import argparse
parser = argparse.ArgumentParser()
parser.add_argument('--format', required=True, choices=['binary', 'xml', 'text'])
args, rem_args = parser.parse_known_args()
if (args.format == "text"):
    parser.add_argument('--encoding', required=True)
    parser.parse_args(rem_args, namespace=args) 
    # this does not work since required argument has already been consumed