What class is this mocking?  If possible, you should use `mock.MagicMock([ClassName])` instead of `mock.Mock()`.  The difference is that `MagicMock` will raise an exception if you try to call a method that is not defined by the mocked class.

There are some cases in which this is not possible: in particular, if the mocked class has fields that are added dynamically at runtime.  Is this the case for the descriptor class?