mono.py

import argparse

parser = argparse.ArgumentParser(
    description='Encrypt or decrypt text files with monoalphabetic substitution cipher',
    epilog="Example usage: "
           "$ python3 mono.py --encrypt '/QWERTZ*LKJHGFDSAÄÖÜMNBVCX' ex2_mono.plaintext.txt "
           "--out ex2_mono.outputtext.txt"
)
group = parser.add_mutually_exclusive_group(required=True)
group.add_argument('--encrypt', metavar='KEY', action='store', dest='encrypt_key', type=str,
                    help='key used for encryption')
group.add_argument('--decrypt', metavar='KEY', action='store', dest='decrypt_key', type=str,
                    help='key used for decryption')
parser.add_argument('--out', metavar="OUTPUT_FILE", dest='output', default='-', action='store',
                    type=argparse.FileType('w', encoding='UTF-8'),
                    help='text is written to the given output file or, if not specified, to standard output')
parser.add_argument('INPUT_FILE', type=argparse.FileType('r'), help='the input file to encrypt/decrypt')
args = parser.parse_args()

if args.encrypt_key:
    encrypt(args.encrypt_key, args.INPUT_FILE, args.output)
else:
    decrypt(args.decrypt_key, args.INPUT_FILE, args.output)
# end if