Vigenère cipher encryption

vigenere.py

import string 
a = string.ascii_lowercase
p = "mei"
k = "biru"
if len(k) > len(p): k = k[:len(p)]
if len(k) < len(p):
    i = 0
    while len(k) != len(p):
        k += k[i % len(k)]
        i += 1 
pIndex = [a.index(char) for char in p if char.isalpha()]
kIndex = [a.index(char) for char in k if char.isalpha()]
index = [pIndex, kIndex]
index = [sum(a) % 26 for a in zip(*index)]
print(str().join([chr(i + 97) for i in index]))