Providing default arguments to argparse.ArgumentParser with a config file.

clicfgtest.py

#!/usr/bin/env python
from __future__ import print_function

import argparse
import json
import sys


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

    parser.add_argument(
        '--conf', type=argparse.FileType('r'),
        help='Configuration file'
    )
    args, remaining = parser.parse_known_args()
    defaults = {}
    if args.conf:
        try:
            defaults = json.load(args.conf)
        except ValueError:
            print('Error: Could not parse config file', file=sys.stderr)
            sys.exit(1)
        finally:
            args.conf.close()

    parser.add_argument(
        '--method', type=str, required=('method' not in defaults),
        choices=['GET', 'PUT', 'POST', 'DELETE'],
        help='HTTP method'
    )
    parser.add_argument(
        '--value', type=str, default='foo',
        help="An argument with a default value"
    )

    parser.set_defaults(**defaults)

    return parser.parse_args(remaining)


def main():
    print(process_args())


if __name__ == '__main__':
    main()