Unit 11.4A · Term 4

Classes & Objects

Object-Oriented Programming (OOP) is a paradigm where code is organized around "objects" rather than actions. An object contains both data (attributes) and code (methods). This lesson introduces the foundation of OOP: Classes and Objects.

Learning Objectives

  • 11.4.1.1 Create classes and instances of classes
  • 11.4.1.3 Use special method __init__ to set default properties
  • 11.4.1.2 Develop methods for the class

Lesson Presentation

11.4A-classes-objects.pdf · Slides for classroom use

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Conceptual Anchor

The Blueprint Analogy

Think of a Class as a blueprint for a house. It describes what the house should have (walls, windows) and do (open door).
An Object (or Instance) is the actual house built from that blueprint. You can build unlimited houses from one blueprint, and each house can have different colors or furniture, but they all follow the same structure.

Rules & Theory

Defining a Class (11.4.1.1)

class Car: pass # Empty class # Creating instances (Objects) my_car = Car() your_car = Car()

The __init__ Method (11.4.1.3)

The __init__ method (constructor) runs automatically when you create a new object. It initializes the object's attributes.

class Car: def __init__(self, brand, model): self.brand = brand # Attribute self.model = model # Attribute # Creating objects with data c1 = Car("Toyota", "Camry") c2 = Car("BMW", "X5") print(c1.brand) # Toyota print(c2.brand) # BMW

Methods (11.4.1.2)

Methods are functions defined inside a class that define the behavior of the object. The first parameter must always be self.

class Car: def __init__(self, brand, model): self.brand = brand self.model = model self.speed = 0 def drive(self): self.speed += 10 print(f"{self.brand} is driving at {self.speed} km/h") def stop(self): self.speed = 0 print(f"{self.brand} stopped.") my_car = Car("Tesla", "Model S") my_car.drive() # Tesla is driving at 10 km/h my_car.drive() # Tesla is driving at 20 km/h my_car.stop() # Tesla stopped.

What is self?

self is a reference to the current object being used. When you call my_car.drive(), Python automatically passes my_car as the self argument.

Worked Examples

1 Student Class

class Student: def __init__(self, name, grade): self.name = name self.grade = grade def promote(self): self.grade += 1 print(f"{self.name} is now in Grade {self.grade}") ali = Student("Ali", 10) print(ali.grade) # 10 ali.promote() # Ali is now in Grade 11

2 Bank Account

class BankAccount: def __init__(self, owner, balance=0): self.owner = owner self.balance = balance def deposit(self, amount): self.balance += amount print(f"Deposited {amount}. New balance: {self.balance}") def withdraw(self, amount): if amount <= self.balance: self.balance -= amount print(f"Withdrew {amount}. New balance: {self.balance}") else: print("Insufficient funds!") acct = BankAccount("Dana", 1000) acct.deposit(500) # 1500 acct.withdraw(2000) # Insufficient funds!

Pitfalls & Common Errors

Forgetting self

Defining a method as def drive(): instead of def drive(self): causes a TypeError when called.

__init__ Underscores

It is a double underscore on both sides! __init__, not _init_.

Graded Tasks

Remember

What is the difference between a class and an object? What is self?

Understand

Why do we need the __init__ method? What happens if we don't define it?

Apply

Create a class Rectangle with attributes length and width, and methods area() and perimeter().

Create

Design a Book class with title, author, and page count. Add a method read(pages) that tracks current page progress.