p1 = argparse.ArgumentParser()
p1.add_argument('-a', choices=['x', 'y', 'z'])
p2 = argparse.ArgumentParser()
p2.add_argument('-b', required=True)
ns, rest = p1.parse_known_args()
if ns.a == 'x':
    p2.parse_args(rest, ns)
python argparser for multiple arguments for partial choices