Method Overriding

Video Materials

Now that we’ve created some classes that follow different inheritance relationships, let’s explore how we can use those relationships to change some methods in each class through the use of method overriding.

Vehicle Class

First, let’s look at a couple of methods in the Vehicle class, the move() and describe() methods. As we might recall, the describe() method is an abstract method since it is in italicized text in the UML diagram. We’ll discuss how to build an abstract method later in this chapter, but for now we’ll just implement it as a normal method that does nothing. Likewise, we haven’t discussed what a hash symbol # means in front of the speed attribute or the constructor, so we’ll just make them public for now.

class Vehicle:
  
  def __init__(self, name):
    self.__name = name
    self.speed = 1.0
    
  @property
  def name(self):
    return self.__name
    
  def move(self, distance):
    print("Moving")
    return distance / self.speed
  
  def describe(self):
    return None

That’s a very simple implementation of the Vehicle class. The move() method simply accepts a distance to move as a floating point number, and then divides that by the speed of the vehicle to get the time it takes to go that distance. For the default vehicle, we’ll assume that it moves at a speed of 1.0.

Inheritance and Constructors

Now that we’ve created some methods in the Vehicle class, let’s go back to the Airplane class and see how we can build it. First, we’ll need to add the attributes and the constructor for this class:

from Vehicle import *

class Airplane(Vehicle):
  
  def __init__(self, name, wingspan, capacity):
    self.__name = name
    self.__wingspan = wingspan
    self.__capacity = capacity

Before we go any further, let’s stop there and run this code to make sure that it works. For starters, let’s add some quick code to the Main class in Main.py that instantiates an Airplane object and tries to print the name attribute, using the property defined in the Vehicle class:

from Airplane import *

class Main:
  
  @staticmethod
  def main():
    a = Airplane("Test", 123, 45)
    print(a.name)
    
# main guard
if __name__ == "__main__":
  Main.main()

Once we have that code, we can use the Python interpreter to run it. So, to do that, we can use the Python - Run File option in the run menu above with the Main.py file selected, or we can open the terminal in Codio and use the following two commands to open the directory containing these files and then execute Main:

cd ~/workspace/12p-inherit/vehicle
python3 Main.py

When we do that, we’ll get some errors as shown in this screenshot:

In this error, it says that the name attribute in the Vehicle class doesn’t exist. This is because we haven’t actually called the constructor for the Vehicle class, so that attribute has not been defined yet. So, when we try to access the name property defined in the Vehicle class, it can’t give us the value we want. This is because the name attribute in the Vehicle class is actually defined in the constructor for that class.

So, to set the value of the name attribute, we’ll need to somehow provide that value to the constructor of the Vehicle class. But wait, why do we need to call the constructor and create a Vehicle object? Aren’t we just trying to create an Airplane object?

One of the major things to recall when inheriting from a class is that each object instantiated from the child class is also an instance of the parent class. We can think of it as though the child object contains an instance of the parent object. Because of that, when we try to create an instance of the child class—Airplane in this example—that class’s constructor must also be able to call the constructor for the parent class, or Vehicle.

We run into a snag, however, because we’ve provided a constructor in Vehicle that requires an argument. In that case, we must provide an argument to the constructor for Vehicle in order to instantiate that object, since the default constructor is no longer available. How can we do that?

Thankfully, there is a quick and easy way to handle this in Python. Inside of our constructor, we can use the special method super() to reference the parent class, allowing us to call that class’s constructor—its __init__() method—directly. We can provide any needed arguments to that method call, which are then provided to the constructor in the parent class. So, let’s update our constructor to use a call to super().__init__():

from Vehicle import *

class Airplane(Vehicle):
  
  def __init__(self, name, wingspan, capacity):
    super().__init__(name)
    self.__wingspan = wingspan
    self.__capacity = capacity

Now, inside of the constructor for Airplane, we have added the line super.__init__(name), which calls the constructor of the parent class Vehicle, providing name as the argument for the parameter. This will resolve the error. We can try it for ourselves by running the code in the Main class again and seeing that it produces the correct output.

Method Overriding

Finally, we can explore how to override a method in our child class. In fact, it is as simple as providing a method declaration that uses the same method signature as the original method. A method signature in programming describes the method name and the order of parameters defined in the function. So, we can override the move() and describe() methods in Airplane using code similar to the following:

from Vehicle import *

class Airplane(Vehicle):
  
  def __init__(self, name, wingspan, capacity):
    super().__init__(name)
    self.__wingspan = wingspan
    self.__capacity = capacity
    
  def __landing_gear(self, set):
    if set:
      print("Landing gear down")
    else:
      print("Landing gear up")
  
  def move(self, distance):
    self.__landing_gear(False)
    print("Moving")
    self.__landing_gear(True)
    return distance / self.speed
  
  def describe(self):
    return "I am an airplane with a wingspan of {} and capacity {}".format(self.__wingspan, self.__capacity)

In this code, we see that we have included method declarations for both move() and describe() that use the exact same method signatures as the ones declared in Vehicle.

Calling Overridden Methods

To really understand how this works, let’s look at a quick main() method that explores how each of these work.

from Vehicle import *
from Airplane import *

class Main:
  
  @staticmethod
  def main():
    vehicle = Vehicle("Boat")
    airplane = Airplane("Plane", 175, 53)
    print(vehicle.move(10))
    print(airplane.move(10))
    print(vehicle.describe())
    print(airplane.describe())
       
# main guard
if __name__ == "__main__":
  Main.main()

This code will simply call each method, printing whatever values are returned by the methods themselves. We must also remember that some of the methods may also print information, so we’ll see that output before we see the return value printed.

To run this code, we should see the following output:

In this screenshot, we can see that calling the move() method on the Vehicle object just prints the message “Moving”, while calling it on the Airplane object causes the messages about landing gear to be printed as well. This shows that our Airplane object is using the code from the overridden move() method correctly.

In a later part of this chapter, we’ll discuss polymorphism and how overridden methods have a major impact on the functionality of objects stored in that way.

Try it!

Let’s see if we can do the same for the overridden methods in the Truck and Car classes. First, we’ll start with this code for the MotorVehicle class:

from Vehicle import *

class MotorVehicle(Vehicle):
  
  def __init__(self, name):
    super().__init__(name)
    self.number_of_wheels = 2
    self.engine_volume = 125
    
  def honk_horn(self):
    return ""

See if you can complete the code for the Truck and Car classes to do the following:

• Truck.describe(self) should return “I’m a big semi truck hauling cargo”
• Truck.honk_horn(self) should return “Honk”
• Car.describe(self) should return either “I’m a sedan” if it has 4 doors, “I’m a coupe” if it has 2 doors, or “I’m different” if it has any other number of doors
• Car.honk_horn(self) should return “Beep”

You’ll also need to write the constructors for each class. Inside the constructor, you should store the parameters provided to the appropriate attributes.

The first test below will determine if your code is structured properly, and the second test will check the functionality of each method.