Vigenere cipher over Latin alphabet in Python3

vigenere.py

#!/usr/bin/env python3

from itertools import cycle

A = [chr(x) for x in range(ord('a'), ord('z')+1)]

def encrypt(a,b):
    return A[(A.index(a) + A.index(b)) % len(A)]

def decrypt(a,b):
    return A[(A.index(a) - A.index(b)) % len(A)]

def vigenere(plaintext, key, op):
    return ''.join([op(x,y) for x,y in list(zip(plaintext, cycle(key)))])

E = (
        ('aaaa', 'aaaa',    'aaaa'),
        ('aaaa', 'b',       'bbbb'),
        ('aaaa', 'y',       'yyyy'),
        ('aaaa', 'yh',      'yhyh'),
        ('dddd', 'y',       'bbbb'),
        ('dddd', 'yh',      'bkbk'),
        ('attackatdawn',    'lemon', 'lxfopvefrnhr'),
        ('cryptoisshortforcryptography', 'abcd', 'csastpkvsiqutgqucsastpiuaqjb'),
        ('therecouldyetbeanothergeographerwhowoulddisagree', 'carbon', 'vhvsspqucemrvbvbbbvhvsurqgibdugrnicjqucervuaxssr'),
    )

for plaintext, key, expected in E:
    ciphertext = vigenere(plaintext, key, encrypt)
    decrypted = vigenere(ciphertext, key, decrypt)
    print(plaintext, key, expected, ciphertext, ciphertext == expected, decrypted, decrypted == plaintext)