Encrypts a message using a Caesar cipher.

caesar.py

#!/usr/bin/env python3
"""Encrypts a message using a Caesar cipher.

https://en.wikipedia.org/wiki/Caesar_cipher
"""

from string import ascii_uppercase


class Caesar():
    def __init__(self, shift):
        self.shift = shift
        self.letters = ascii_uppercase

    def __repr__(self):
        return "Performs a Caesar encryption."

    def get_message(self):
        print("Enter message:")
        message = input().upper()
        return message

    def encrypt(self, message):
        cipher = ""

        # Special characters don't get encrypted.
        # find() returns -1 if a string is not contained
        # in the letters from A-Z.
        for character in message:
            index = self.letters.find(character)

            if index == -1:
                cipher += character
            else:
                cipher_index = index + shift
                # After Z, start again from A:
                if cipher_index > 25:
                    cipher_index -= 26

                cipher += self.letters[cipher_index]

        return cipher


# Set cipher shift (max. 25).
shift = 3

c = Caesar(shift)
message = c.get_message()
cipher = c.encrypt(message)

print("\nEncrypted message:")
print(cipher)