Well, I'm not talking about every C extension in the world, I'm talking about thin bindings around C routines which is what numpy has for fft for example, someone wrote a basic equivalent for numpypy in a few hours with no prior numpypy/cffi knowledge.

Unfortunately, NumPy uses deep knowledge of the CPython API in quite a few places, which is one of the reasons implementing NumPyPy has been so challenging.

Another approach should be used for those, but to be clear I wasn't talking about the entire numpy library, I'm talking about things like numpy.fft