argparse — requiring either 2 values or none for an optional argument

Question

I\'m trying to make an optional argument for a script that can either take no values or 2 values, nothing else. Can you accomplish this using argparse?

# desired         


        

Answer 1

Just handle that case yourself:

parser.add_argument("-a", "--action", nargs='*', action="store", help="do some action")
args = parser.parse_args()

if args.action is not None:
    if len(args.action) not in (0, 2):
        parser.error('Specify no or two actions')

    # action was specified but either there were two actions or no action
else:
    # action was not specified

Of course you should update the help text in that case so that the user has a chance to know this before running into the error.