Hi David,
I just fired up the docs and found this:
"The mutex module defines a class that allows mutual-exclusion via acquiring and releasing locks. It does not require (or imply) threading or multi-tasking, though it could be useful for those purposes."

The docs dont say about threads using mutex object, but instead say if you want to use threading you can use mutex obj.

How are you using mutex with threads, can you please provide some information.

If muobj is an instance of mutex class.
muobj.testandset() for process-a will set the lock.
muobj.testandset() for process-b will be dealt with self.lock = True and wont be able to set.