A simple implementation of a base64 decoder that supports alternative alphabets

base64decode.py

def b64decode(string,alphabet):
        string = string.replace("=","")
        ret = ""
        left = 0
        for i in range(0, len(string)):
                if left == 0:
                        left = 6
                else:
                        value1 = alphabet.index(string[i - 1]) & (2 ** left - 1)
                        value2 = alphabet.index(string[i]) >> (left - 2)
                        value = (value1 << (8 - left)) | value2
                        ret += chr(value)
                        left -= 2
        return ret