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Exercise 2

Write the code needed to make the following code work as shown:

print(Car.vehicles())     # 0
car1 = Car()
print(Car.vehicles())     # 1
car2 = Car()
car3 = Car()
car4 = Car()
print(Car.vehicles())     # 4
truck1 = Truck()
truck2 = Truck()
print(Truck.vehicles())   # 6
boat1 = Boat()
boat2 = Boat()
print(Boat.vehicles())    # 8

Solution

class Vehicle:
    number_of_vehicles = 0

    def __init__(self):
        Vehicle.number_of_vehicles += 1

    @classmethod
    def vehicles(cls):
        return Vehicle.number_of_vehicles

class Car(Vehicle):

    def __init__(self):
        super().__init__()

class Truck(Vehicle):

    def __init__(self):
        super().__init__()

class Boat(Vehicle):

    def __init__(self):
        super().__init__()

# Test code omitted

For this problem, the easiest solution is to use a Vehicle superclass that keeps track of how many objects it creates. We need a class variable and a __init__ method that increments the class variable each time a new vehicle is constructed.

We also need a __init__ method in the Car, Truck, and Boat classes, all of which call the Vehicle.__init__ method using super().__init__. These subclass __init__ methods aren't strictly required in this rudimentary code, but it's good practice.

Finally, notice that we can call Vehicle.vehicles using any of the four classes used in this problem.

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