Argparse argument with default and optional value

---

• Instead of default we use the const parameter here
• We tell argparse that the value of the parameter is optional by nargs='?'

examples/argparse/argument_with_optional_value.py

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('--level',     help='Some level', type=int, const=10, nargs='?')
args = parser.parse_args()

print(args.level)

$ python argument_with_optional_value.py
None

$ python argument_with_optional_value.py --level
10

$ python argument_with_optional_value.py --level 20
20