Topic 7 · Object-Oriented Programming (HL)

BLUEPRINT — defines attributes + methods. Does nothing on its own.

Each separate THING created from a class. `d1 = Dog('Rex')`. Many objects per class.

Constructor — runs automatically when object is created. Sets up initial attributes via `self.x = ...`.

Reference to the current object. `self.name` is this object's name; different objects have different selves.

Data stored INSIDE an object. e.g. self.balance, self.name. Each object has its own values.

Function defined INSIDE a class. Called as `obj.method()`. First parameter is always `self`.

`class Sub(Super)` — Sub IS-A Super and inherits Super's attributes + methods.

Subclass defines a method with the SAME NAME as the parent → subclass version is used for its objects.

Same method call → different behaviour depending on object's actual class. Run-time resolution.

Encapsulation · Abstraction · Inheritance · Polymorphism.

Bundle data + methods together. HIDE internals (private attributes) + expose CONTROLLED interface (public methods).

Single underscore `_x` = convention. Double underscore `__x` = name-mangled (effectively private).

Method that RETURNS a private attribute's value. Read-only access.

Method that SETS a private attribute, usually with VALIDATION (e.g. reject negative balance).

Encapsulation benefit — validation in setters prevents invalid states.

Can change internals (e.g. data structure used) WITHOUT breaking external code that uses the public interface.

Object CONTAINS another as an attribute. `Car has-a Engine`. Flexible, replaceable.

Subclass IS A kind of superclass. `Dog is-a Animal`. Use only when relationship is genuinely is-a.

Prefer COMPOSITION over INHERITANCE unless IS-A is genuinely true. Composition is more flexible.

Expose simple INTERFACE; hide complex implementation. Lets multiple implementations satisfy same interface.

Code should be OPEN for extension (new subclasses) but CLOSED for modification (existing code unchanged). Polymorphism enables it.

Confusing override (same name, replaces parent) with overload (same name, different parameters — NOT supported by Python directly).