Introduction

In the previous lesson, you learned about encapsulation:

  • Objects protect their internal data
  • Interaction happens through methods

Polymorphism builds directly on this idea.

Polymorphism means:

  • Different objects respond to the same method name
  • The behaviour depends on the object’s class
  • The code using the object does not need to know its type

In this lesson, you will learn how:

  • Inheritance enables polymorphism
  • Overridden methods create different behaviour
  • One interface can work with many object types

Polymorphism in Simple Terms

If two objects both have a method called describe(), Python does not care what class they belong to.

It only cares that the method exists.

Example: Animals Speaking 

class Animal:
    def speak(self):
        print("The animal makes a sound")

class Dog(Animal):
    def speak(self):
        print("Woof")

class Cat(Animal):
    def speak(self):
        print("Meow")

All three classes share the same method name:

  • speak()

But each behaves differently.

Predict:

What do you think will be printed?

class Animal:
    def speak(self):
        print("The animal makes a sound")

class Dog(Animal):
    def speak(self):
        print("Woof")

class Cat(Animal):
    def speak(self):
        print("Meow")

animals = [Dog(), Cat(), Animal()]

for animal in animals:
    animal.speak()

Think about:

  • Does Python check the object’s class?
  • Or does it just call the method?

Run:

Run the program below to check your prediction.

Why This Is Useful

Without polymorphism, you might write code like this:

if type == "dog":
    print("Woof")
elif type == "cat":
    print("Meow")

With polymorphism:

  • No if statements needed
  • Code is easier to extend
  • New classes can be added without changing existing code

Investigate:

Let’s use polymorphism with a more practical example.

Try this:

  • Create a list containing different account types
  • Call the same method on each object

Think about:

  • Why does this work without checking the class type?
  • How would adding a new account type affect the loop?

Modify:

The following program is meant to demonstrate polymorphism but contains an issue.

Fix the code so that each object responds correctly.

The problem

  • The overridden method is missing self
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