If PyPy gets good enough, some of the need for C extensions should go away. Things such as "numpy" are written in C because numbers in Python are always boxed, that is, contained in a object. PyPy tries to figure out when that's not necessary, so it can use hardware arithmetic.

PyPy has to work very hard to do this, because you can muck with almost anything from anywhere with "setattr()" Even from other threads. So any guess PyPy makes has to be un-doable at any time. Python, the language, is what a naive interpreter such as CPython can do easily. In CPython, almost everything is a dictionary, and there are a lot of lookups. PyPy gets rid of much of that overhead. I'm enormously impressed that the PyPy crowd got it to work as well as they did.