[python] Python argparse: default value or specified value

I would like to have a optional argument that will default to a value if only the flag is present with no value specified, but store a user-specified value instead of the default if the user specifies a value. Is there already an action available for this?

An example:

python script.py --example
# args.example would equal a default value of 1
python script.py --example 2
# args.example would equal a default value of 2

I can create an action, but wanted to see if there was an existing way to do this.

The difference between:

parser.add_argument("--debug", help="Debug", nargs='?', type=int, const=1, default=7)

and

parser.add_argument("--debug", help="Debug", nargs='?', type=int, const=1)

is thus:

myscript.py => debug is 7 (from default) in the first case and "None" in the second

myscript.py --debug => debug is 1 in each case

myscript.py --debug 2 => debug is 2 in each case

Actually, you only need to use the default argument to add_argument as in this test.py script:

import argparse

if __name__ == '__main__':

    parser = argparse.ArgumentParser()
    parser.add_argument('--example', default=1)
    args = parser.parse_args()
    print(args.example)

test.py --example
% 1
test.py --example 2
% 2

Details are here.