Argparse: How to create the equivalent of const with nargs='*'

Question

Is there a way to produce the equivalent of const (that we can use with nargs='?', see reference question here for an example), but for nargs='*'. Meaning that I would want:

import argparse

argparser = argparse.ArgumentParser()
argparser.add_argument('--option', nargs='*', const=[1, 2, 3])
print(argparser.parse_args())

And then during usage:

my_script.py               #  Namespace(option=None)
my_script.py --option      #  Namespace(option=[1, 2, 3])
my_script.py --option 4 5  #  Namespace(option=[4, 5])

Currently I get ValueError: nargs must be '?' to supply const

Without the const the second case should produce option=[]. You could easily detect and replace that after parsing. — hpaulj
– hpaulj, Commented Jun 29, 2022 at 15:49
The error message you get comes from an explicit test in the store Action class "if const is not None and nargs != OPTIONAL:". In _get_values 'OPTIONAL' has a special step to handle const. Other nargs do not. _get_values is called before the Action's __call__. — hpaulj
– hpaulj, Commented Jun 29, 2022 at 15:55
@hpaulj Yes, but checking "empty but not None" feels a bit odd. const is more straightforward. Why is argparse not enabling const with *? It's just inconsistent... — mara004
– mara004, Commented Jan 14, 2024 at 18:56
@mara004, I used to follow the bug/issues religiously, and don't recall any suggesting expending the use of const. I don't know the thinking of the original developer, but my guess is the const` was included for use by 'store_const' and its sublclass 'store_true'.. Its use in the 3 way '?' is a bonus. If argparse doesn't do everything you want it's perfect ok (even encouraged) to write a custom action class, or to do your own post-parsing checking. — hpaulj
– hpaulj, Commented Jan 14, 2024 at 21:45
Part of the style issue with checking against [] is that a) we can't use default=[] anymore; b) We rely on the detail of nargs="*" producing a list, not a tuple. So you'd have to do if args.myarg is None: {default}; elif not args.myarg: {const}. Not very idiomatic IMO. — mara004
– mara004, Commented Jan 15, 2024 at 20:57

Yaakov Bressler · Accepted Answer · 2022-06-29 15:00:31Z

1

You can use a custom action to achieve this outcome:

import argparse

p = argparse.ArgumentParser()

class customAction(argparse.Action):
    """
    Customized argparse action, will set the
    value in the following way:

        1) If no option_string is supplied: set to None

        2) If option_string is supplied:

            2A) If values are supplied:
                set to list of values

            2B) If no values are supplied:
                set to default value (`self.const`)

    NOTES:
        If `const` is not set, default value (2A) will be None
    """
    def __call__(self, parser, namespace, values, option_string=None):
        if option_string:
            setattr(namespace, self.dest, self.const)
        elif not values:
            setattr(namespace, self.dest, None)
        else:
            setattr(namespace, self.dest, values)


p.add_argument('--option',
    nargs='*',
    action=customAction,
    dest='option',
    type=int,
    const=[1, 2, 3]
)

edited Jun 29, 2022 at 15:00

answered Jun 29, 2022 at 14:43

Yaakov Bressler

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1 Comment

EmmaRenauld Over a year ago

Thanks! This was a little 'overkill' for the context of my work but it does work!

mara004 · Accepted Answer · 2024-01-14 21:49:38Z

1

Simplification of @YaakovBressler's code:

class ConstWithMultiArgs (argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        obj = values if values else self.const if option_string else self.default
        setattr(namespace, self.dest, obj)

edited Jan 14, 2024 at 21:49

answered Jan 14, 2024 at 19:36

mara004

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