I feel guilty of having said, some months ago, that "Well, I wouldn't say bz2module is the best module out there, but as you say it's probably good enough" (my words).

It's true that bz2module is not awful in terms of coding style or quality; the main issue with it is that uses outdated ways of doing I/O compared to what Python 3 recommends (that is, compose various building blocks together to allow for better re-use).

A point of reference is gzip, where the core compression routines (mostly wrappers) are in zlibmodule (in C), but the high-level file object is written in pure Python (and is composeable with other building blocks for fast buffered IO).