2.2.1 Inheritance

2.2.1 Inheritance

Inheritance (Python OOP)

Inheritance lets you create a new class based on an existing class.

• The existing class is the parent (or base) class.
• The new class is the child (or derived) class.

The child class automatically gets the parent’s attributes and methods, and can add or modify behavior.

Why Use Inheritance?

• Avoid repeating code (DRY: Don’t Repeat Yourself)
• Model real relationships (a Car is a Vehicle)
• Extend features safely without rewriting everything

Example 1: Vehicle -> Car and Bike

class Vehicle:
    def __init__(self, brand):
        self.brand = brand

    def start(self):
        return f"{self.brand} vehicle started"


class Car(Vehicle):
    def __init__(self, brand, doors):
        super().__init__(brand)
        self.doors = doors

    def honk(self):
        return "Beep beep!"


class Bike(Vehicle):
    def ring_bell(self):
        return "Ring ring!"


car = Car("Toyota", doors=4)
bike = Bike("Giant")

print(car.start())
print(car.honk())
print(bike.start())
print(bike.ring_bell())

What to notice:

• Car and Bike reuse start() from Vehicle.
• Car adds doors and honk().
• Bike adds ring_bell().

Example 2: Overriding (Changing Parent Behavior)

class Animal:
    def speak(self):
        return "Some sound"


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


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


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

Dog and Cat override the parent’s speak().

super() (Calling the Parent)

Use super() when you still want the parent logic.

class User:
    def __init__(self, username):
        self.username = username


class Admin(User):
    def __init__(self, username, permissions):
        super().__init__(username)
        self.permissions = permissions


admin = Admin("root", permissions=["read", "write", "delete"])
print(admin.username, admin.permissions)

When NOT to Use Inheritance (Beginner Rule)

Don’t force inheritance just to reuse a few lines.

• If the relationship is not “is-a” (Car is a Vehicle), inheritance may be wrong.
• Sometimes composition is better: “has-a” (Car has an Engine).

Quick Practice

Make a base class Employee with:

• name
• method get_salary()

Create two child classes:

• FullTimeEmployee (fixed monthly salary)
• PartTimeEmployee (hourly_rate * hours)

Call get_salary() for both.