How to do it...

1. Prepare the following task and save it as prepare_task_step1.py:

import argparse


def main(number, other_number):
    result = number * other_number
    print(f'The result is {result}')


if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument('-n1', type=int, help='A number', default=1)
    parser.add_argument('-n2', type=int, help='Another number', default=1)

    args = parser.parse_args()

    main(args.n1, args.n2)

2. Update the file to define a config file that contains both arguments, and save it as prepare_task_step2.py. Note that defining a config file overwrites any command-line parameters:

import argparse
import configparser


def main(number, other_number):
    result = number * other_number
    print(f'The result is {result}')


if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument('-n1', type=int, help='A number', default=1)
    parser.add_argument('-n2', type=int, help='Another number', default=1)

    parser.add_argument('--config', '-c', type=argparse.FileType('r'),
                        help='config file')

    args = parser.parse_args()
    if args.config:
        config = configparser.ConfigParser()
        config.read_file(args.config)
        # Transforming values into integers
        args.n1 = int(config['DEFAULT']['n1'])
        args.n2 = int(config['DEFAULT']['n2'])

    main(args.n1, args.n2)

3. Create the config file config.ini:

[ARGUMENTS]
n1=5
n2=7

4. Run the command with the config file. Note that the config file overwrites the command-line parameters, as described in step 2:

$ python3 prepare_task_step2.py -c config.ini
The result is 35
$ python3 prepare_task_step2.py -c config.ini -n1 2 -n2 3
The result is 35

5. Add a parameter to store the result in a file, and save it as prepare_task_step5.py:

import argparse
import sys
import configparser


def main(number, other_number, output):
    result = number * other_number
    print(f'The result is {result}', file=output)


if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument('-n1', type=int, help='A number', default=1)
    parser.add_argument('-n2', type=int, help='Another number', default=1)

    parser.add_argument('--config', '-c', type=argparse.FileType('r'),
                        help='config file')
    parser.add_argument('-o', dest='output', type=argparse.FileType('w'),
                        help='output file',
                        default=sys.stdout)

    args = parser.parse_args()
    if args.config:
        config = configparser.ConfigParser()
        config.read_file(args.config)
        # Transforming values into integers
        args.n1 = int(config['DEFAULT']['n1'])
        args.n2 = int(config['DEFAULT']['n2'])

    main(args.n1, args.n2, args.output)

6. Run the result to check that it's sending the output to the defined file. Note that there's no output outside the result files:

$ python3 prepare_task_step5.py -n1 3 -n2 5 -o result.txt
$ cat result.txt
The result is 15
$ python3 prepare_task_step5.py -c config.ini -o result2.txt
$ cat result2.txt
The result is 35