def METHOD_NAME(self):
    """
    Make sure no one writes:
        try:
            self._type_checkers[type](...)
        except KeyError:
    ignoring the fact that the function itself can raise that.
    """
    error = KeyError("Stuff")
    def raises_keyerror(checker, instance):
        raise error
    with self.assertRaises(KeyError) as context:
        TypeChecker({"foo": raises_keyerror}).is_type(4, "foo")
    self.assertIs(context.exception, error)