argparse: how to configure multiple choice, multiple value, optional argument?

Question

I'm trying to set up an argument that accepts one or more values from a given list of choices, but is not obligatory. I'm trying this (with a couple of variants that also don't work as expected):

parser.add_argument("FLAGS", nargs='*', choices=["X","Y","Z","ALL"])

I expect to get a list of values from the list of choices, or an empty list if nothing was given (that, I think, should be enforced by nargs='*'). But regardless of whether I add default="" or not, when I don't pass any argument it fails with:

error: argument FLAGS: invalid choice: []

How to achieve what I need?

Answer 1

This probably doesn't fit your needs, but you can do it easily with an option like --flags.

parser.add_argument(
    "--flags",
    nargs='*',
    default=[],  # Instead of "None"
    choices=["X", "Y", "Z", "ALL"])

args = parser.parse_args()
print(args)

$ tmp.py
Namespace(flags=[])

$ tmp.py --flags
Namespace(flags=[])

$ tmp.py --flags X
Namespace(flags=['X'])

$ tmp.py --flags X Z
Namespace(flags=['X', 'Z'])

$ tmp.py --flags foobar
usage: tmp.py [-h] [--flags [{X,Y,Z,ALL} ...]]
tmp.py: error: argument --flags: invalid choice: 'foobar' (choose from 'X', 'Y', 'Z', 'ALL')

$ tmp.py --help
usage: tmp.py [-h] [--flags [{X,Y,Z,ALL} ...]]

optional arguments:
  -h, --help            show this help message and exit
  --flags [{X,Y,Z,ALL} ...]