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

Have your option take a single, optional comma-separated string. You'll use a custom type to convert that string to a list and verify that it has exactly two items.

def pair(value):
    rv = value.split(',')
    if len(rv) != 2:
        raise argparse.ArgumentParser()
    return rv

parser.add_argument("-a", "--action", nargs='?',
                    type=pair, metavar='val1,val2',
                    help="do some action")
print parser.parse_args()

Then you'd use it like

$ ./script.py -a
Namespace(action=None)
$ ./script.py -a val1,val2
Namespace(action=['val1','val2'])
$ ./script.py -a val1
usage: tmp.py [-h] [-a [ACTION]]
script.py: error: argument -a/--action: invalid pair value: 'val1'
$ ./script.py -a val1,val2,val3
usage: tmp.py [-h] [-a [ACTION]]
script.py: error: argument -a/--action: invalid pair value: 'val1,val2,val3'

You can adjust the definition of pair to use a different separator and to return something other than a list (a tuple, for example).

The metavar provides a better indication that the argument to action is a pair of values, rather than just one.

$ ./script.py -h
usage: script.py [-h] [-a [val1,val2]]

optional arguments:
  -h, --help            show this help message and exit
  -a [val1,val2], --action [val1,val2]
                        do some action