I'm not calling Py_Finalize from a non-main thread. What I called "thread B" is the main thread. It's the script that first imports the threading module that runs in a non-main thread (and running user-defined scripts in non-main threads is hopefully not unsafe, or there wouldn't be much point in supporting multithreading at all in Python).

It didn't even occur to me that this could be reproduced in pure Python code, so I didn't include an example in my original post. Of course, it can - see attachment. Tested on Python 2.3.5 on Mac OS X.