Argparse action-handler trick to add new command line arguments at parsing time depending on value of some other argument

conditional_argparse.py

import argparse
from typing import Dict


class ActionHandler(argparse.Action):
    def __init__(self, *args, **kwargs):
        super(ActionHandler, self).__init__(*args, **kwargs)
        # ran_once is used to flag if __call__ has been run yet. This avoids an infinite loop.
        self.ran_once = False

    def __call__(self, parser, args, values, option_string=None):
        if not self.ran_once:
            self.ran_once = True
            if values == "alpha":
                # Add an extra option called '--bar'
                parser.add_argument("--bar", default=1, type=int)
            elif values == "bravo":
                # Add an extra option called '--baz'
                parser.add_argument("--baz", default=2.0, type=float)
            # Recursive parse, storing results in the same 'args' Namespace object so it's transparent to the caller.
            parser.parse_known_args(namespace=args)


def my_parse_args(parser):
    # Pre-parse onece to populate any extra subparser stuff. Using 'parse_known_args' is permissive
    # and doesn't raise any errors for unrecognized values.
    parser.parse_known_args()
    # Parse for real!
    return parser.parse_args()


parser = argparse.ArgumentParser()
parser.add_argument("--foo", default="alpha", choices=["alpha", "bravo"], action=ActionHandler)
args = my_parse_args(parser)
print(args)


# Test cases to try out
# $ python3 -m conditional_argparse                        # should see (foo=alpha, bar=1)
# $ python3 -m conditional_argparse --foo alpha --bar 2    # should see (foo=alpha, bar=2)
# $ python3 -m conditional_argparse --foo alpha --baz 5.0  # Error! baz is only defined if foo=bravo
# $ python3 -m conditional_argparse --foo bravo --baz 5.0  # should see (foo=alpha, baz=5.0)
# $ python3 -m conditional_argparse --foo bravo --bar 2    # Error! bar is only defined if foo=alpha