You're talking past chrisseaton's point here. If you want two C extensions to interoperate with bare-metal performance, you can't just do from lib1 import m

You're talking past chrisseaton's point here. If you want two C extensions to interoperate with bare-metal performance, you can't just do from lib1 import makedata from lib2 import processdata data = makedata() print(processdata(data)) Because makedata needs to provide a c->py bridge and processdata needs a py->c bridge, so your process inherently has python in the middle unless lib2 has intimate knowledge of lib1. It can absolutely be done (I've written plenty of c extensions that handle numpy arrays, for example) but if somebody hasn't done the work, you don't get it for free. If your c extension expects a list of lists of floats, the numpy array totally supports that interface... but (last I checked) the overhead there is way slower than calling list(map(list, data)) and throwing that into your numpy-naive c extension.