From if Statements to Classes: How Refactoring Taught Me Testable Code

From Inline Code to OOP: What Building a Python Calculator Taught Me About Testability I am a self-taught learner. So when I decided to build a Python calculator, I didn't start with classes or design patterns. I started with the messiest, most beginner thing possible — everything crammed into one script. And honestly? That was the right decision. This article is about how I evolved that calculator through three versions, and what each step taught me about writing code that's actually testable — which matters a lot when you're learning QA automation like I am. 👉 GitHub: https://github.com/enayetrashid/python-calculator-evolution Version 1: The "Just Make It Work" Phase My first version was pure chaos by design. No functions, no classes; just raw logic from top to bottom. a = float ( input ( " Enter first number: " )) b = float ( input ( " Enter second number: " )) operation = input ( " Enter operation (+, -, *, /): " ) if operation == " + " : print ( a + b ) elif operation == " - " : p