word = "to be or not to be that is the question"
key = [3, 9, 1]

def shift(a, k):
    # a is a letter, k is a number
    # ex. shift('b', 2) = 'd'
    #     shift('y', 3) = 'b'

    # don't shift non-alphanumeric
    if (not a.isalpha()):
        return a

    # shift the letters ascii value (so a is 0, b is 1, ...)
    i = ord(a) - ord('a')
    i += k
    # if we went past 'z', wrap around
    i %= 26

    # return the letter back at the right ascii value
    return chr(i + ord('a'))

def polyalphabetic(w, k):
    k_index = 0
    result = []
    for char in w:
        if (not char.isalpha()):
            # char isn't alphanumeric, so we don't shift it at all
            result.append(char)
            continue
        result.append(shift(char, k[k_index]))

        k_index += 1      # go to the next key
        k_index %= len(k) # if we're past the end, go back to 0

    return "".join(result)

if __name__ == "__main__":
    print polyalphabetic(word, key)