Push Down Method

若超類別中有個方法只與某子類別有關，那下移它是個好選擇，
但僅限於呼叫方已知它要用哪個特定子類別時。

# Before
class Vehicle:
    def start_engine(self):
        print("Engine started.")

class Car(Vehicle):
    pass

class Motorcycle(Vehicle):
    pass

# After
class Vehicle:
    pass

class Car(Vehicle):
    def start_engine(self):
        print("Car engine started.")

class Motorcycle(Vehicle):
    def start_engine(self):
        print("Motorcycle engine started.")