Inheritance (Part 2)

Add New Properties to Child Class

An EVCar object needs two properties that are not defined in the Car class: battery_size and charge_level.

To include these, we must define an __init__ method inside the EVCar class. The idea is to inherit all of the property names from the Car class and just add the two new ones.

The general syntax for this is:

class ChildClassName(ParentClassName):

   def __init__(self, parent_properties, new_property):
      # First, set the properties found in ParentClassName.
      ParentClassName.__init__(self, parent_properties)
      # Next, assign a value to the new property.
      self.new_property = new_property

When we call the child class, the __init__ method runs in two parts. First, it calls the parent class and completes that initialization. Next, control returns to the __init__ method in the child class, and values are assigned to any additional properties.

1. Line 3 defines the initializer method for the child class. Note that the parameters include self and variables for both the parent and child properties.

2. Line 5 calls the __init__ method from the parent class. This assigns values to all properties inherited from that class.

3. Line 7 creates a new property name and assigns it a value.

Example

Let’s see how this works for our EVCar class:

class Car:
   def __init__(self, make, model, year, color):
      self.make = make
      self.model = model
      self.year = year
      self.color = color

   def honk_horn(self):
      return "Beep!"

class EVCar(Car):
   def __init__(self, make, model, year, color, battery_size):
      # Call the Car __init__ method and assign the inherited properties.
      Car.__init__(self, make, model, year, color)
      # Initialize properties specific to the EVCar class.
      self.battery_size = battery_size
      self.charge_level = 100  # Begin with 100% charge.

   def recharge(self):
      # Code for recharging the battery.
      return "Fully charged!"

def main():
   my_car = Car('Saturn', 'SW-1', 1999, 'green')
   dream_car = EVCar('Tesla', 'Model S', 2020, 'blue', '100 kWh')

   print("my_car:", vars(my_car))
   print("dream_car:", vars(dream_car))

if __name__ == '__main__':
   main()

Console Output

my_car: {'make': 'Saturn', 'model': 'SW-1', 'year': 1999, 'color': 'green'}
dream_car: {'make': 'Tesla', 'model': 'Model S', 'year': 2020, 'color': 'blue', 'battery_size': '100 kWh', 'charge_level': 100}

Lines 27 and 28 print out the property names and values for the Car and EVCar objects. Note that dream_car includes two properties that are missing from my_car.

Change Existing Methods

On the previous page, we saw that EVCar inherits the honk_horn() method from the Car parent class. Any EVCar object can call this method without a problem. We also saw how to add a new method to a child class, like the recharge() and refuel() methods for EVCar and GasCar, respectively.

What if we want the parent and child class to share a method name, but we want the two functions to do different things?

Python and other programming languages allow us to override a method inherited from the parent class. The child class includes a method with the same name. However, instead of using the code from the parent class, the method contains a different set of instructions. These execute instead of the parent code.

Let’s see how this works with the honk_horn() method.

Example

We can call the honk_horn() method on a GasCar object, but the sound produced depends on the make of the car.

class GasCar(Car):
   def refuel(self):
      # Code for refilling the gas tank.
      return "Tank full."

   def honk_horn(self):
      if self.make == "BMW":
         return "HONK!!!"
      elif self.make == "Toyota":
         return "Beep, beep."
      elif self.make == "Plymouth":
         return "ah-OOOO-gah"
      else:
         return Car.honk_horn(self)

def main():
   hybrid = GasCar('Toyota', 'Prius', 2018, 'gray')
   classic = GasCar('Plymouth', 'De Luxe', 1947, 'blue')
   guzzler = GasCar('Ford','F-150', 2019, 'red')

   print(hybrid.honk_horn())
   print(classic.honk_horn())
   print(guzzler.honk_horn())

if __name__ == '__main__':
   main()

Console Output

Beep, beep.
ah-OOOO-gah
Beep!

In this case, code in the the GasCar class (lines 16 - 24) overrides the honk_horn() method inherited from Car. When we call the method on hybrid, classic, and guzzler, the code in the child class executes. This gives our three objects the chance to return something different than a Car object.

Notice that if make isn’t 'BMW', 'Toyota', or Plymouth, then the else block executes:

else:
   return Car.honk_horn(self)

Line 24 essentially says: Return the same thing as a Car object. Thus, the GasCar honk_horn() method includes the Car behavior as a backup.

Try It!

In the editor below:

1. Define a honk_horn() method in the EVCar class. For now, just have it return a string value that is different from 'Beep!'.

2. In main(), confirm that my_car and dream_car behave differently when you use the objects to call the honk_horn() method.

3. Expand the code in the child honk_horn() method to be more interesting than a single return statement. Be sure to include logic that calls the parent method if certain conditions are met.

Wrap-Up

Inheritance allows us to define new data types by extending the code from previously defined types, like the relationship between Car and EVCar.

A child class like EVCar inherits all the properties and methods of its parent class. It can then define its own new properties and methods.

A child class can override methods inherited from the parent class but still access the original behavior.