Argparse: add method for retrieving the parsers of actions

[dogversioning]

Feature or enhancement

Proposal:

I have a CLI interface with an argparse.ArgumentParser composed of a bunch of different actions. I was trying to validate if an argument had been set to a default value, and with just the main parser available, I was having some issues using get_default() working from the main parser, rather than the parsers for the various actions.

I ended up going with the following comparison, in order to reduce the amount of refactoring I would need to do, where args is the var holding the object returned from parse_args():

        args.my_param == parser._actions[1].choices[args.action]].get_default(args.my_param)

I could also have solved this by adding a wrapper class holding all the subparsers in an object of some kind.

But - I think it would be nice to have a public method for accessing these action parsers, mostly for enabling this kind of use case.

Has this already been discussed elsewhere?

Not that I could find

Links to previous discussion of this feature:

No response

[mpkocher]

Exposing _actions will expose a bunch of private symbols.

In [2]: p = get_parser()

In [3]: p._actions
Out[3]: 
[_HelpAction(option_strings=['-h', '--help'], dest='help', nargs=0, const=None, default='==SUPPRESS==', type=None, choices=None, required=False, help='show this help message and exit', metavar=None, deprecated=False),
 _SubParsersAction(option_strings=[], dest='commands', nargs='A...', const=None, default=None, type=None, choices={'alpha': ArgumentParser(prog='ipython alpha', usage=None, description='Run alpha', formatter_class=<class 'argparse.ArgumentDefaultsHelpFormatter'>, conflict_handler='error', add_help=True), 'beta': ArgumentParser(prog='ipython beta', usage=None, description='Run Beta', formatter_class=<class 'argparse.ArgumentDefaultsHelpFormatter'>, conflict_handler='error', add_help=True), 'gamma': ArgumentParser(prog='ipython gamma', usage=None, description='Run gamma', formatter_class=<class 'argparse.ArgumentDefaultsHelpFormatter'>, conflict_handler='error', add_help=True)}, required=False, help=None, metavar=None, deprecated=False),
 _VersionAction(option_strings=['--version'], dest='version', nargs=0, const=None, default='==SUPPRESS==', type=None, choices=None, required=False, help="show program's version number and exit", metavar=None, deprecated=False)]

Not sure it's a good idea to encourage patterns like [action for action in p.actions() if isinstance(action, argparse._HelpAction)] or similar by using private symbols.

Alternatively, a model defining defaults in a central location might a useful.

from dataclasses import dataclass
import sys
from argparse import ArgumentParser


@dataclass(frozen=True)
class DefaultsAlpha:
    max_records: int = 10


@dataclass(frozen=True)
class DefaultsBeta:
    filter_score: int = 20


DEFAULTS = dict(alpha=DefaultsAlpha(), beta=DefaultsBeta())


def get_parser() -> ArgumentParser:
    p = ArgumentParser()
    sp = p.add_subparsers(dest='commands')

    a = sp.add_parser('alpha')
    a.add_argument('-m', '--max-records', type=int, default=DefaultsAlpha.max_records)

    b = sp.add_parser('beta')
    b.add_argument('-f', '--filter-score', type=int, default=DefaultsBeta.filter_score)

    return p


def main(argv: list[str]) -> int:
    pa = get_parser().parse_args(argv)
    defaults = DEFAULTS[pa.commands]

    if pa.commands == 'alpha':
        if pa.max_records == defaults.max_records:
            print("using default")

    return 0


if __name__ == '__main__':
    sys.exit(main(sys.argv[1:]))

[dogversioning]

Alternatively, a model defining defaults in a central location might a useful.

During review, we did end up going with something like this, though I think the specifics in this are better, so I may update to this pattern.

But, I still think that a 'get all the subparsers' method in a safe way without exposing the private classes would be a nice utility helper.