Lesson 2: Classes & Objects

⏱ ~40 min Lesson 2 of 14 💚 Free

What You'll Learn

Define a class with attributes and methods using __init__
Create and use object instances
Understand self and instance vs. class variables
Use dunder (magic) methods like __str__ and __repr__

What Is a Class?

A class is a blueprint for creating objects. It defines what data an object holds (attributes) and what it can do (methods). An object (or instance) is a specific thing created from that blueprint.

Think of a class as a cookie cutter — it defines the shape. Each cookie you cut out is an object. The cookies share the same shape (class) but can have different decorations (attribute values).

Anatomy of a Class

__init__ — The constructor method. Python calls it automatically when you create a new instance. Use it to set initial attribute values.

self — A reference to the specific instance being operated on. Always the first parameter of instance methods. Python passes it automatically.

Instance variables — Variables stored on each object (e.g., self.balance). Each instance has its own copy.

Class variables — Variables shared by all instances (defined outside __init__). Useful for constants or counters.

Dunder Methods

"Dunder" means double-underscore. Python defines special meaning for many __name__ methods. The most useful for beginners:

__str__ — Called by print(obj) and str(obj). Return a human-friendly string.
__repr__ — Called in the REPL or debugging. Return an unambiguous string.
__len__ — Called by len(obj).
__eq__ — Called by obj1 == obj2.

Quick Check

1. What is the purpose of __init__?

It deletes an object from memory

It initializes an object's attributes when the object is created

It defines a class variable shared by all instances

It prints the object to the console

2. What does self refer to inside a method?

The class itself

The parent class

The specific instance the method is called on

A built-in Python keyword with no special meaning

3. Which dunder method is called when you use print(obj)?

__repr__

__str__

__init__

__eq__

4. Two instances a = Dog("Rex") and b = Dog("Buddy") — do they share the same name attribute value?

Yes, all instances share every attribute

No, each instance has its own copy of instance variables

Only if name is defined as a class variable

It depends on how __init__ is written