Check if ArgParse optional argument is set, default value supplied

I'm trying to check if an optional argument is supplied by the user but I want to set its default value if it is not supplied. If I try to use if statement by exploiting None, the default in help string says None which is incorrect

import argparse
parser = argparse.ArgumentParser(usage=__file__ + " [--options]",
                                 description=__doc__, 
         formatter_class=argparse.ArgumentDefaultsHelpFormatter)

parser.add_argument("--scheme", choices=["A", "B"])
parser.add_argument("--min-A", type=float, help="Default=5")
parser.add_argument("--min-B", type=float, help="Default=10")
args = parser.parse_args()

if args.scheme == "A":
    if args.min_A:
        min_A = args.min_A
    else:
        min_A = 5
else:
    if args.min_A:
        parser.error("--min-A can only be provided when --scheme is A")

#similar code for B

Here, the help string automatically puts default as None. If I put default as 5, then I'm not able to check whether --min-A has been provided by user because it stores the default instead of None when the option is absent in input.

formatter_class=argparse.ArgumentDefaultsHelpFormatter

^ This custom formatter class is your problem. You can just remove it. Then your help text will render like that:

usage: myscript.py [--options]

options:
  -h, --help      show this help message and exit
  --scheme {A,B}
  --min-A MIN_A   Default=5
  --min-B MIN_B   Default=10