parser=argparse.ArgumentParser(prog='PROG')>>> parser.add_argument('-x')>>> parser.add_argument('foo',nargs='?')>>> # no negative number options, so -1 is a positional argument>>> parser.parse_args(['-x','-1'])Namespace(foo=None, x='-1')>>> # no negative number options, so -1 and -5 are positional arguments>>> parser.parse_args(['-x','-1','-5'])Namespace(foo='-5', x='-1')>>> parser=argparse.ArgumentParser(prog='PROG')>>> parser.add_argument('-1',dest='one')>>> parser.add_argument('foo',nargs='?')>>> # negative number options present, so -1 is an option>>> parser.parse_args(['-1','X'])Namespace(foo=None, one='X')>>> # negative number options present, so -2 is an option>>> parser.parse_args(['-2'])usage: PROG [-h] [-1 ONE] [foo]PROG: error: unrecognized arguments: -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 argumentIf you have positional arguments that must begin with - and don’t look like negative numbers, you can insert the pseudo-argument '--' which tells
that everything after that is a positional argument:
>>> parser.parse_args(['--','-f'])Namespace(foo='-f', one=None)See also
the argparse howto on ambiguous arguments
for more details.
Changed in version 3.14: Negative-number matching was expanded to include numbers in scientific notation (-2.5e-6), numbers containing underscores (-1_234.5), and complex numbers (-1.2e-3j).
Argument abbreviations (prefix matching)
The
method
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, -baconAn error is produced for arguments that could produce more than one options. This feature can be disabled by setting
to False.
Beyond sys.argv
Sometimes it may be useful to have an
parse arguments other than those of
. This can be accomplished by passing a list of strings to
. This is useful for testing at the interactive prompt:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument(... 'integers',metavar='int',type=int,choices=range(10),... nargs='+',help='an integer in the range 0..9')>>> parser.add_argument(... '--sum',dest='accumulate',action='store_const',const=sum,... default=max,help='sum the integers (default: find the max)')>>> parser.parse_args(['1','2','3','4'])Namespace(accumulate=<built-in function max>, integers=[1, 2, 3, 4])>>> parser.parse_args(['1','2','3','4','--sum'])Namespace(accumulate=<built-in function sum>, integers=[1, 2, 3, 4])The Namespace object
classargparse.Namespace
Simple class used by default by
to create an object holding attributes and return it.
This class is deliberately simple, just an
subclass with a readable string representation. If you prefer to have dict-like view of the attributes, you can use the standard Python idiom,
:
>>> 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
assign attributes to an already existing object, rather than a new Namespace object. This can be achieved by specifying the namespace= keyword argument:
>>> classC:... pass...>>> c=C()>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--foo')>>> parser.parse_args(args=['--foo','BAR'],namespace=c)>>> c.foo'BAR'Other utilities
Subcommands
ArgumentParser.add_subparsers(*[, title][, description][, prog][, parser_class][, action][, dest][, required][, help][, metavar])
Many programs split up their functionality into a number of subcommands, for example, the svn program can invoke subcommands like svncheckout, svnupdate, and svncommit. 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.
supports the creation of such subcommands with the add_subparsers() method. The add_subparsers() method is normally called with no arguments and returns a special action object. This object has a single method, add_parser(), which takes a command name and any ArgumentParser constructor arguments, and returns an ArgumentParser object that can be modified as usual.
Description of parameters:
title - title for the sub-parser group in help output; by default “subcommands” if description is provided, otherwise uses title for positional arguments
description - description for the sub-parser group in help output, by default None
prog - usage information that will be displayed with subcommand help, by default the name of the program and any positional arguments before the subparser argument
parser_class - class which will be used to create sub-parser instances, by default the class of the current parser (e.g.
)
- the basic type of action to be taken when this argument is encountered at the command line
- name of the attribute under which subcommand name will be stored; by default None and no value is stored
- Whether or not a subcommand must be provided, by default False (added in 3.7)
- help for sub-parser group in help output, by default None
- string presenting available subcommands in help; by default it is None and presents subcommands in form {cmd1, cmd2, ..}
Some example usage:
>>> # create the top-level parser>>> parser=argparse.ArgumentParser(prog='PROG')>>> parser.add_argument('--foo',action='store_true',help='foo help')>>> subparsers=parser.add_subparsers(help='subcommand help')>>>>>> # create the parser for the "a" command>>> parser_a=subparsers.add_parser('a',help='a help')>>> parser_a.add_argument('bar',type=int,help='bar help')>>>>>> # create the parser for the "b" command>>> parser_b=subparsers.add_parser('b',help='b help')>>> parser_b.add_argument('--baz',choices=('X','Y','Z'),help='baz help')>>>>>> # parse some argument lists>>> parser.parse_args(['a','12'])Namespace(bar=12, foo=False)>>> parser.parse_args(['--foo','b','--baz','Z'])Namespace(baz='Z', foo=True)Note that the object returned by
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.
If a subparser defines an argument with the same dest as the parent parser, the two share a single namespace attribute, so the parent’s value won’t be retained. Users should give them distinct dest values to keep both.
Similarly, when a help message is requested from a subparser, only the help for that particular parser will be printed. The help message will not include parent parser or sibling parser messages. (A help message for each subparser command, however, can be given by supplying the help= argument to add_parser() as above.)
>>> parser.parse_args(['--help'])usage: PROG [-h] [--foo] {a,b} ...positional arguments: {a,b} subcommand help a a help b b helpoptions: -h, --help show this help message and exit --foo foo help>>> parser.parse_args(['a','--help'])usage: PROG a [-h] barpositional arguments: bar bar helpoptions: -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 helpThe 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 exitsubcommands: valid subcommands {foo,bar} additional helpFurthermore, add_parser() supports an additional aliases argument, which allows multiple strings to refer to the same subparser. This example, like svn, aliases co as a shorthand for checkout:
>>> parser=argparse.ArgumentParser()>>> subparsers=parser.add_subparsers()>>> checkout=subparsers.add_parser('checkout',aliases=['co'])>>> checkout.add_argument('foo')>>> parser.parse_args(['co','bar'])Namespace(foo='bar')add_parser() supports also an additional deprecated argument, which allows to deprecate the subparser.
>>> importargparse>>> parser=argparse.ArgumentParser(prog='chicken.py')>>> subparsers=parser.add_subparsers()>>> run=subparsers.add_parser('run')>>> fly=subparsers.add_parser('fly',deprecated=True)>>> parser.parse_args(['fly'])chicken.py: warning: command 'fly' is deprecatedNamespace()Added in version 3.13.
One particularly effective way of handling subcommands is to combine the use of the add_subparsers() method with calls to
so that each subparser knows which Python function it should execute. For example:
>>> # subcommand functions>>> deffoo(args):... print(args.x*args.y)...>>> defbar(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
do the job of calling the appropriate function after argument parsing is complete. Associating functions with actions like this is typically the easiest way to handle the different actions for each of your subparsers. However, if it is necessary to check the name of the subparser that was invoked, the dest keyword argument to the add_subparsers() call will work:
>>> parser=argparse.ArgumentParser()>>> subparsers=parser.add_subparsers(dest='subparser_name')>>> subparser1=subparsers.add_parser('1')>>> subparser1.add_argument('-x')>>> subparser2=subparsers.add_parser('2')>>> subparser2.add_argument('y')>>> parser.parse_args(['2','frobble'])Namespace(subparser_name='2', y='frobble')Changed in version 3.7: New required keyword-only parameter.
Changed in version 3.14: Subparser’s prog is no longer affected by a custom usage message in the main parser.
FileType objects
classargparse.FileType(mode='r', bufsize=-1, encoding=None, errors=None)
The FileType factory creates objects that can be passed to the type argument of
. Arguments that have FileType objects as their type will open command-line arguments as files with the requested modes, buffer sizes, encodings and error handling (see the
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
for readable FileType objects and
for writable FileType objects:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('infile',type=argparse.FileType('r'))>>> parser.parse_args(['-'])Namespace(infile=<_io.TextIOWrapper name='<stdin>' encoding='UTF-8'>)Note
If one argument uses FileType and then a subsequent argument fails, an error is reported but the file is not automatically closed. This can also clobber the output files. In this case, it would be better to wait until after the parser has run and then use the
-statement to manage the files.
Changed in version 3.4: Added the encoding and errors parameters.
Deprecated since version 3.14.
Argument groups
ArgumentParser.add_argument_group(title=None, description=None, *[, argument_default][, conflict_handler])
By default,
groups command-line arguments into “positional arguments” and “options” when displaying help messages. When there is a better conceptual grouping of arguments than this default one, appropriate groups can be created using the add_argument_group() method:
>>> parser=argparse.ArgumentParser(prog='PROG',add_help=False)>>> group=parser.add_argument_group('group')>>> group.add_argument('--foo',help='foo help')>>> group.add_argument('bar',help='bar help')>>> parser.print_help()usage: PROG [--foo FOO] bargroup: bar bar help --foo FOO foo helpThe add_argument_group() method returns an argument group object which has an
method just like a regular
. When an argument is added to the group, the parser treats it just like a normal argument, but displays the argument in a separate group for help messages. The add_argument_group() method accepts title and description arguments which can be used to customize this display:
>>> parser=argparse.ArgumentParser(prog='PROG',add_help=False)>>> group1=parser.add_argument_group('group1','group1 description')>>> group1.add_argument('foo',help='foo help')>>> group2=parser.add_argument_group('group2','group2 description')>>> group2.add_argument('--bar',help='bar help')>>> parser.print_help()usage: PROG [--bar BAR] foogroup1: group1 description foo foo helpgroup2: group2 description --bar BAR bar helpThe optional, keyword-only parameters
and
allow for finer-grained control of the behavior of the argument group. These parameters have the same meaning as in the
constructor, but apply specifically to the argument group rather than the entire parser.
Note that any arguments not in your user-defined groups will end up back in the usual “positional arguments” and “optional arguments” sections.
Within each argument group, arguments are displayed in help output in the order in which they are added.
Deprecated since version 3.11, removed in version 3.14: Calling add_argument_group() on an argument group now raises an exception. This nesting was never supported, often failed to work correctly, and was unintentionally exposed through inheritance.
Deprecated since version 3.14: Passing
to add_argument_group() is now deprecated.
Mutual exclusion
ArgumentParser.add_mutually_exclusive_group(required=False)
Create a mutually exclusive group. argparse will make sure that only one of the arguments in the mutually exclusive group was present on the command line:
>>> parser=argparse.ArgumentParser(prog='PROG')>>> group=parser.add_mutually_exclusive_group()>>> group.add_argument('--foo',action='store_true')>>> group.add_argument('--bar',action='store_false')>>> parser.parse_args(['--foo'])Namespace(bar=True, foo=True)>>> parser.parse_args(['--bar'])Namespace(bar=False, foo=False)>>> parser.parse_args(['--foo','--bar'])usage: PROG [-h] [--foo | --bar]PROG: error: argument --bar: not allowed with argument --fooThe 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 requiredNote that currently mutually exclusive argument groups do not support the title and description arguments of
. 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 exitGroup title: Group description --foo FOO foo help --bar BAR bar helpDeprecated since version 3.11, removed in version 3.14: Calling
or add_mutually_exclusive_group() on a mutually exclusive group now raises an exception. This nesting was never supported, often failed to work correctly, and was unintentionally exposed through inheritance.
Parser defaults
ArgumentParser.set_defaults(**kwargs)
Most of the time, the attributes of the object returned by
will be fully determined by inspecting the command-line arguments and the argument actions. set_defaults() allows some additional attributes that are determined without any inspection of the command line to be added:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('foo',type=int)>>> parser.set_defaults(bar=42,baz='badger')>>> parser.parse_args(['736'])Namespace(bar=42, baz='badger', foo=736)Note that defaults can be set at both the parser level using set_defaults() and at the argument level using
. If both are called for the same argument, the last default set for an argument is used:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--foo',default='bar')>>> parser.set_defaults(foo='spam')>>> parser.parse_args([])Namespace(foo='spam')Parser-level defaults can be particularly useful when working with multiple parsers. See the
method for an example of this type.
ArgumentParser.get_default(dest)
Get the default value for a namespace attribute, as set by either
or by
:
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--foo',default='badger')>>> parser.get_default('foo')'badger'Printing help
In most typical applications,
will take care of formatting and printing any usage or error messages. However, several formatting methods are available:
ArgumentParser.print_usage(file=None)
Print a brief description of how the
should be invoked on the command line. If file is None,
is assumed.
ArgumentParser.print_help(file=None)
Print a help message, including the program usage and information about the arguments registered with the
. If file is None,
is assumed.
There are also variants of these methods that simply return a string instead of printing it:
ArgumentParser.format_usage()
Return a string containing a brief description of how the
should be invoked on the command line.
ArgumentParser.format_help()
Return a string containing a help message, including the program usage and information about the arguments registered with the
.
Partial parsing
ArgumentParser.parse_known_args(args=None, namespace=None)
Sometimes a script only needs to handle a specific set of command-line arguments, leaving any unrecognized arguments for another script or program. In these cases, the parse_known_args() method can be useful.
This method works similarly to
, but it does not raise an error for extra, unrecognized arguments. Instead, it parses the known arguments and returns a two item tuple that contains the populated namespace and the list of any unrecognized arguments.
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--foo',action='store_true')>>> parser.add_argument('bar')>>> parser.parse_known_args(['--foo','--badger','BAR','spam'])(Namespace(bar='BAR', foo=True), ['--badger', 'spam'])Warning
rules apply to
. The parser may consume an option even if it’s just a prefix of one of its known options, instead of leaving it in the remaining arguments list.
Customizing file parsing
ArgumentParser.convert_arg_line_to_args(arg_line)
Arguments that are read from a file (see the fromfile_prefix_chars keyword argument to the
constructor) are read one argument per line. convert_arg_line_to_args() can be overridden for fancier reading.
This method takes a single argument arg_line which is a string read from the argument file. It returns a list of arguments parsed from this string. The method is called once per line read from the argument file, in order.
A useful override of this method is one that treats each space-separated word as an argument. The following example demonstrates how to do this:
classMyArgumentParser(argparse.ArgumentParser):defconvert_arg_line_to_args(self,arg_line):returnarg_line.split()Note that with this override an argument can no longer contain spaces, since each space-separated word becomes a separate argument.
Exiting methods
ArgumentParser.exit(status=0, message=None)
This method terminates the program, exiting with the specified status and, if given, it prints a message to
before that. The user can override this method to handle these steps differently:
classErrorCatchingArgumentParser(argparse.ArgumentParser):defexit(self,status=0,message=None):ifstatus:raiseException(f'Exiting because of an error: {message}')exit(status)ArgumentParser.error(message)
This method prints a usage message, including the message, to
and terminates the program with a status code of 2.
Intermixed parsing
ArgumentParser.parse_intermixed_args(args=None, namespace=None)
ArgumentParser.parse_known_intermixed_args(args=None, namespace=None)
A number of Unix commands allow the user to intermix optional arguments with positional arguments. The
and parse_known_intermixed_args() methods support this parsing style.
These parsers do not support all the argparse features, and will raise exceptions if unsupported features are used. In particular, subparsers, and mutually exclusive groups that include both optionals and positionals are not supported.
The following example shows the difference between
and
: the former returns ['2','3'] as unparsed arguments, while the latter collects all the positionals into rest.
>>> parser=argparse.ArgumentParser()>>> parser.add_argument('--foo')>>> parser.add_argument('cmd')>>> parser.add_argument('rest',nargs='*',type=int)>>> parser.parse_known_args('doit 1 --foo bar 2 3'.split())(Namespace(cmd='doit', foo='bar', rest=[1]), ['2', '3'])>>> parser.parse_intermixed_args('doit 1 --foo bar 2 3'.split())Namespace(cmd='doit', foo='bar', rest=[1, 2, 3])parse_known_intermixed_args() returns a two item tuple containing the populated namespace and the list of remaining argument strings.
raises an error if there are any remaining unparsed argument strings.
Added in version 3.7.
Registering custom types or actions
ArgumentParser.register(registry_name, value, object)
Sometimes it’s desirable to use a custom string in error messages to provide more user-friendly output. In these cases, register() can be used to register custom actions or types with a parser and allow you to reference the type by their registered name instead of their callable name.
The register() method accepts three arguments - a registry_name, specifying the internal registry where the object will be stored (e.g., action, type), value, which is the key under which the object will be registered, and object, the callable to be registered.
The following example shows how to register a custom type with a parser:
>>> importargparse>>> parser=argparse.ArgumentParser()>>> parser.register('type','hexadecimal integer',lambdas:int(s,16))>>> parser.add_argument('--foo',type='hexadecimal integer')_StoreAction(option_strings=['--foo'], dest='foo', nargs=None, const=None, default=None, type='hexadecimal integer', choices=None, required=False, help=None, metavar=None, deprecated=False)>>> parser.parse_args(['--foo','0xFA'])Namespace(foo=250)>>> parser.parse_args(['--foo','1.2'])usage: PROG [-h] [--foo FOO]PROG: error: argument --foo: invalid 'hexadecimal integer' value: '1.2'Exceptions
exceptionargparse.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.
exceptionargparse.ArgumentTypeError
Raised when something goes wrong converting a command line string to a type.
Guides and Tutorials