def test_subtract_mileage_less_than_zero(self):
tracker = TripTracker()
tracker.subtract_mileage(10)
self.assertEqual(tracker.mileage, 0)
def test_subtract_mileage(self):
tracker = TripTracker()
tracker.add_mileage(10)
self.assertEqual(tracker.mileage, 10)
tracker.subtract_mileage(5)
self.assertEqual(tracker.mileage, 5)