Caesar cipher

caesar.py

#!/usr/bin/env python3
import argparse
import string
import sys

def rotate(letters, offset):
    return letters[offset:] + letters[:offset]

def cipher(message, map_dict):
    return ''.join(map_dict.get(c, c) for c in message)

class CaesarCipher:

    def __init__(self, offset):
        lower, upper = string.ascii_lowercase, string.ascii_uppercase
        offset = offset % len(lower)

        plain = lower + upper
        rotated = rotate(lower, offset) + rotate(upper, offset)
        pairs = tuple(zip(plain, rotated))

        self._encipher = {x: y for (x, y) in pairs}
        self._decipher = {y: x for (x, y) in pairs}

    def encipher(self, message):
        return cipher(message, self._encipher)

    def decipher(self, message):
        return cipher(message, self._decipher)

def main():
    parser = argparse.ArgumentParser(
        description="Encipher or decipher text from standard input.")
    parser.add_argument('-r', '--rotate', type=int)
    parser.add_argument('-d', '--decipher', action='store_true')
    args = parser.parse_args()

    if not args.rotate:
        parser.print_help()
        return

    caesar = CaesarCipher(args.rotate)
    action = caesar.decipher if args.decipher else caesar.encipher

    message = sys.stdin.read()
    sys.stdout.write(action(message))

if __name__ == '__main__':
    sys.exit(main())