One more thing:
in the sample code, the thread initializes no PyThreadState. So the ctypes callback creates a temporary PyThreadState just for the duration of the call.

This explains the difference between threading.local and _threading_local:

- threading.local() uses the PyThreadState to store its data: You get a new value on each call.
- _threading_local.local() uses a global dictionary indexed by the thread id: the same value is retained across call.