Rot 7 program in pyhton

ROT-7.py

__author__ = 'Rahul Bali'

import string
from itertools import cycle

a = input()
i = 0
rot7=""
b = list(string.ascii_lowercase)
while i < len(a):
    pos = 0
    alpha=a[i]
    if(alpha!=" "):
        pos = b.index(alpha)
        rotkey=pos+7
        if(rotkey>25):
            rotkey-=26
        final=b[rotkey]
        rot7+=final
    else:
        rot7+=alpha
    i += 1
print(rot7)