Phase 5: Object-Oriented & Modular Thinking · ~35 minutes · Python · uv

Classes and Objects — Bundling Data and Behavior

self isn't a keyword or a convention you follow out of habit. Forget it in a method definition and Python tells you exactly what's missing.

Hiring signal: Explains self correctly as the instance itself, not memorized boilerplate, and can diagnose a missing-self TypeError on sight

What you will learn

Define a class with __init__, instance attributes, and methods
Explain self as the instance itself, automatically passed — not magic, not optional boilerplate
Diagnose a real TypeError from a malformed method definition missing self
Predict an attribute's value after a method call that mutates it

Introduction

Type: Learn Languages: Python Prerequisites: Phase 04 (Computational Thinking & Algorithms) Time: ~35 minutes

Objective

Learning objectives

Define a class with __init__, instance attributes, and methods
Explain self as the instance itself, automatically passed — not magic, not optional boilerplate
Diagnose a real TypeError from a malformed method definition missing self
Predict an attribute's value after a method call that mutates it

What you're building

Take this Phase 02-style dict-based record:

student = {"name": "Ada", "grades": [92, 88, 95]}

def average_grade(student):
    return sum(student["grades"]) / len(student["grades"])

def add_grade(student, grade):
    student["grades"].append(grade)

A script (student_class.py) that:

Converts this into a Student class with __init__(self, name, grades), an average_grade(self) method, and an add_grade(self, grade) method
Creates at least two Student instances and demonstrates that mutating one (via add_grade) doesn't affect the other
Contains a comment showing the exact TypeError that would occur if average_grade were defined without self, and explains in one sentence why

A class defines def deposit(self, amount): self.balance += amount. Given account = Account(balance=100), what does account.deposit(50) actually do, step by step?

account.deposit(50) is shorthand for Account.deposit(account, 50) — self is automatically bound to account, and amount receives 50. Inside the method, self.balance += amount is exactly Phase 01's assignment-as-instruction pattern, applied to an attribute instead of a plain variable: it computes account.balance + 50 first, then rebinds account.balance to that new value. No new object is created; the same account instance is mutated in place, exactly like bump_grade mutated the same Student instance above.

Unlock the full lesson

You've read the first 2 sections. The rest of this lesson covers The Problem, Check Yourself, Key Terms & Next — plus a hands-on lab, quiz, and project artifact.

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