python decoder ring

decoder.py

import string

alphabet = "abcdefghijklmnopqrstuvwxyz"
ring     = "abcdefghijklmnopqrstuvwxyz"

if __name__ == "__main__":
    offset = -1
    while not (0 <= offset <= len(alphabet)):
        choice = raw_input("What is your key's offset?: ")
        offset = int(choice)
        
    print "Shifting ring by %d places..." % offset
    
    ring = ring[offset:] + ring[0:offset]
    print "(psst keep it safe!) new ring: %r" % ring
    
    choice =  raw_input("Encrypt = 1 Decrypt = 2: ")
    message = raw_input("What is the message?: ")
    out = ""
    translation = None
    if choice in ['1','2']:
        if choice == '1':
            translation = string.maketrans(alphabet, ring)
        else:
            translation = string.maketrans(ring, alphabet)
        
        out = message.translate(translation)
        print "Message: %s" % out    
    else:
        print "Not a valid option. Exiting"