Stage 5 of 6 · 10 lessons · ~2 months

Object-oriented design and clean code

Organise programs so they stay readable as they grow: classes where they help, separation of concerns, small functions, tests first on the logic that matters, and documentation someone else can follow.

Start: Classes, objects and encapsulation

Lessons· one lesson for each topic, in this order

Start with· follow one of these

CoursePaid
Grokking SOLID Design Principles
Design Gurus · Beginner · $50 one-time
Five principles with code examples, including when applying them makes code worse
GuideFree
Refactoring.Guru: Code Smells and Refactoring
Refactoring.Guru · Beginner
Free catalogue of code smells and the small refactorings that fix each one

Practise

Exercism
Request mentoring on a solved exercise and ask specifically about naming and structure

Keep handy

pytest Documentation
Get started in 20 minutes: plain assert, fixtures and running tests from the terminal

Also worth your time

Beginner's Guide to Object-Oriented Programming (OOP) (opens in a new tab)Read
Classes, objects and the four pillars explained with small code examples
A Philosophy of Software DesignGo deeperPaid
Short book on deep modules and complexity; a useful counterweight to rigid rules

Before moving on: Refactor one of your earlier programs: split input, logic and output into separate modules, remove the duplication, write unit tests for the core logic first, and add a README that explains how to run it.

Tauseef's advice

Use a class when it groups data with the behaviour that changes it, not because a tutorial made everything a class. Treat DRY, KISS, YAGNI and SOLID as questions to ask while refactoring, not rules for a first draft, and write the test before you fix any bug.