Python OOP Masterclass: Mastering "Self", Inheritance, and Abstract Classes (2026)

Day 11: The Architecture of State — Classes, OOP, and Dunders 38 min read Series: Logic & Legacy Day 11 / 30 Level: Beginner to Architect In the wild, code is chaotic. Variables float aimlessly, and functions manipulate data they do not own. To build a legacy, you must bind your data (state) and your functions (behavior) into an unbreakable fortress. This is Object-Oriented Programming (OOP) in Python . ⚠️ The 3 Fatal Beginner Mistakes Before we build, we must unlearn. Here is how beginners destroy their own architecture when attempting to write classes: The Mutable Trap : Defining a list like inventory = [] directly under the class keyword. Result: Every user shares the exact same inventory. User A adds an item, and it magically appears in User B's cart. A catastrophic data bleed. Forgetting self : Writing def attack(target): instead of def attack(self, target): . Result: TypeError: method takes 1 positional argument but 2 were given . Python secretly passes the object itself as the f