I wanted a way to get my argparser to read args from a config file, so came up with this.

ConfigAction.py

from argparse import Action, FileType, _ensure_value


class ConfigAction(Action):
    """
    Given a json file ConfigAction will populate the parser with properties
    defined in the json. This won't override settings from other actions or
    allow settings not defined in the parser initialisation.
    """

    def __init__(self,
                 option_strings,
                 dest,
                 default=None,
                 required=False,
                 help=None):
        super(ConfigAction, self).__init__(
            option_strings=option_strings,
            dest=dest,
            nargs=1,
            default=default,
            type=FileType('r'),
            required=required,
            help=help)

    def __call__(self, parser, namespace, values, option_string=None):
        cfg_values = {}
        # get all
        destinations = [getattr(x, 'dest') for x in parser._actions]
        for i in values:
            cfg_values.update(json.load(i))
        for k, v in cfg_values.items():
            if k in destinations:
                if parser.get_default(k) == _ensure_value(namespace, k, v):
                    setattr(namespace, k, v)

inyerfile.py

def main():
    parser = argparse.ArgumentParser(
        description='Do some things'
    )
    parser.register('action', 'config', ConfigAction)
    parser.add_argument('--config', action='config')