Topic 7 · Object-Oriented Programming (HL)

Chapter 2 · Going deeper & exam practice

📋 Reference · always available

Encapsulation

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

Private (Python)

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

Accessor (getter)

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

Mutator (setter)

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

Data integrity

Encapsulation benefit — validation in setters prevents invalid states.

Implementation hiding

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

Composition (has-a)

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

Inheritance (is-a)

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

Rule of thumb

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

Abstraction

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

Open-Closed Principle

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

Common exam trap

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