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 output:
# ./script.py -a --> works
# ./script.py -a val1 --> error
# ./script.py -a val1 val2 --> works

version 1 -- accepts 0 or 1 values:

parser = argparse.ArgumentParser()
parser.add_argument("-a", "--action", nargs="?", const=True, action="store", help="do some action")
args = parser.parse_args()

# output:
# ./script.py -a --> works
# ./script.py -a val1 --> works
# ./script.py -a val1 val2 --> error

version 2 - accepts exactly 2 values:

parser = argparse.ArgumentParser()
parser.add_argument("-a", "--action", nargs=2, action="store", help="do some action")
args = parser.parse_args()

# output:
# ./script.py -a --> error
# ./script.py -a val1 --> error
# ./script.py -a val1 val2 --> works

How do you combine these 2 different versions so that the script accepts 0 or 2 values for the argument, but rejects it when it only has 1 value?

Answer 1

You'll have to do your own error checking here. Accept 0 or more value, and reject anything other than 0 or 2:

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

if args.action is not None and len(args.action) not in (0, 2):
    parser.error('Either give no values for action, or two, not {}.'.format(len(args.action)))

Note that args.action is set to None when no -a switch was used:

>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument("-a", "--action", nargs='*', action="store", help="do some action")
_StoreAction(option_strings=['-a', '--action'], dest='action', nargs='*', const=None, default=None, type=None, choices=None, help='do some action', metavar=None)
>>> args = parser.parse_args([])
>>> args.action is None
True
>>> args = parser.parse_args(['-a'])
>>> args.action
[]

Answer 2

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.