http://stackoverflow.com/questions/25626109/python-argparse-conditionally-required-arguments

asks about implementing a 'conditionally-required-arguments' case in `argparse`.  The post-parsing test is simple enough:

    if args.argument and (args.a is None or args.b is None):
        # raise argparse error here

I believe the clearest and shortest expression using Groups is:

    p = ArgumentParser(formatter_class=UsageGroupHelpFormatter)
    g1 = p.add_usage_group(kind='nand', dest='nand1')
    g1.add_argument('--arg', metavar='C')
    g11 = g1.add_usage_group(kind='nand', dest='nand2')
    g11.add_argument('-a')
    g11.add_argument('-b')

The usage is (using !() to mark a 'nand' test):

    usage: issue25626109.py [-h] !(--arg C & !(-a A & -b B))

This uses a 'nand' group, with a 'not-all' test (False if all its actions are present, True otherwise).