How to do it...

1. Create a script that will accept a single integer as a positional argument, and will print a hash symbol that amount of times. The recipe_cli_step1.py script is as follows, but note we are following the structure presented previously, and the main function is just printing the argument:

import argparse

def main(number):
    print('#' * number)

if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument('number', type=int, help='A number')
    args = parser.parse_args()
    
    main(args.number)

2. Call the script and see how the parameter is presented. Calling the script with no arguments displays the automatic help. Use the automatic argument -h to display the extended help:

$ python3 recipe_cli_step1.py
usage: recipe_cli_step1.py [-h] number
recipe_cli_step1.py: error: the following arguments are required: number
$ python3 recipe_cli_step1.py -h
usage: recipe_cli_step1.py [-h] number
positional arguments:
 number A number
optional arguments:
 -h, --help show this help message and exit

3. Calling the script with the extra parameters works as expected:

$ python3 recipe_cli_step1.py 4
####
$ python3 recipe_cli_step1.py not_a_number
usage: recipe_cli_step1.py [-h] number
recipe_cli_step1.py: error: argument number: invalid int value: 'not_a_number'

4. Change the script to accept an optional argument for the character to print. The default will be '#'. The recipe_cli_step2.py  script will look like this:

import argparse

def main(character, number):
    print(character * number)

if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument('number', type=int, help='A number')
    parser.add_argument('-c', type=str, help='Character to print',
                        default='#')

args = parser.parse_args()
main(args.c, args.number)

5. The help is updated, and using the -c flag allows us to print different characters:

$ python3 recipe_cli_step2.py -h
usage: recipe_cli_step2.py [-h] [-c C] number

positional arguments:
 number A number

optional arguments:
 -h, --help show this help message and exit
 -c C Character to print
$ python3 recipe_cli_step2.py 4
####
$ python3 recipe_cli_step2.py 5 -c m
mmmmm

6. Add a flag that changes the behavior when present. The recipe_cli_step3.py script is as follows:

import argparse

def main(character, number):
    print(character * number)

if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument('number', type=int, help='A number')
    parser.add_argument('-c', type=str, help='Character to print',
                        default='#')
    parser.add_argument('-U', action='store_true', default=False,
                        dest='uppercase',
                        help='Uppercase the character')
    args = parser.parse_args()

    if args.uppercase:
        args.c = args.c.upper()

    main(args.c, args.number)

7. Calling it uppercases the character if the -U flag is added:

$ python3 recipe_cli_step3.py 4 -c f
ffff
$ python3 recipe_cli_step3.py 4 -c f -U
FFFF