Cause Python's argparse to execute action for default

Question

I am using argparse\'s action to add various data to a class. I would like to use that action on the default value if that arg is not provided at the command line. Is this possi

Answer 1

I needed to prompt the user if an option was not specified - that's how I did it:

class _PromptUserAction(argparse.Action):

    def __call__(self, parser, namespace, values, option_string=None):
        if values == self.default:
            print 'Please specify', self.dest
            values = raw_input('>')
        setattr(namespace, self.dest, values)

class Parser:
    def __init__(self, desc, add_h=True):
        self.parser = argparse.ArgumentParser(description=desc, add_help=add_h,
                        formatter_class=argparse.ArgumentDefaultsHelpFormatter)
        #actions to be run on options if not specified (using default to check)
        self.actions = []

    @staticmethod
    def new(description, add_help=True):
        return Parser(description, add_help)

    # ...

    def milestone(self, help_='Specify the milestone for latest release.'):
        action = self.parser.add_argument('-m', '--milestone',
                                          dest='milestone',
                                          action=_PromptUserAction,
                                          default='PROMPT', # needed I think
                                          type=str,
                                          help=help_)
        self.actions.append(action)
        return self

    def parse(self):
        """
        Return an object which can be used to get the arguments as in:
            parser_instance.parse().milestone

        :return: ArgumentParser
        """
        args = self.parser.parse_args()
        # see: http://stackoverflow.com/a/21588198/281545
        dic = vars(args)
        ns = argparse.Namespace()
        for a in self.actions:
            if dic[a.dest] == a.default:
                a(self.parser, ns, a.default) # call action
        # duh - can I avoid it ?
        import sys
        return self.parser.parse_args(sys.argv[1:],ns)

I am interested if this can somehow be done without having to reparse the args (the import sys part). Maybe some constructor options for argparse.Action ?