b64decode(s, altchars=None)
Decode a Base64 encoded string.
s is the string to decode. Optional altchars must be a string of at least
length 2 (additional characters are ignored) which specifies the
alternative alphabet used instead of the '+ and
/' characters.
The decoded string is returned. A TypeError is raised if s were incorrectly padded or if there are non-alphabet characters present in the string.