simple caesar shift in python

caesar.py

alphabet = 'abcdefghijklmnopqrstuvwxyz'

def shift(text, n):
    answer = ''
    for c in text:
        if c in alphabet:
            answer += alphabet[(alphabet.index(c)+n)%26]
            continue
        answer += c
    return answer

def compare(text1, text2):
    for c1, c2 in zip(text1, text2):
        diff = alphabet.index(c1) - alphabet.index(c2)
        if diff < 0:
            diff += 26
        opp = diff - 26
        print opp, '\t', diff