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
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
[]
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
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.
How about the required argument:parser.add_argument("-a", "--action", nargs=2, action="store", help="do some action", required=False)