def test_derive(self): landing_gross_weight = LandingGrossWeight() landing_gross_weight.set_flight_attr = Mock() touchdown_gross_weight = KPV('Gross Weight At Touchdown', items=[KeyPointValue(5, 15, 'a'), KeyPointValue(12, 120, 'b')]) landing_gross_weight.derive(touchdown_gross_weight) landing_gross_weight.set_flight_attr.assert_called_once_with(120)