> I don't really _like_ that this is a .h file acting as a C template to inject
> effectively the same static code into each module that wants to use it...
> Which I think is the concern Victor is expressing in a comment above.

I think so too.

The defines of BOB_BUFFER_TYPE/BOB_SIZE_TYPE/BOB_SIZE_MAX are ugly. If put the core code together, these defines can be put in a thin wrapper in _bz2module.c/_lzmamodule.c/zlibmodule.c files. This can be done now, but it's ideal to improve it more thoroughly in 3.11.

_PyBytesWriter has different behavior, user may access existing data as plain data, which is impossible for _BlocksOutputBuffer. An API/code can be carefully designed, efficient/flexible/elegant, then the code may be used in some sites in CPython.