argparse: Default value for nargs='*'?

Question

I tried to use this statement :

parser = argparse.ArgumentParser()
parser.add_argument('-m', '--music', nargs='*', default=False, const=True)
args = parser.parse_args()
print(args.music)

But got this error:

`builtins.ValueError: nargs must be '?' to supply const`

what i want to do is :

• if -m is in args list but whithout any value, args.music will give me True
• if -m is in args list and have 'N' values, args.music will give me a list of all values
• if -m is not in args list, args,music will return False

the second and lastOne worked but, when i try to use const i got an error

Answer 1

You could use a custom action:

import argparse
class EmptyIsTrue(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        if len(values) == 0:
            values = True
        setattr(namespace, self.dest, values)

parser = argparse.ArgumentParser()
parser.add_argument('-m', '--music', nargs='*', default=False, action=EmptyIsTrue)
print(parser.parse_args([]))
# Namespace(music=False)

print(parser.parse_args(['-m']))
# Namespace(music=True)

print(parser.parse_args('-m 1 2'.split()))
# Namespace(music=['1', '2'])

If you have only one argument to handle this way, then

arg.music = True if len(arg.music) == 0 else arg.music

is simpler. If you have many such arguments, then defining a custom action could reduce the repetition, and help ensure all those arguments are treated the same way.