Option accepted with and without value

Question

I have a small script and I need it to be able to accept parameter with value and withou value.

./cha.py --pretty-xml
./cha.py --pretty-xml=5

I

Answer 1

Use the const keyword:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument("--pretty-xml", nargs="?", type=int, dest="xml_space", const=4)
print(parser.parse_args([]))
print(parser.parse_args(['--pretty-xml']))
print(parser.parse_args(['--pretty-xml=5']))

results in

Namespace(xml_space=None)
Namespace(xml_space=4)
Namespace(xml_space=5)

Answer 2

Leave out the default parameter and use a custom Action instead:

class PrettyXMLAction(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        if not values:
            values = 4
        setattr(namespace, self.dest, values)

parser.add_argument('--pretty-xml', nargs='?', type=int, dest='xml_space', action=PrettyXMLAction)

Demo:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--pretty-xml', nargs='?', type=int, dest='xml_space', action=PrettyXMLAction)
PrettyXMLAction(option_strings=['--pretty-xml'], dest='xml_space', nargs='?', const=None, default=None, type=None, choices=None, help=None, metavar=None)
>>> parser.parse_args('--pretty-xml'.split())
Namespace(xml_space=4)
>>> parser.parse_args('--pretty-xml=5'.split())
Namespace(xml_space=5)
>>> parser.parse_args(''.split())
Namespace(xml_space=None)