> Note that the code may cause a context switch even today if the C code callbacks into Python bytecode (via a __del__ method) and the bytecode is long enough (100 instructions I think). However, that is extremely unlikely and much C extension code is not written with such situations in mind.

As someone who works professionally on a parallel / async runtime that supports thousands of continuously running servers, "extremely unlikely" means that actually it breaks all the time, but it's also impossible to debug.