Simple bijective function (base(n) encode/decode) in Python

bijective.py

# Simple bijective function
#   Basically encodes any integer into a base(n) string,
#     where n is ALPHABET.length.
#   Based on pseudocode from http://stackoverflow.com/questions/742013/how-to-code-a-url-shortener/742047#742047

ALPHABET = list("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")
  # make your own alphabet using:
  # (('a'..'z').to_a + ('A'..'Z').to_a + (0..9).to_a).shuffle.join

def bijective_encode(i):
    # from http://refactormycode.com/codes/125-base-62-encoding
    if i == 0:
        return ALPHABET[0]
    s = ''
    base = len(ALPHABET)
    while i > 0:
        s += ALPHABET[i % base]
        i /= base
    return s[::-1] # reverse string


def bijective_decode(s):
    # based on base2dec() in Tcl translation 
    # at http://rosettacode.org/wiki/Non-decimal_radices/Convert#Ruby
    i = 0
    base = len(ALPHABET)
    for char in s:
        i = i * base + ALPHABET.index(char)
    return i

# Two little demos:

numbers = 95769
result = bijective_encode(number)
print result #xyz

letters = 'abc'
new_number = bijective_decode(letters)
print new_number #66

the 2 demo samples are wrong.