Inheritance (Part 1)

Throughout this textbook, we try to follow the simple goal of Don’t Repeat Yourself (DRY). Loops, functions, modules, objects, etc. allow us to create and then reuse blocks of code. Classes are no different.

Imagine we have two objects that are very similar but not quite the same. One example would be an electric car compared to one that runs on gasoline. Both are cars, and they share a number of properties and behaviors. However, there are definite differences between the two.

We could code separate EVCar and GasCar classes, but notice how each one repeats some of the same code:

Example

class EVCar:
   def __init__(self, make, model, year, color, battery_size):
      self.make = make
      self.model = model
      self.year = year
      self.color = color
      self.battery_size = battery_size
      self.charge_level = 100  # Begin with 100% charge.

   def honk_horn(self):
      return "Beep!"

   def recharge(self):
      # Code for recharging the battery.

   def drive(self):
      # Code for decreasing charge_level after moving.

class GasCar:
   def __init__(self, make, model, year, color, tank_size):
      self.make = make
      self.model = model
      self.year = year
      self.color = color
      self.tank_size = tank_size
      self.fuel_level = tank_size   # Begin with a full tank.

   def honk_horn(self):
      return "Beep!"

   def refuel(self):
      # Code for refilling the gas tank.

   def drive(self):
      # Code for decreasing fuel_level after moving.

Inheritance refers to one class taking the same properties and methods from a different one. In the example above, the two classes share the make, model, year, and color property names. The classes also share the same code for the honk_horn method.

Instead of repeating statements, we define a parent class to hold all of the shared code. Next, we add child classes, which will inherit the shared code. Finally, we add new properties and methods to the child classes. This makes them distinct from each other, but they are both related to the same parent.

Let’s work through an example.

Define a Parent Class

First, we want to define the parent class called Car. It holds the properties and one method that EVCar and GasCar share.

Example

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!"

The Car class is fully functional, and we can use it to create multiple Car objects with different makes, models, years, and colors.

Define a Child Class

Now let’s add a child class (also called a subclass) that inherits the code contained in Car.

The general syntax to define a child class is:

class ChildClassName(ParentClassName):

By placing ParentClassName inside the parentheses (), we link ChildClassName to all of the code the parent contains.

Let’s start with EVCar, and for now we will only add one new method:

Example

The following program creates one Car object and one EVCar object. Notice that the EVCar class does not include an __init__ method.

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 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')

   print(type(my_car), type(dream_car))
   print(my_car.make, my_car.honk_horn())
   print(dream_car.make, dream_car.honk_horn())

   print(dream_car.recharge())
   print(my_car.recharge())

if __name__ == '__main__':
   main()

Console Output

<class '__main__.Car'> <class '__main__.EVCar'>
Saturn Beep!
Tesla Beep!
Fully charged!
File "main.py", line 25, in main
   print(my_car.recharge())
AttributeError: 'Car' object has no attribute 'recharge'

1. Lines 17 and 18 call the two classes and create the Car and EVCar objects. Note that both statements include the same number and types of arguments.

2. Line 20 shows us that my_car and dream_car are different data types.

3. Line 21 prints the make property and calls the honk_horn method for the Car object.

4. Line 22 shows us that even without its own __init__ method, properties were assigned to the EVCar object. dream_car can also call the Car method honk_horn!

5. Lines 24 and 25 show us that dream_car can call the EVCar method recharge, but my_car cannot.

This is what inheritance does! It allows a subclass to use the __init__ and other methods defined in the parent class.

Try It!

In the editor below:

1. Add the GasCar child class to the program. It should inherit from the Car parent class.

2. Add a simple refuel method that returns the string 'Tank full'.

3. In main(), test your new class:

   1. Create a GasCar object and print its data type.

   2. Print one or more of its properties, and call the honk_horn method.

   3. Call the refuel method and print the returned string.

   4. Verify that the my_car and dream_car objects cannot call the refuel() method.

Similar, but Different

Notice that EVCar and GasCar objects need to include properties that are NOT defined in the Car class: battery_size, charge_level, tank_size, and fuel_level.

On the next page, we will learn how to add new properties to a child class.