I'm not sure how generally applicable this is, but it might be useful as a starting point:  Attached is a bit of code I've been using: it's a reset_warnings_registry() context manager, which backs up & clears the registry state for the duration of the context, then restores it afterwards.  

It's particularly useful for unittests, just create & call the __enter__() method during setUp(), and attach __exit__() via addCleanup.  Each test is then gets it's own isolated warnings registry.