The add_argument() method

add_argument(name or flags...[, action][, nargs][, const][, default][, type][, choices][, required][, help][, metavar][, dest])

Define how a single command line argument should be parsed. Each parameter has its own more detailed description below, but in short they are:

  • name or flags - Either a name or a list of option strings, e.g. foo or -f, --foo
  • action - The basic type of action to be taken when this argument is encountered at the command-line.
  • nargs - The number of command-line arguments that should be consumed.
  • const - A constant value required by some action and nargs selections.
  • default - The value produced if the argument is absent from the command-line.
  • type - The type to which the command-line arg 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().

The following sections describe how each of these are used.

name or flags

The add_argument() method needs to know whether you’re expecting an optional argument, e.g. -f or --foo, or a positional argument, e.g. a list of filenames. 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 parse_args() is called, optional arguments will be identified by the - prefix, and the remaining arguments will be assumed to be positional:

>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-f', '--foo')
>>> parser.add_argument('bar')
>>> parser.parse_args(['BAR'])
Namespace(bar='BAR', foo=None)
>>> parser.parse_args(['BAR', '--foo', 'FOO'])
Namespace(bar='BAR', foo='FOO')
>>> parser.parse_args(['--foo', 'FOO'])
usage: PROG [-h] [-f FOO] bar
PROG: error: too few arguments

action

ArgumentParser objects associate command-line args with actions. These actions can do just about anything with the command-line args associated with them, though most actions simply add an attribute to the object returned by parse_args(). When you specify a new argument using the add_argument() method, you can indicate how the command-line args should be handled by specifying the action keyword argument. The supported actions are:

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

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--foo')
    >>> parser.parse_args('--foo 1'.split())
    Namespace(foo='1')
    
  • 'store_const' - This stores the value specified by the const keyword argument. Note that the const keyword argument defaults to None, so you’ll almost always need to provide a value for it. The 'store_const' action is most commonly used with optional arguments that specify some sort of flag. For example:

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--foo', action='store_const', const=42)
    >>> parser.parse_args('--foo'.split())
    Namespace(foo=42)
    
  • 'store_true' and 'store_false' - These store the values True and False respectively. These are basically special cases of 'store_const'. For example:

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--foo', action='store_true')
    >>> parser.add_argument('--bar', action='store_false')
    >>> parser.parse_args('--foo --bar'.split())
    Namespace(bar=False, foo=True)
    
  • 'append' - This stores a list, and appends each argument value to the list. This is useful when you want to allow an option to be specified multiple times. Example usage:

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--foo', action='append')
    >>> parser.parse_args('--foo 1 --foo 2'.split())
    Namespace(foo=['1', '2'])
    
  • 'append_const' - This stores a list, and appends the value specified by the const keyword argument to the list. Note that the const keyword argument defaults to None, so you’ll almost always need to provide a value for it. The 'append_const' action is typically useful when you want multiple arguments 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=[<type 'str'>, <type 'int'>])
    
  • 'version' - This expects a version= keyword argument in the add_argument() call, and prints version information and exits when invoked.

    >>> import argparse
    >>> parser = argparse.ArgumentParser(prog='PROG')
    >>> parser.add_argument('-v', '--version', action='version', version='%(prog)s 2.0')
    >>> parser.parse_args(['-v'])
    PROG 2.0
    

You can also specify an arbitrary action by passing an object that implements the Action API. The easiest way to do this is to extend argparse.Action, supplying an appropriate __call__ method. The __call__ method accepts four parameters:

  • parser - The ArgumentParser object which contains this action.
  • namespace - The namespace object that will be returned by parse_args(). Most actions add an attribute to this object.
  • values - The associated command-line args, with any type-conversions applied. (Type-conversions are specified with the type keyword argument to add_argument().
  • option_string - The option string that was used to invoke this action. The option_string argument is optional, and will be absent if the action is associated with a positional argument.

So for example:

>>> class FooAction(argparse.Action):
...     def __call__(self, parser, namespace, values, option_string=None):
...     print '%r %r %r' % (namespace, values, option_string)
...     setattr(namespace, self.dest, values)
...
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', action=FooAction)
>>> parser.add_argument('bar', action=FooAction)
>>> args = parser.parse_args('1 --foo 2'.split())
Namespace(bar=None, foo=None) '1' None
Namespace(bar='1', foo=None) '2' '--foo'
>>> args
Namespace(bar='1', foo='2')

nargs

ArgumentParser objects usually associate a single command-line argument with a single action to be taken. In the situations where you’d like to associate a different number of command-line arguments with a single action, you can use the nargs keyword argument to add_argument(). The supported values are:

  • N (an integer). N args 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 arg will be consumed from the command-line if possible, and produced as a single item. If no command-line arg 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 arg. In this case the value from const will be produced. Some examples to illustrate this:

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--foo', nargs='?', const='c', default='d')
    >>> parser.add_argument('bar', nargs='?', default='d')
    >>> parser.parse_args('XX --foo YY'.split())
    Namespace(bar='XX', foo='YY')
    >>> parser.parse_args('XX --foo'.split())
    Namespace(bar='XX', foo='c')
    >>> parser.parse_args(''.split())
    Namespace(bar='d', foo='d')
    

    One of the more common uses of nargs='?' is to allow optional input and output files:

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('infile', nargs='?', type=argparse.FileType('r'), default=sys.stdin)
    >>> parser.add_argument('outfile', nargs='?', type=argparse.FileType('w'), default=sys.stdout)
    >>> parser.parse_args(['input.txt', 'output.txt'])
    Namespace(infile=<open file 'input.txt', mode 'r' at 0x...>, outfile=<open file 'output.txt', mode 'w' at 0x...>)
    >>> parser.parse_args([])
    Namespace(infile=<open file '<stdin>', mode 'r' at 0x...>, outfile=<open file '<stdout>', mode 'w' at 0x...>)
    
  • '*'. All command-line args present are gathered into a list. Note that it generally doesn’t make much sense to have more than one positional argument with nargs='*', but multiple optional arguments with nargs='*' is possible. For example:

    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('--foo', nargs='*')
    >>> parser.add_argument('--bar', nargs='*')
    >>> parser.add_argument('baz', nargs='*')
    >>> parser.parse_args('a b --foo x y --bar 1 2'.split())
    Namespace(bar=['1', '2'], baz=['a', 'b'], foo=['x', 'y'])
    
  • '+'. Just like '*', all command-line args present are gathered into a list. Additionally, an error message will be generated if there wasn’t at least one command-line arg present. For example:

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

If the nargs keyword argument is not provided, the number of args consumed is determined by the action. Generally this means a single command-line arg will be consumed and a single item (not a list) will be produced.

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() 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 parse_args(). See the action description for examples.
  • When add_argument() is called with option strings (like -f or --foo) and nargs='?'. This creates an optional argument that can be followed by zero or one command-line args. When parsing the command-line, if the option string is encountered with no command-line arg following it, the value of const will be assumed instead. See the nargs description for examples.

The const keyword argument defaults to None.

default

All optional arguments and some positional arguments may be omitted at the command-line. The default keyword argument of add_argument(), whose value defaults to None, specifies what value should be used if the command-line arg is not present. For optional arguments, the default value is used when the option string was not present at the command line:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', default=42)
>>> parser.parse_args('--foo 2'.split())
Namespace(foo='2')
>>> parser.parse_args(''.split())
Namespace(foo=42)

For positional arguments with nargs ='?' or '*', the default value is used when no command-line arg was present:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('foo', nargs='?', default=42)
>>> parser.parse_args('a'.split())
Namespace(foo='a')
>>> parser.parse_args(''.split())
Namespace(foo=42)

If you don’t want to see an attribute when an option was not present at the command line, you can supply default=argparse.SUPPRESS:

>>> 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, ArgumentParser objects read command-line args in as simple strings. However, quite often the command-line string should instead be interpreted as another type, e.g. float, int or file. The type keyword argument of add_argument() allows any necessary type-checking and type-conversions to be performed. Many common builtin types 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=file)
>>> parser.parse_args('2 temp.txt'.split())
Namespace(bar=<open file 'temp.txt', mode 'r' at 0x...>, foo=2)

To ease the use of various types of files, the argparse module provides the factory FileType which takes the mode= and bufsize= arguments of the file object. 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=<open file 'out.txt', mode 'w' at 0x...>)

If you need to do some special type-checking or type-conversions, you can provide your own types by passing to type= a callable that takes a single string argument and returns the type-converted value:

>>> def perfect_square(string):
...     value = int(string)
...     sqrt = math.sqrt(value)
...     if sqrt != int(sqrt):
...         msg = "%r is not a perfect square" % string
...         raise argparse.ArgumentTypeError(msg)
...     return value
...
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('foo', type=perfect_square)
>>> parser.parse_args('9'.split())
Namespace(foo=9)
>>> parser.parse_args('7'.split())
usage: PROG [-h] foo
PROG: error: argument foo: '7' is not a perfect square

Note that if your type-checking function is just checking for a particular set of values, it may be more convenient to use the choices keyword argument:

>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('foo', type=int, choices=xrange(5, 10))
>>> parser.parse_args('7'.split())
Namespace(foo=7)
>>> parser.parse_args('11'.split())
usage: PROG [-h] {5,6,7,8,9}
PROG: error: argument foo: invalid choice: 11 (choose from 5, 6, 7, 8, 9)

See the choices section for more details.

choices

Some command-line args should be selected from a restricted set of values. ArgumentParser objects can be told about such sets of values by passing a container object as the choices keyword argument to add_argument(). When the command-line is parsed with parse_args(), arg values will be checked, and an error message will be displayed if the arg was not one of the acceptable values:

>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('foo', choices='abc')
>>> parser.parse_args('c'.split())
Namespace(foo='c')
>>> parser.parse_args('X'.split())
usage: PROG [-h] {a,b,c}
PROG: error: argument foo: invalid choice: 'X' (choose from 'a', 'b', 'c')

Note that inclusion in the choices container is checked after any type conversions have been performed, so the type of the objects in the choices container should match the type specified:

>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('foo', type=complex, choices=[1, 1j])
>>> parser.parse_args('1j'.split())
Namespace(foo=1j)
>>> parser.parse_args('-- -4'.split())
usage: PROG [-h] {1,1j}
PROG: error: argument foo: invalid choice: (-4+0j) (choose from 1, 1j)

Any object that supports the in operator can be passed as the choices value, so dict objects, set objects, custom containers, etc. are all supported.

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 change this behavior, i.e. to make an option required, the value True should 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.

Warning: Required options are generally considered bad form - normal users expect options to be optional. You should avoid the use of required options whenever possible.

help

A great command-line interface isn’t worth anything if your users can’t figure out which option does what. So for the end-users, help is probably the most important argument to include in your add_argument() calls. The help value should be a string containing a brief description of what the argument specifies. When a user requests help (usually by using -h or --help at the command-line), these help descriptions will be displayed with each argument:

>>> parser = argparse.ArgumentParser(prog='frobble')
>>> parser.add_argument('--foo', action='store_true',
...         help='foo the bars before frobbling')
>>> parser.add_argument('bar', nargs='+',
...         help='one of the bars to be frobbled')
>>> parser.parse_args('-h'.split())
usage: frobble [-h] [--foo] bar [bar ...]

positional arguments:
  bar     one of the bars to be frobbled

optional arguments:
  -h, --help  show this help message and exit
  --foo   foo the bars before frobbling

The help strings can include various format specifiers to avoid repetition of things like the program name or the argument default. The available specifiers include the program name, %(prog)s and most keyword arguments to 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

metavar

When ArgumentParser objects generate help messages, they need some way to refer to each expected argument. By default, ArgumentParser objects use the dest value as the “name” of each object. By default, for positional argument actions, the dest value is used directly, and for optional argument actions, the dest value is uppercased. So if we have a single positional argument with dest='bar', that argument will be referred to as bar. And if we have a single optional argument --foo that should be followed by a single command-line arg, that arg will be referred to as FOO. You can see this behavior in the example below:

>>> 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

If you would like to provide a different name for your argument in help messages, you can supply a value for the metavar keyword argument to add_argument():

>>> 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. If you’d like to specify a different display name for each of the arguments, you can provide a tuple to metavar:

>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-x', nargs=2)
>>> parser.add_argument('--foo', nargs=2, metavar=('bar', 'baz'))
>>> parser.print_help()
usage: PROG [-h] [-x X X] [--foo bar baz]

optional arguments:
  -h, --help     show this help message and exit
  -x X X
  --foo bar baz

dest

Most 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'.split())
Namespace(bar='XXX')

For optional argument actions, the value of dest is normally inferred from the option strings. ArgumentParser objects generate 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')

If you would like to use a different attribute name from the one automatically inferred by the ArgumentParser, you can supply it with an explicit dest parameter:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', dest='bar')
>>> parser.parse_args('--foo XXX'.split())
Namespace(bar='XXX')

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