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.
<<< import base64 <<< encoded = 'YmFzZTY0IGVuY29kZWQgc3RyaW5n' <<< data = base64.b64decode(encoded) <<< data 'base64 encoded string'