Inheritance and Polymorphism

You will reuse parent class code and call matching methods on different objects.

10 min read 10 quiz questions

Start with a parent class

Inheritance lets a child class reuse a parent class. The child can keep a method, add a method, or replace a method with its own version.

class Parent:
    def action(self):
        print('Parent action')

class Child(Parent):
    pass

Example: override a method and use super()

class Animal:
    def __init__(self, name):
        self.name = name

    def speak(self):
        print(f'{self.name} makes a sound.')

class Dog(Animal):
    def speak(self):
        print(f'{self.name} says woof!')

    def describe(self):
        super().speak()

pet = Dog('Milo')
pet.speak()
pet.describe()

Dog.speak() overrides, or replaces, the parent version. Inside describe(), super().speak() calls the parent class method.

Check an object's type

Example: isinstance() with inheritance

class Animal:
    pass

class Cat(Animal):
    pass

whiskers = Cat()
print(isinstance(whiskers, Cat))
print(isinstance(whiskers, Animal))

isinstance(object, ClassName) checks whether an object is made from a class or one of its child classes. A Cat is also an Animal.

One method, many object types

Example: polymorphism in a loop

class Dog:
    def speak(self):
        return 'Woof'

class Cat:
    def speak(self):
        return 'Meow'

class Duck:
    def speak(self):
        return 'Quack'

animals = [Dog(), Cat(), Duck()]
for animal in animals:
    print(animal.speak())

This is polymorphism: the loop uses the same speak() call, while each object responds in its own way. The classes do not need identical internal code.

Key points

A child class can reuse a parent class.
Overriding replaces an inherited method.
super() calls a parent implementation.
isinstance() recognizes parent-child relationships.
Polymorphism lets one call work with many objects.

Worked example

Every vehicle honks, but buses charge fare. Bus inherits Vehicle and adds its own twist — code reuse plus specialisation.

Example

class Vehicle:
    def describe(self):
        return "A vehicle"

class Bus(Vehicle):
    def fare(self, km):
        return km * 2.5

bus = Bus()
print(bus.describe(), bus.fare(10))

How it works, step by step

class Bus(Vehicle) means Bus gets everything Vehicle has.
describe() was never written twice — inheritance supplied it.
fare() exists only on Bus: specialisation.

References & Further Reading

Related entries from PyLabs's own reference library.

super() (Access a parent class from inside a child class.)
isinstance() (Check whether a value is an instance of a type.)
issubclass() (Check whether one class inherits from another.)

Your notes

Practice exercise

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Create a parent class Vehicle with a move method that prints Moving. Create a child class Bike that overrides it to print Pedaling. Create a Bike and call move().

# Write your code below

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