> That's true, but I thought one of the goals of PEP 3121 is to
> separate
> states between sub-interpreters. So that one can't corrupt another.
> I'm not
> sure how much it matters in practice in this case of the pyexpat
> capsule;
> need to look into it more.

pyexpat's "capi" object is a static struct inside pyexpat.c, so that
wouldn't change anything.
Separating states between sub-interpreters only matters when said state
is mutable, which it isn't here.

> I don't see a call to PyState_AddModule. What am I missing?

It is called implicitly when an extension module is imported.