Differentiate between optional and mandatory in Pythons argparse

Question

I am a bit confused with argparse but using it for years. Let's see this example:

$ ./argpars_debug.py foo --help
usage: argpars_debug.py [-h] [-d] [--version] input

positional arguments:
  input

options:
  -h, --help   show this help message and exit
  -d, --debug
  --version    show program's version number and exit

Problem

The input argument is mandatory. But using --version without input is possible.

$ ./argpars_debug.py --version
0.1.2

But using --debug without input is not possible:

$ ./argpars_debug.py --debug
usage: argpars_debug.py [-h] [-d] [--version] input
argpars_debug.py: error: the following arguments are required: input

My Goal

I would like to use --debug with and without input.

$ ./argpars_debug.py --debug
$ ./argpars_debug.py foo --debug

I can do this with --version. But I don't understand how I can modify this behavior.

The minimal working example

#!/usr/bin/env python3
import argparse
import sys

def main():
    parser = argparse.ArgumentParser()

    parser.add_argument('input', type=str)
    parser.add_argument('-d', '--debug', action='store_true')
    parser.add_argument('--version', action='version', version='0.1.2')

    return parser.parse_args()


if __name__ == '__main__':
    main()
    print(f'{sys.argv=}')