There's a pattern I have developed for my python scripts of writing a object like that :

    # where a and b are functions
    FOO = { a.__name__: a, b.__name__: b }

and with argparse do

    parser = argparse.ArgumentParser()
    parser.add_argument(
        "-f",
        "--foo",
        choices=FOO.keys(),
        default=next(iter(FOO)),
        help="choose your type of foo",
    )