Summary  
This chapter covers composition in object-oriented programming, where a class contains instances of other classes (has-a relationships) to assemble complex objects with loose coupling and modular, swappable components instead of using inheritance.

General domain of usage  
Object-oriented software design

**Composition** represents a **has-a relationship**, where a class contains an instance of another class instead of inheriting from it. For example, a **Car has an Engine**. This differs from **inheritance**, which models an **'is-a' relationship**, such as a **Truck is a Vehicle**.

class Engine:
    def start(self):
        return "Engine started"

class Car:
    def __init__(self, brand, engine):
        self.brand = brand
        self.engine = engine   # Car HAS an Engine

    def drive(self):
        return f"{self.brand}: {self.engine.start()}"


engine = Engine()
car = Car("Toyota", engine)

print(car.drive())

`Car` does not inherit from `Engine`. Instead, it **contains** an `Engine` object and uses it to perform its work. This shows a **has-a** relationship, not **is-a**, making the design more flexible and easier to change later.

You should use composition when you need **flexible parts**, **clear boundaries**, and **runtime replacement** of behavior. **Prefer inheritance** when a **true is-a** hierarchy is obvious and stable.


Note

What is the main benefit of using composition?

Learn how to build clean, secure, and scalable applications with Object-Oriented Programming in Python. Cover the fundamentals of classes and objects, then moves into advanced concepts such as inheritance, composition, polymorphism, and encapsulation. With clear explanations and practical examples, you'll develop the ability to design Python programs that are powerful, maintainable, and ready for real-world use.

Learn the core building blocks of object-oriented programming in Python. This section covers classes, objects, methods, and variables, giving you a solid base to understand how OOP organizes and structures code.

Explore how classes can share and extend functionality. You'll learn different inheritance patterns, from simple hierarchies to multiple inheritance and abstract classes, and see how they enable code reuse and scalability.

Discover how to build flexible systems by combining objects. Instead of relying only on inheritance, you'll see how composition lets you assemble behaviors in a modular, adaptable way.

Understand how different objects can share a common interface while behaving in their own way. This section shows how method overriding, interfaces, and protocols make your code more dynamic and reusable.

Learn how to safeguard internal data and control access to class attributes. You'll explore private attributes, getters and setters, and data-hiding techniques that keep your classes secure and maintainable.

Introduction to Composition

Introduction to Composition