Composition

Motivation

Composition solves the problem of inheritance hierarchies that become rigid and fragile. Instead of forcing behavior into a class tree, objects can be built by combining focused collaborators.

Where it fits

Composition belongs to software design and object-oriented programming. It supports modularity, testability, and dependency injection.

class EmailReceiptOrderService(PaymentOrderService):  def checkout(self, order):      self.charge(order)      self.save(order)      self.send_email(order)

class OrderService:  def __init__(self, payment, repository, notifier):      self.payment = payment      self.repository = repository      self.notifier = notifier  def checkout(self, order):      self.payment.charge(order.total)      self.repository.save(order)      self.notifier.confirm(order)

Common mistakes

• Replacing every inheritance relationship even when subtyping is valid.
• Passing too many collaborators into one object because responsibilities are still unclear.