Caesar / Vigenere cypher

from string import ascii_lowercase

abc = ascii_lowercase

def caesar_code(message, shift):
    output = ""
    shift = shift % len(abc)
    for char in message.lower():
        output += char if char not in abc else _find_shifted_letter(char, shift)
    return output


def _find_shifted_letter(letter, shift):
    index = abc.index(letter) + shift
    index = index if index < len(abc) else index - len(abc)
    return abc[index]


def vigenere_encode(message, code):
    code_gen = _code_generator(code)
    output = ""
    for letter in message.lower():
        output += letter if letter not in abc else caesar_code(next(code_gen), abc.index(letter))
    return output


def vigenere_decode(message, code):
    code_gen = _code_generator(code)
    output = ""
    for letter in message.lower():
        output += letter if letter not in abc else caesar_code("a", abc.index(letter) - abc.index(next(code_gen)))
    return output

def _code_generator(code):
    i = 0
    while True:
        yield code[i % len(code)].lower()
        i += 1