I was more interested in source file resolution than bytecode caching. If Python 3.5 would prefer "spam.cpython-35.py" or "spam.cpython-3.py" over "spam.py" and Python 2 preferred "spam.py", then I can more easily separate the code that won't parse in the alternative.

Happy to be told it's unrelated and I should raise it separately, but from my POV resolving .pyd filenames looks very similar to resolving .py files.