All public classes not designed for pickling explicitly. I tested only operator.methodcaller, mmap.mmap, sqlite3 classes (Connect, Cursor, Row), _socket.socket, select.epoll, _csv.Dialect, but should be more. Instances of these classes can be "pickled", but unpickling either raise an exception (usually TypeError), or returns default or underinitialized object.

For other classes the patch changes raised exception type. E.g. pickling zlib.compressobj() raised

_pickle.PicklingError: Can't pickle <class 'zlib.Compress'>: attribute lookup Compress on zlib failed

and with the patch it raises

TypeError: can't pickle zlib.Compress objects