intermediate30 min

Object-Oriented Programming

Classes, objects, inheritance, and encapsulation

What is OOP?

Object-Oriented Programming (OOP) is a way of organizing code by bundling data and behavior into reusable blueprints called classes.

Think of a class like a cookie cutter — it defines the shape, and each cookie you make is an object (an instance of that class).

Why OOP?

Organize complex programs into logical pieces
Reuse code through inheritance
Encapsulate data — keep related variables and functions together

Your First Class

Creating a Class and Objects

Output

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Breakdown:

class Dog: — defines a new class named Dog
__init__(self, name, age) — the constructor, runs when you create an object
self — refers to the specific instance being created or used
self.name = name — stores the parameter as an instance attribute

Check Your Understanding

What is the purpose of the __init__ method?

Instance Methods

Methods are functions that belong to a class. They always take self as their first parameter:

A BankAccount Class

Output

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Inheritance

Classes can inherit from other classes — getting all their methods and attributes for free:

Inheritance Hierarchy

Output

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Cat and Fish inherit from Animal
They override the speak() method with their own version
They can add their own unique methods (like purr() and swim())

Check Your Understanding

What happens when a child class defines a method with the same name as the parent class?

Class Variables vs Instance Variables

Class vs Instance Variables

Output

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Class variables are shared across all instances — change once, affects everyone
Instance variables (with self.) are unique to each object

Exercise

Create a `Rectangle` class with a constructor that takes `width` and `height`. Add a method `area()` that returns `width * height`, and a method `perimeter()` that returns `2 * (width + height)`. Create a rectangle with width 5 and height 3, then print its area and perimeter.

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Key Takeaways

A class is a blueprint for creating objects
The __init__ method initializes new objects
self refers to the current instance
Inheritance lets child classes reuse parent code
Class variables are shared; instance variables are unique
Method overriding lets child classes customize inherited behavior

OOP is the foundation of almost every large Python project. Understanding classes sets you up to read and write professional-grade code!

Questions & Discussion

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