Parse arguments with defaults

another_script.py

from run_something import main as run_something

run_something({'input':'a.txt',
               'output':'b.txt'})
# Output: default 5


run_something({'input':'a.txt',
               'default':10,
               'output':'b.txt'})
# Output: default 10

parsing_utils.py

from argparse import Namespace

def parse_args(argument_parser, prefetched_args=None):
    if not prefetched_args:
        # If prefetched arguments are empty - just use regular parsing
        prefetched_args = argument_parser.parse_args()
    else:
        # if prefetched arguments are presented - use them and add default from argument_parser
        if isinstance(prefetched_args, dict):
            prefetched_args = Namespace(**prefetched_args)
        for action in argument_parser._actions:
            prefetched_args.__dict__[action.dest] = prefetched_args.__dict__.get(action.dest, action.default)
    return prefetched_args

run_something.py

import argparse
from parsing_utils import parse_args

def main(args=None):
    ap = argparse.ArgumentParser()
    ap.add_argument('-i', '--input', required=True)
    ap.add_argument('-d', '--default', default=5)
    ap.add_argument('-o', '--output', required=True)

    args = parse_args(ap, args)
    
    print('default', args.default)



if __name__ == '__main__':
    main()

# python run.py -i a.txt -o b.txt
# Output: default 5