16.4. argparse
argparse
New in version 3.2.
Source code: Lib/argparse.py
Tutorial
This page contains the API reference information. For a more gentle introduction to Python command-line parsing, have a look at the argparse tutorial.
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
sys.argv
. The
argparse
module also automatically generates help and usage messages and issues errors when users give the program invalid arguments.
16.4.1. 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:
$ python 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:
$ python prog.py 1 2 3 4
4
$ python prog.py 1 2 3 4 --sum
10
If invalid arguments are passed in, it will issue an error:
$ python prog.py a b c
usage: prog.py [-h] [--sum] N [N ...]
prog.py: error: argument N: invalid int value: 'a'
The following sections walk you through this example.
16.4.1.1. Creating a parser
The first step in using the
argparse
is creating an
ArgumentParser
object:
>>>
The
ArgumentParser
object will hold all the information necessary to parse the command line into Python data types.
16.4.1.2. Adding arguments
Filling an
ArgumentParser
with information about program arguments is done by making calls to the
add_argument()
method. Generally, these calls tell the
ArgumentParser
how to take the strings on the command line and turn them into objects. This information is stored and used when
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
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
sum()
function, if
--sum
was specified at the command line, or the
max()
function if it was not.
16.4.1.3. Parsing arguments
ArgumentParser
parses arguments through the
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
Namespace
object will be built up from attributes parsed out of the command line:
>>>
>>> parser.parse_args(['--sum', '7', '-1', '42'])
Namespace(accumulate=<built-in function sum>, integers=[7, -1, 42])
In a script,
parse_args()
will typically be called with no arguments, and the
ArgumentParser
will automatically determine the command-line arguments from
sys.argv
.
16.4.2. ArgumentParser objects
- class
- Create a new
ArgumentParser
- prog - The name of the program (default:
sys.argv[0]
- usage - The string describing the program usage (default: generated from arguments added to parser)
- description - Text to display before the argument help (default: none)
- epilog - Text to display after the argument help (default: none)
- parents - A list of
ArgumentParser
- formatter_class - A class for customizing the help output
- prefix_chars - The set of characters that prefix optional arguments (default: ‘-‘)
- fromfile_prefix_chars - The set of characters that prefix files from which additional arguments should be read (default:
None
- argument_default - The global default value for arguments (default:
None
- conflict_handler - The strategy for resolving conflicting optionals (usually unnecessary)
- add_help - Add a
-h/--help
True
- allow_abbrev - Allows long options to be abbreviated if the abbreviation is unambiguous. (default:
True
- prog - The name of the program (default:
argparse.
ArgumentParser
The following sections describe how each of these are used.
16.4.2.1. prog
By default,
ArgumentParser
objects use
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
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
16.4.2.2. usage
By default,
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.
16.4.2.3. description
Most calls to the
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.
16.4.2.4. epilog
Some programs like to display additional description of the program after the description of the arguments. Such text can be specified using the
epilog=
argument to
ArgumentParser
:
>>>
>>> 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
ArgumentParser
.
16.4.2.5. parents
Sometimes, several parsers share a common set of arguments. Rather than repeating the definitions of these arguments, a single parser with all the shared arguments and passed to
parents=
argument to
ArgumentParser
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
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.
16.4.2.6. formatter_class
ArgumentParser
objects allow the help formatting to be customized by specifying an alternate formatting class. Currently, there are four such classes:
- class
argparse.
RawDescriptionHelpFormatter
argparse.
RawTextHelpFormatter
argparse.
ArgumentDefaultsHelpFormatter
argparse.
MetavarTypeHelpFormatter
RawDescriptionHelpFormatter
and
RawTextHelpFormatter
give more control over how textual descriptions are displayed. By default,
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
RawDescriptionHelpFormatter
as
formatter_class=
indicates that description and epilog are already correctly formatted and should not be line-wrapped:
>>>
>>> parser = argparse.ArgumentParser(
... prog='PROG',
... formatter_class=argparse.RawDescriptionHelpFormatter,
... description=textwrap.dedent('''\
... Please do not mess up this text!
... --------------------------------
... I have indented it
... exactly the way
... I want it
... '''))
>>> parser.print_help()
usage: PROG [-h]
Please do not mess up this text!
--------------------------------
I have indented it
exactly the way
I want it
optional arguments:
-h, --help show this help message and exit
RawTextHelpFormatter
maintains whitespace for all sorts of help text, including argument descriptions. However, multiple new lines are replaced with one. If you wish to preserve multiple blank lines, add spaces between the newlines.
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)
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
16.4.2.7. 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.
16.4.2.8. 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
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
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.
16.4.2.9. argument_default
Generally, argument defaults are specified either by passing a default to
add_argument()
or by calling the
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
ArgumentParser
. For example, to globally suppress attribute creation on
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()
16.4.2.10. allow_abbrev
Normally, when you pass an argument list to the
parse_args()
method of an
ArgumentParser
, it recognizes abbreviations of long options.
This feature can be disabled by setting
allow_abbrev
to
False
:
>>>
>>> parser = argparse.ArgumentParser(prog='PROG', allow_abbrev=False)
>>> parser.add_argument('--foobar', action='store_true')
>>> parser.add_argument('--foonley', action='store_false')
>>> parser.parse_args(['--foon'])
usage: PROG [-h] [--foobar] [--foonley]
PROG: error: unrecognized arguments: --foon
New in version 3.5.
16.4.2.11. conflict_handler
ArgumentParser
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): --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
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
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.
16.4.2.12. 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
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
16.4.3. The add_argument() method
- 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
-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
parse_args()
- name or flags - Either a name or a list of option strings, e.g.
ArgumentParser.
add_argument
The following sections describe how each of these are used.
16.4.3.1. name or flags
The
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
add_argument()
must therefore be either a series of flags, or a simple argument name. For example, an optional argument could be created like:
>>>
while a positional argument could be created like:
>>>
When
parse_args()
is called, optional arguments will be identified by the
-
prefix, and the remaining arguments will be assumed to be positional:
>>>
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-f', '--foo')
>>> parser.add_argument('bar')
>>> parser.parse_args(['BAR'])
Namespace(bar='BAR', foo=None)
>>> parser.parse_args(['BAR', '--foo', 'FOO'])
Namespace(bar='BAR', foo='FOO')
>>> parser.parse_args(['--foo', 'FOO'])
usage: PROG [-h] [-f FOO] bar
PROG: error: the following arguments are required: bar
16.4.3.2. action
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
parse_args()
. The
action
keyword argument specifies how the command-line arguments should be handled. The supplied actions are:
-
'store'
- This just stores the argument’s value. This is the default action. For example:
>>>
>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--foo') >>> parser.parse_args('--foo 1'.split()) Namespace(foo='1') -
'store_const'
'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'
'store_false'
'store_const'
True
False
False
True
respectively. For example:
>>>
>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--foo', action='store_true') >>> parser.add_argument('--bar', action='store_false') >>> parser.add_argument('--baz', action='store_false') >>> parser.parse_args('--foo --bar'.split()) Namespace(foo=True, bar=False, baz=True) -
'append'
- This stores a list, and appends each argument value to the list. 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'
None
'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']) Namespace(verbose=3) -
'help'
ArgumentParser
-
'version'
version=
add_argument()
call, and prints version information and exits when invoked:
>>>
>>> import argparse >>> parser = argparse.ArgumentParser(prog='PROG') >>> parser.add_argument('--version', action='version', version='%(prog)s 2.0') >>> parser.parse_args(['--version']) PROG 2.0
You may also specify an arbitrary action by passing an Action subclass or other object that implements the same interface. The recommended way to do this is to extend
Action
, overriding the
__call__
method and optionally the
__init__
method.
An example of a custom action:
>>>
>>> class FooAction(argparse.Action):
... def __init__(self, option_strings, dest, nargs=None, **kwargs):
... if nargs is not None:
... raise ValueError("nargs not allowed")
... super(FooAction, self).__init__(option_strings, dest, **kwargs)
... def __call__(self, parser, namespace, values, option_string=None):
... print('%r %r %r' % (namespace, values, option_string))
... setattr(namespace, self.dest, values)
...
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', action=FooAction)
>>> parser.add_argument('bar', action=FooAction)
>>> args = parser.parse_args('1 --foo 2'.split())
Namespace(bar=None, foo=None) '1' None
Namespace(bar='1', foo=None) '2' '--foo'
>>> args
Namespace(bar='1', foo='2')
For more details, see
Action
.
16.4.3.3. 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
N
arguments from the command line will be gathered together into a list. For example:
>>>
Note that>>> 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'])nargs=1
-
'?'
. 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:
>>>
One of the more common uses of>>> 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')nargs='?'
is to allow optional input and output files:
>>>
>>> parser = argparse.ArgumentParser() >>> parser.add_argument('infile', nargs='?', type=argparse.FileType('r'), ... default=sys.stdin) >>> parser.add_argument('outfile', nargs='?', type=argparse.FileType('w'), ... default=sys.stdout) >>> parser.parse_args(['input.txt', 'output.txt']) Namespace(infile=<_io.TextIOWrapper name='input.txt' encoding='UTF-8'>, outfile=<_io.TextIOWrapper name='output.txt' encoding='UTF-8'>) >>> parser.parse_args([]) Namespace(infile=<_io.TextIOWrapper name='<stdin>' encoding='UTF-8'>, outfile=<_io.TextIOWrapper name='<stdout>' encoding='UTF-8'>) -
'*'
nargs='*'
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']) -
'+'
'*'
, all command-line args present are gathered into a list. Additionally, an error message will be generated if there wasn’t at least one command-line argument present. For example:
>>>
>>> parser = argparse.ArgumentParser(prog='PROG') >>> parser.add_argument('foo', nargs='+') >>> parser.parse_args(['a', 'b']) Namespace(foo=['a', 'b']) >>> parser.parse_args([]) usage: PROG [-h] foo [foo ...] PROG: error: the following arguments are required: foo
-
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.
16.4.3.4. const
The
const
argument of
add_argument()
is used to hold constant values that are not read from the command line but are required for the various
ArgumentParser
actions. The two most common uses of it are:
- When
add_argument()
action='store_const'
action='append_const'
const
parse_args()
- When
add_argument()
-f
--foo
nargs='?'
const
With the
'store_const'
and
'append_const'
actions, the
const
keyword argument must be given. For other actions, it defaults to
None
.
16.4.3.5. default
All optional arguments and some positional arguments may be omitted at the command line. The
default
keyword argument of
add_argument()
, whose value defaults to
None
, specifies what value should be used if the command-line argument is not present. For optional arguments, the
default
value is used when the option string was not present at the command line:
>>>
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', default=42)
>>> parser.parse_args(['--foo', '2'])
Namespace(foo='2')
>>> parser.parse_args([])
Namespace(foo=42)
If the
default
value is a string, the parser parses the value as if it were a command-line argument. In particular, the parser applies any type conversion argument, if provided, before setting the attribute on the
Namespace
return value. Otherwise, the parser uses the value as is:
>>>
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--length', default='10', type=int)
>>> parser.add_argument('--width', default=10.5, type=int)
>>> parser.parse_args()
Namespace(length=10, width=10.5)
For positional arguments with nargs equal to
?
or
*
, the
default
value is used when no command-line argument was present:
>>>
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('foo', nargs='?', default=42)
>>> parser.parse_args(['a'])
Namespace(foo='a')
>>> parser.parse_args([])
Namespace(foo=42)
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')
16.4.3.6. type
By default,
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
float
or
int
. The
type
keyword argument of
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)
See the section on the default keyword argument for information on when the
type
argument is applied to default arguments.
To ease the use of various types of files, the argparse module provides the factory FileType which takes the
mode=
,
bufsize=
,
encoding=
and
errors=
arguments of the
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'])
Namespace(foo=9)
>>> parser.parse_args(['7'])
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'])
Namespace(foo=7)
>>> parser.parse_args(['11'])
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.
16.4.3.7. 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
add_argument()
. When the command line is parsed, argument values will be checked, and an error message will be displayed if the argument was not one of the acceptable values:
>>>
>>> parser = argparse.ArgumentParser(prog='game.py')
>>> parser.add_argument('move', choices=['rock', 'paper', 'scissors'])
>>> parser.parse_args(['rock'])
Namespace(move='rock')
>>> parser.parse_args(['fire'])
usage: game.py [-h] {rock,paper,scissors}
game.py: error: argument move: invalid choice: 'fire' (choose from 'rock',
'paper', 'scissors')
Note that inclusion in the choices 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='doors.py')
>>> parser.add_argument('door', type=int, choices=range(1, 4))
>>> print(parser.parse_args(['3']))
Namespace(door=3)
>>> parser.parse_args(['4'])
usage: doors.py [-h] {1,2,3}
doors.py: error: argument door: invalid choice: 4 (choose from 1, 2, 3)
Any object that supports the
in
operator can be passed as the choices value, so
dict
objects,
set
objects, custom containers, etc. are all supported.
16.4.3.8. 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
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
,
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.
16.4.3.9. help
The
help
value is a string containing a brief description of the argument. When a user requests help (usually by using
-h
or
--help
at the command line), these
help
descriptions will be displayed with each argument:
>>>
>>> parser = argparse.ArgumentParser(prog='frobble')
>>> parser.add_argument('--foo', action='store_true',
... help='foo the bars before frobbling')
>>> parser.add_argument('bar', nargs='+',
... help='one of the bars to be frobbled')
>>> parser.parse_args(['-h'])
usage: frobble [-h] [--foo] bar [bar ...]
positional arguments:
bar one of the bars to be frobbled
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
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
%%
.
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
16.4.3.10. metavar
When
ArgumentParser
generates help messages, it needs some way to refer to each expected argument. By default, ArgumentParser objects use the dest value as the “name” of each object. By default, for positional argument actions, the dest value is used directly, and for optional argument actions, the dest value is uppercased. So, a single positional argument with
dest='bar'
will be referred to as
bar
. A single optional argument
--foo
that should be followed by a single command-line argument will be referred to as
FOO
. An example:
>>>
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo')
>>> parser.add_argument('bar')
>>> parser.parse_args('X --foo Y'.split())
Namespace(bar='X', foo='Y')
>>> parser.print_help()
usage: [-h] [--foo FOO] bar
positional arguments:
bar
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
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
16.4.3.11. dest
Most
ArgumentParser
actions add some value as an attribute of the object returned by
parse_args()
. The name of this attribute is determined by the
dest
keyword argument of
add_argument()
. 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.
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')
16.4.3.12. 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
add_argument()
.
- class
argparse.
Action
Action objects are used by an ArgumentParser to represent the information needed to parse a single argument from one or more strings from the command line. The Action class must accept the two positional arguments plus any keyword arguments passed to
ArgumentParser.add_argument()
except for the
action
itself.
Instances of Action (or return value of any callable to the
action
parameter) should have attributes “dest”, “option_strings”, “default”, “type”, “required”, “help”, etc. defined. The easiest way to ensure these attributes are defined is to call
Action.__init__
.
Action instances should be callable, so subclasses must override the
__call__
method, which should accept four parameters:
-
parser
-
namespace
Namespace
parse_args()
setattr()
-
values
add_argument()
-
option_string
option_string
The
__call__
method may perform arbitrary actions, but will typically set attributes on the
namespace
based on
dest
and
values
.
16.4.4. The parse_args() method
-
Convert argument strings to objects and assign them as attributes of the namespace. Return the populated namespace.
Previous calls to
add_argument()
add_argument()
- args - List of strings to parse. The default is taken from
sys.argv
- namespace - An object to take the attributes. The default is a new empty
Namespace
- args - List of strings to parse. The default is taken from
ArgumentParser.
parse_args
16.4.4.1. Option value syntax
The
parse_args()
method supports several ways of specifying the value of an option (if it takes one). In the simplest case, the option and its value are passed as two separate arguments:
>>>
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-x')
>>> parser.add_argument('--foo')
>>> parser.parse_args(['-x', 'X'])
Namespace(foo=None, x='X')
>>> parser.parse_args(['--foo', 'FOO'])
Namespace(foo='FOO', x=None)
For long options (options with names longer than a single character), the option and value can also be passed as a single command-line argument, using
=
to separate them:
>>>
>>> parser.parse_args(['--foo=FOO'])
Namespace(foo='FOO', x=None)
For short options (options only one character long), the option and its value can be concatenated:
>>>
>>> parser.parse_args(['-xX'])
Namespace(foo=None, x='X')
Several short options can be joined together, using only a single
-
prefix, as long as only the last option (or none of them) requires a value:
>>>
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-x', action='store_true')
>>> parser.add_argument('-y', action='store_true')
>>> parser.add_argument('-z')
>>> parser.parse_args(['-xyzZ'])
Namespace(x=True, y=True, z='Z')
16.4.4.2. Invalid arguments
While parsing the command line,
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
16.4.4.3. Arguments containing -
-
The
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
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
parse_args()
that everything after that is a positional argument:
>>>
>>> parser.parse_args(['--', '-f'])
Namespace(foo='-f', one=None)
16.4.4.4. Argument abbreviations (prefix matching)
The
parse_args()
method by default allows long options to be abbreviated to a prefix, if the abbreviation is unambiguous (the prefix matches a unique option):
>>>
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-bacon')
>>> parser.add_argument('-badger')
>>> parser.parse_args('-bac MMM'.split())
Namespace(bacon='MMM', badger=None)
>>> parser.parse_args('-bad WOOD'.split())
Namespace(bacon=None, badger='WOOD')
>>> parser.parse_args('-ba BA'.split())
usage: PROG [-h] [-bacon BACON] [-badger BADGER]
PROG: error: ambiguous option: -ba could match -badger, -bacon
An error is produced for arguments that could produce more than one options. This feature can be disabled by setting allow_abbrev to
False
.
16.4.4.5. Beyond sys.argv
sys.argv
Sometimes it may be useful to have an ArgumentParser parse arguments other than those of
sys.argv
. This can be accomplished by passing a list of strings to
parse_args()
. This is useful for testing at the interactive prompt:
>>>
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument(
... 'integers', metavar='int', type=int, choices=range(10),
... nargs='+', help='an integer in the range 0..9')
>>> parser.add_argument(
... '--sum', dest='accumulate', action='store_const', const=sum,
... default=max, help='sum the integers (default: find the max)')
>>> parser.parse_args(['1', '2', '3', '4'])
Namespace(accumulate=<built-in function max>, integers=[1, 2, 3, 4])
>>> parser.parse_args(['1', '2', '3', '4', '--sum'])
Namespace(accumulate=<built-in function sum>, integers=[1, 2, 3, 4])
16.4.4.6. The Namespace object
- class
- Simple class used by default by
parse_args()
argparse.
Namespace
This class is deliberately simple, just an
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,
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
ArgumentParser
assign attributes to an already existing object, rather than a new
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'
16.4.5. Other utilities
16.4.5.1. Sub-commands
- Many programs split up their functionality into a number of sub-commands, for example, the
svn
svn checkout
svn update
svn commit
ArgumentParser
add_subparsers()
add_subparsers()
add_parser()
ArgumentParser
ArgumentParser
object that can be modified as usual.
Description of parameters:
- title - title for the sub-parser group in help output; by default “subcommands” if description is provided, otherwise uses title for positional arguments
- description - description for the sub-parser group in help output, by default
None
- prog - usage information that will be displayed with sub-command help, by default the name of the program and any positional arguments before the subparser argument
- parser_class - class which will be used to create sub-parser instances, by default the class of the current parser (e.g. ArgumentParser)
- action - the basic type of action to be taken when this argument is encountered at the command line
- dest - name of the attribute under which sub-command name will be stored; by default
None
- help - help for sub-parser group in help output, by default
None
- metavar - string presenting available sub-commands in help; by default it is
None
Some example usage:
>>>
Note that the object returned by>>> # 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)parse_args()
a
foo
bar
b
foo
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=
add_parser()
as above.)
>>>
The>>> 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 helpadd_subparsers()
title
description
keyword arguments. When either is present, the subparser’s commands will appear in their own group in the help output. For example:
>>>
Furthermore,>>> 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 helpadd_parser
aliases
svn
co
checkout
:
>>>
One particularly effective way of handling sub-commands is to combine the use of the>>> 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_subparsers()
set_defaults()
so that each subparser knows which Python function it should execute. For example:
>>>
This way, you can let>>> # 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))parse_args()
dest
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')
ArgumentParser.
add_subparsers
16.4.5.2. FileType objects
- class
- The
FileType
ArgumentParser.add_argument()
FileType
open()
function for more details):
>>>
FileType objects understand the pseudo-argument>>> 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'>)'-'
sys.stdin
FileType
sys.stdout
FileType
objects:
>>>
New in version 3.4: The encodings and errors keyword arguments.>>> parser = argparse.ArgumentParser() >>> parser.add_argument('infile', type=argparse.FileType('r')) >>> parser.parse_args(['-']) Namespace(infile=<_io.TextIOWrapper name='<stdin>' encoding='UTF-8'>)
argparse.
FileType
16.4.5.3. Argument groups
- By default,
ArgumentParser
add_argument_group()
method:
>>>
The>>> 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 helpadd_argument_group()
add_argument()
ArgumentParser
add_argument_group()
method accepts title and description arguments which can be used to customize this display:
>>>
Note that any arguments not in your user-defined groups will end up back in the usual “positional arguments” and “optional arguments” sections.>>> 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
ArgumentParser.
add_argument_group
16.4.5.4. Mutual exclusion
- 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:
>>>
The>>> 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 --fooadd_mutually_exclusive_group()
method also accepts a required argument, to indicate that at least one of the mutually exclusive arguments is required:
>>>
Note that currently mutually exclusive argument groups do not support the title and description arguments of>>> 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 requiredadd_argument_group()
ArgumentParser.
add_mutually_exclusive_group
16.4.5.5. Parser defaults
- Most of the time, the attributes of the object returned by
parse_args()
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-level defaults can be particularly useful when working with multiple parsers. See the>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--foo', default='bar') >>> parser.set_defaults(foo='spam') >>> parser.parse_args([]) Namespace(foo='spam')add_subparsers()
ArgumentParser.
set_defaults
- Get the default value for a namespace attribute, as set by either
add_argument()
set_defaults()
:
>>>
>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--foo', default='badger') >>> parser.get_default('foo') 'badger'
ArgumentParser.
get_default
16.4.5.6. Printing help
In most typical applications,
parse_args()
will take care of formatting and printing any usage or error messages. However, several formatting methods are available:
- Print a brief description of how the
ArgumentParser
None
sys.stdout
ArgumentParser.
print_usage
- Print a help message, including the program usage and information about the arguments registered with the
ArgumentParser
None
sys.stdout
ArgumentParser.
print_help
There are also variants of these methods that simply return a string instead of printing it:
- Return a string containing a brief description of how the
ArgumentParser
ArgumentParser.
format_usage
- Return a string containing a help message, including the program usage and information about the arguments registered with the
ArgumentParser
ArgumentParser.
format_help
16.4.5.7. Partial parsing
ArgumentParser.
parse_known_args
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
parse_known_args()
method can be useful. It works much like
parse_args()
except that it does not produce an error when extra arguments are present. Instead, it returns a two item tuple containing the populated namespace and the list of remaining argument strings.
>>>
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', action='store_true')
>>> parser.add_argument('bar')
>>> parser.parse_known_args(['--foo', '--badger', 'BAR', 'spam'])
(Namespace(bar='BAR', foo=True), ['--badger', 'spam'])
Warning
Prefix matching rules apply to
parse_known_args()
. The parser may consume an option even if it’s just a prefix of one of its known options, instead of leaving it in the remaining arguments list.
16.4.5.8. Customizing file parsing
- Arguments that are read from a file (see the fromfile_prefix_chars keyword argument to the
ArgumentParser
convert_arg_line_to_args()
can be overridden for fancier reading.
This method takes a single argument arg_line which is a string read from the argument file. It returns a list of arguments parsed from this string. The method is called once per line read from the argument file, in order.
A useful override of this method is one that treats each space-separated word as an argument. The following example demonstrates how to do this:
class MyArgumentParser(argparse.ArgumentParser): def convert_arg_line_to_args(self, arg_line): return arg_line.split()
ArgumentParser.
convert_arg_line_to_args
16.4.5.9. Exiting methods
- This method terminates the program, exiting with the specified status and, if given, it prints a messagebefore that.
ArgumentParser.
exit
- This method prints a usage message including the message to the standard error and terminates the program with a status code of 2.
ArgumentParser.
error
16.4.6. Upgrading optparse code
Originally, the
argparse
module had attempted to maintain compatibility with
optparse
. However,
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
optparse
had either been copy-pasted over or monkey-patched, it no longer seemed practical to try to maintain the backwards compatibility.
The
argparse
module improves on the standard library
optparse
module in a number of ways including:
- Handling positional arguments.
- Supporting sub-commands.
- Allowing alternative option prefixes like
+
/
- Handling zero-or-more and one-or-more style arguments.
- Producing more informative usage messages.
- Providing a much simpler interface for custom
type
action
A partial upgrade path from
optparse
to
argparse
:
- Replace all
optparse.OptionParser.add_option()
ArgumentParser.add_argument()
- Replace
(options, args) = parser.parse_args()
args = parser.parse_args()
ArgumentParser.add_argument()
options
argparse
args
- Replace
optparse.OptionParser.disable_interspersed_args()
nargs
parse_known_args()
- Replace callback actions and the
callback_*
type
action
- Replace string names for
type
- Replace
optparse.Values
Namespace
optparse.OptionError
optparse.OptionValueError
ArgumentError
- Replace strings with implicit arguments such as
%default
%prog
%(default)s
%(prog)s
- Replace the OptionParser constructor
version
parser.add_argument('--version',action='version', version='<the version>')
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