At a glance, it looks like ENTER_PYTHON will *restore* the GIL on the current thread, but it may be coming in on a thread that's never seen the GIL before.

"The GIL" is actually the Python thread state, which is actually a per-thread data structure that's either active/locked or inactive/unlocked. If the current thread doesn't have a thread state, PyGILState_Ensure will create one, while ENTER_PYTHON will not.

So the underlying issue is probably that the callbacks are coming in from a thread that they shouldn't be, and really ought to be marshalled back into the correct event loop first.