only allow one arg from a list python code example

Example 1: arg parse array argument

parser.add_argument("--list", nargs="+", default=["a", "b"])

SHELL
$ python3 example.py --list a b c
# List would look like ['a','b','c']

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:])