Victor Stinner wrote:

Premature optimization is the root of all evil. Most C extensions use premature optimization which are causing us a lot of troubles nowadays when we want to make the C API evolve and cause issues to PyPy which has issues to reimplement the C API on top of their different object model with a different GC.

I'm intimately familiar with several C extensions in the math/science space and this is not the impression I get at all. Most people are happy if they get things to work, because the actual problems are much harder than focusing on private vs. public API. As for PyPy, if I understood correctly, Armin Rigo was skeptical of the proposed plan and favored publishing an API as a third party package. Stefan Krah