import argparse

if __name__ == '__main__':
    parser = argparse.ArgumentParser(
        formatter_class=argparse.ArgumentDefaultsHelpFormatter
    )

    sub_parser = parser.add_subparsers(title='sub commands',
                                       help='sub commands help')

    foo = sub_parser.add_parser('foo')
    foo.add_argument(
        '--a',
        action='store',
        default='1234',
        help='A'
    )

    parser.parse_args(['foo', '--help'])

usage: test_args.py foo [-h] [--a A]

optional arguments:
  -h, --help  show this help message and exit
  --a A       A

Process finished with exit code 0