I guess https://bugs.python.org/issue39728 is a perfect example of "previous exception not adding any value". :-) And I think it isn't a coincidence that it happens in "your" module.

The morale: we think about exceptions in different ways, and it's hard to say what's the right way. Maybe we should just change the wording to imply that __context__ exception didn't spontaneously "occur", but was explicitly "raised" by a handler.