A couple of minor comments:

* "self.fail" with an appropriate error message is a clearer way to
indicate an incorrect logic path has been hit in a test case

* the details of the exception chaining don't quite look right, as I
believe "raise X from Y" sets both the cause and the context, with the
suppress_context attribute set to turn off the display of the latter. I
suggest checking that in Python code to confirm ny recollection, and then
adjusting the test and implementation if necessary to match the Python
level equivalent