passing None in argparse argument list code example

Example 1: argparse list of options

parser.add_argument('--list',
                    default='all',
                    const='all',
                    nargs='?',
                    choices=['servers', 'storage', 'all'],
                    help='list servers, storage, or both (default: %(default)s)')

Example 2: argparse accept only few options

...
parser.add_argument('--val',
                    choices=['a', 'b', 'c'],
                    help='Special testing value')

args = parser.parse_args(sys.argv[1:])