Section 1/4has-a vs is-a

has-a vs is-a

A common OOP mistake is using inheritance when composition is correct. A Car should not inherit from Engine.

Composition

Composition is a "has-a" relationship where one object ownsanother. The contained object is created inside the container — and if the container is destroyed, the contained object is too. This is called a strong has-arelationship.

has-a vs is-a

is-a → use inheritance: a Dog is an Animal
has-a → use composition: a Car has an Engine

A common OOP mistake is using inheritance when composition is correct. A Car should not inherit from Engine.

Composition in Code

class Engine:
    def __init__(self, hp):
        self.horsepower = hp
class Car:
    def __init__(self, make, hp):
        self.make   = make
        self.engine = Engine(hp)  # Car CREATES the Engine
    def get_power(self):
        return self.engine.horsepower
c = Car("Toyota", 400)
print(c.get_power())  # → 400

→

The Engine is created inside Car.__init__. It has no independent existence — this is the defining feature of composition.

Lifecycle Ownership

Composition = the container controls the lifetime of the contained object. When the Car is deleted, its Engine ceases to exist too (no other reference holds it).

Car → Engine (composition — filled diamond in UML)
House → Rooms (composition)
Computer → CPU (composition)

# Task: Build a Car class that uses composition with Engine.
#
# Engine is already complete.
# 1. Write class Car with __init__(self, make, horsepower)
#    - Create an Engine instance INSIDE __init__: self.engine = Engine(horsepower)
#    - Store the make: self.make = make
# 2. Write get_power(self) that returns self.engine.horsepower
# 3. Write __str__(self) that returns "Toyota (400hp)"
class Engine:
    def __init__(self, horsepower):
        self.horsepower = horsepower
    def __str__(self):
        return f"Engine({self.horsepower}hp)"
# Write your Car class here
# class Car:
#     def __init__(self, make, horsepower):
#         ...
c = Car("Toyota", 400)
print(c.make)          # Should print: Toyota
print(c.get_power())   # Should print: 400

Output:

Click "Check" to run your code.

Composition

has-a vs is-a

Composition in Code

Lifecycle Ownership

Your Task

has-a vs is-a

Composition

has-a vs is-a

Composition in Code

Lifecycle Ownership

Your Task

Output: