Used as a decorator to set the validator method for a Parameter. The validator method validates any value set for that parameter. It takes two arguments–self, which refers to the Model instance (remember, this is a method defined on a Model), and the value being set for this parameter. The validator method’s return value is ignored, but it may raise an exception if the value set on the parameter is invalid (typically an InputParameterError should be raised, though this is not currently a requirement).