Is the Classes & Objects lesson free?

Yes. The first 5 lessons on PyRun are free forever — no signup, no card. Classes & Objects runs real Python in your browser via Pyodide.

How long does the Classes & Objects lesson take?

About 10 minutes end-to-end. It's a hands-on lesson — you read a paragraph, edit the code, and run it in the same tab.

Do I need to install Python to try the Classes & Objects lesson?

No install required. PyRun runs real CPython in your browser via Pyodide (Python compiled to WebAssembly). Works on Chrome, Firefox, Safari, and Edge — desktop or mobile.

What level is the Classes & Objects lesson?

Intermediate level. Best after finishing the Beginner track (syntax, control flow, functions, data types).

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Intermediate·10 min·intermediate · oop

Classes & Objects

You’ll be able to

Define classes with class and initialize with __init__
Create instances and access instance attributes
Write instance methods that operate on self
Understand the difference between class and instance state

Why this matters

Every serious framework — Django models, SQLAlchemy declarative base, Pydantic BaseModel, PyTorch nn.Module — is class-based. Understanding self, __init__, and method resolution order is what separates script writers from library authors.

Common pitfalls

Mutable default arguments like def __init__(self, items=[]) — shared across instances; use None sentinel instead.
Forgetting self on method calls or attribute reads — Python won't warn; you'll get NameError at runtime.
Overriding __eq__ without __hash__ — silently breaks set and dict membership.

A class is a blueprint. Each instance holds its own state.

Key pieces

__init__ — runs when you create an instance; sets initial state on self
self — refers to the current instance
__repr__ — controls how the object prints

Try it

Add a transfer(self, other, amount) method that moves money between accounts.
Make balance a property that can't go below zero.

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Define a class Rectangle with __init__(self, width, height) storing both. Add a method area(self) that returns width × height. Then print Rectangle(4, 5).area() — expected: 20.
Exercise 2
Define a Counter class with __init__ starting count at 0. Add increment(self) that bumps by 1, reset(self) that sets back to 0, and value(self) that returns the count. Then create one, increment 3 times, print value, reset, print value. Expected outputs: 3 then 0.
Exercise 3
Define a BankAccount with __init__(self, owner, balance=0), methods deposit(amount) and withdraw(amount) that update balance. withdraw should raise ValueError("insufficient funds") if amount exceeds balance. Add __repr__ returning "BankAccount(<owner>, ₹<balance>)". Then create one, deposit 500, print repr — expected line ends with ₹1500).

← PreviousIn-memory I/O Next →Inheritance & super()

Classes & Objects

You’ll be able to

Define classes with class and initialize with __init__
Create instances and access instance attributes
Write instance methods that operate on self
Understand the difference between class and instance state

Why this matters

Common pitfalls

Mutable default arguments like def __init__(self, items=[]) — shared across instances; use None sentinel instead.
Forgetting self on method calls or attribute reads — Python won't warn; you'll get NameError at runtime.
Overriding __eq__ without __hash__ — silently breaks set and dict membership.

A class is a blueprint. Each instance holds its own state.

Key pieces

__init__ — runs when you create an instance; sets initial state on self
self — refers to the current instance
__repr__ — controls how the object prints

Try it

Add a transfer(self, other, amount) method that moves money between accounts.
Make balance a property that can't go below zero.

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Define a class Rectangle with __init__(self, width, height) storing both. Add a method area(self) that returns width × height. Then print Rectangle(4, 5).area() — expected: 20.
Exercise 2
Define a Counter class with __init__ starting count at 0. Add increment(self) that bumps by 1, reset(self) that sets back to 0, and value(self) that returns the count. Then create one, increment 3 times, print value, reset, print value. Expected outputs: 3 then 0.
Exercise 3
Define a BankAccount with __init__(self, owner, balance=0), methods deposit(amount) and withdraw(amount) that update balance. withdraw should raise ValueError("insufficient funds") if amount exceeds balance. Add __repr__ returning "BankAccount(<owner>, ₹<balance>)". Then create one, deposit 500, print repr — expected line ends with ₹1500).