#! /usr/bin/python

import argparse

class MutuallyExclusiveParameter(argparse.Action):
    def __init__(self, option, **kwargs):
        # print('init', repr(option), repr(kwargs))
        self.option = option
        super().__init__(**kwargs)

    def __call__(self, parser, namespace, values, option_string=None):
        # print('call', repr(values), repr(option_string))
        setattr(namespace, self.option, True)
        setattr(namespace, self.dest, values)

parser = argparse.ArgumentParser(description='Play with options.',
    formatter_class=argparse.ArgumentDefaultsHelpFormatter)
group = parser.add_mutually_exclusive_group()
group.add_argument('-a', dest='num', default=1, type=int,
    action=lambda **kwargs: MutuallyExclusiveParameter('a', **kwargs),
    help='help a')
group.add_argument('-b', dest='num', default=2, type=int,
    action=lambda **kwargs: MutuallyExclusiveParameter('b', **kwargs),
    help='help b')

def p(l):
    print(repr(parser.parse_args(l)))

p([])
p(['-a', '11'])
p(['-b', '22'])
try:
    p(['-h'])
except:
    pass
p(['-a111', '-b222'])