It's a classical time-space trade-off. I'm not sure this is a bug; if it is, the reasonable change (IMO) is to just revert the introduction of these *2 tables, and loop through the output character-by-character.

I'd personally be in favor of such a change (drop the tables). If people think that these functions are too slow, they should propose an extension of the binascii module, rather than micro-optimizing the Python code.