To fix the failing test, you can run cargo fmt in the repository.

I can't seem to see how

zlibd = zlib._ZlibDecompressor()
self.assertRaises(TypeError, zlibd.decompress)

can produce a TypeError when using #[pymethod] on the rust side, unless it already does and I'm reading something incorrectly.

Not only Constructor, it may also requires Initializer. Constructor corresponds to __new__ while Initializer corresponds to __init__

Sorry, I just checked CPython code but it doesn't look like that

I resolved a few issues, but currently I'm getting a bunch of
RuntimeError: Expected payload '_ZlibDecompressor' but '_ZlibDecompressor' found

Additionally _ZlibDecompressor has to be unpicklable for another test to pass, but I'm fine with just disabling that one.

Also it seems to my knowledge __getstate__ isn't a magic method I can use? It would be nice to use it because then I could pass the pickling test rather trivially.

Each class need to implement __getstate__ to support pickling. pickle library will use getstate in priority. Searching for fn getstate will result other types' implementation

Oh... unlike setstate, getstate doesn't have many implementations