Classes and Objects

Group related data and behavior into objects when the domain benefits from it.

Why this matters

Classes are useful when several values and operations belong to one concept such as an Order, User, or Report.

Start with the idea

A class defines structure and behavior. An object is an instance of that class with its own state. self refers to the current object inside instance methods.

IdeaUnderstand the purpose

→

CodeRun the smallest example

→

PracticeChange it yourself

SubjectVision learning pattern: understand the idea before memorizing syntax.

Small working example

Run this example first. Do not change several things at once; confirm the basic behavior, then experiment.

class Cart:
    def __init__(self):
        self.items = []

    def add(self, item):
        self.items.append(item)

cart = Cart()
cart.add("book")
print(cart.items)

Expected output

['book']

What to notice

The example is intentionally small. Focus on the chapter’s main idea before adding extra syntax or framework code.

Common beginner mistake

Mistake

Creating a class for every small piece of data even when a function or simple structure is clearer.

Better approach

Use classes when state and behavior form a meaningful domain unit.

Quick recap

Classes define object behavior.
Instances hold state.
Object-oriented design is a tool, not a requirement for every problem.

Try it yourself

These are deliberately small. If you can complete them without copying the example, you are ready to continue.

Create a simple Counter class.
Add a method that changes its state.