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Class Inheritance

In this section we will go over an example of class inheritance. Class inheritance is when you build classes that are derived from a general base class. There is a parent to child relationship where the child class (or subclass) retains a similar implementation to the base class (parent class). It’s best understood through some examples.

First we will create a Dog and Cat class which are assumed to be pets.

class Cat:
    def __init__(self, name, age, color, owner, likes_dogs):
        self.name=name
        self.age=age
        self.color=color
        self.owner=owner
        self.likes_dogs=likes_dogs
    
    def speak(self):
        print('meow')
    
    def eat(self):
        print('eat')
        
    def sleep(self):
        print('sleep')
        
class Dog:
    def __init__(self, name, age, color, owner, chases_cats):
        self.name=name
        self.age=age
        self.color=color
        self.owner=owner
        self.chases_cats = chases_cats
    
    def speak(self):
        print('woof')
    
    def eat(self):
        print('eat')
        
    def sleep(self):
        print('sleep')

cat = Cat(name='Luna', age=4, color='brown', owner='Chris', likes_dogs=False)
dog = Dog(name='Max', age=2, color='white', owner='Isaac', chases_cats=True)
print(cat.name, cat.color, cat.owner, cat.age, cat.likes_dogs)
print(dog.name, dog.color, dog.owner, dog.age, dog.chases_cats)

Luna brown Chris 4 False
Max white Isaac 2 True

You can probably see that the code for the two classes Cat and Dog is very similar and there is a lot of repeated code. This in general is not good. If you wanted to change the way eat or sleep works then you would have to change it in two different places. The only real difference is the speak method and some arguments in the constructor (__init__). Also if we wanted to create another animal/pet such as Bird we would have to copy out a lot of the same methods and attributes from either Cat or Dog. This would result in even more repeated code.

This is where the idea of class inheritance becomes useful. We can first create a base class called Pet with all the common attributes and methods for pets. There could be more methods and attributes for a Pet class and feel free to add them. I want to keep the example simple.

class Pet:
    def __init__(self, name, age, color, owner):
        self.name=name
        self.age=age
        self.color=color
        self.owner=owner
    
    def eat(self):
        print('eat')
        
    def sleep(self):
        print('sleep')

Then we can subclass this class to create the Dog and Cat classes.

class Cat(Pet):
    def __init__(self, name, age, color, owner, likes_dogs):
        super().__init__(name, age, color, owner)
        self.likes_dogs = likes_dogs
    
    def speak(self):
        print('meow')

class Dog(Pet):
    def __init__(self, name, age, color, owner, chases_cats):
        super().__init__(name, age, color, owner)
        self.chases_cats = chases_cats
    
    def speak(self):
        print('woof')

cat = Cat(name='Luna', age=4, color='brown', owner='Chris', likes_dogs=False)
dog = Dog(name='Max', age=2, color='white', owner='Isaac', chases_cats=True)
print(cat.name, cat.color, cat.owner, cat.age, cat.likes_dogs)
print(dog.name, dog.color, dog.owner, dog.age, dog.chases_cats)

Luna brown Chris 4 False
Max white Isaac 2 True

The main difference is instead of writing class Dog you write class Dog(Pet) which means Dog will inherit from the base class Pet. Also in the __init__ (aka constructor) you have to call the super().__init__ method which simply means to call the constructor of the parent class. Then you can add any additional logic to the child class constructor that is required.

We can even make the code more concise. Notice that we had to repeat all the Pet class initialization arguments. We can use **kwargs to shorten this up.

class Cat(Pet):
    def __init__(self, likes_dogs, **kwargs):
        super().__init__(**kwargs)
        self.likes_dogs = likes_dogs
    
    def speak(self):
        print('meow')

class Dog(Pet):
    def __init__(self, chases_cats, **kwargs):
        super().__init__(**kwargs)
        self.chases_cats = chases_cats
    
    def speak(self):
        print('woof')

cat = Cat(name='Luna', age=4, color='brown', owner='Chris', likes_dogs=False)
dog = Dog(name='Max', age=2, color='white', owner='Isaac', chases_cats=True)
print(cat.name, cat.color, cat.owner, cat.age, cat.likes_dogs)
print(dog.name, dog.color, dog.owner, dog.age, dog.chases_cats)

Luna brown Chris 4 False
Max white Isaac 2 True

We can even add more methods or attributes that are common to only one of the child classes. For example, suppose dogs like to swim and cats do not. Then we can add a swim method to the dog class.

class Dog(Pet):
    def __init__(self, chases_cats, **kwargs):
        super().__init__(**kwargs)
        self.chases_cats = chases_cats
    
    def speak(self):
        print('woof')
    
    def swim(self):
        print('swim')

dog = Dog(name='Max', age=2, color='white', owner='Isaac', chases_cats=True)
dog.eat()
dog.sleep()
dog.swim()

eat
sleep
swim

In summary, the child class will have all the same methods and attributes as the parent class as well as the new ones you define in the child class.

Another thing you can do is override the method from the parent class. For example, we have the method sleep from the parent class which simply prints sleep. We can change this implementation for the Dog class for example.

class Dog(Pet):
    def __init__(self, chases_cats, **kwargs):
        super().__init__(**kwargs)
        self.chases_cats = chases_cats
    
    def speak(self):
        print('woof')
    
    def swim(self):
        print('swim')
        
    def sleep(self, hours):
        print(f'sleeping for {hours} hours.')

cat = Cat(name='Luna', age=4, color='brown', owner='Chris', likes_dogs=False)
dog = Dog(name='Max', age=2, color='white', owner='Isaac', chases_cats=True)
print(cat.name, cat.color, cat.owner, cat.age, cat.likes_dogs)
print(dog.name, dog.color, dog.owner, dog.age, dog.chases_cats)

Luna brown Chris 4 False
Max white Isaac 2 True

cat.sleep()
cat.eat()

sleep
eat

dog.eat()
dog.swim()
dog.sleep(2)

eat
swim
sleeping for 2 hours.

I hope you now understand a little bit about class inheritance in Python now.

There is more that could be said and there are entire courses/books which only focus on object-oriented programming (OOP). Just remember that classes are a great way to organize code into similar functions and methods that work together. Classes are templates for how objects should behave. All the functionality and data related to similar objects can be defined in the class. The best way to learn is to write your own simple programs which use classes and also spend time reading other peoples code.

Class Variables and Methods Ideas for Practice