Clean Architecture

Clean Architecture Expert

Purpose

Structure codebases with clean boundaries between business logic, use cases, infrastructure, and UI — making systems testable and changeable.

The Four Layers

1️⃣ Entities (Innermost)

Pure business objects with business rules:

class Order:
    def __init__(self, id: str, items: list[OrderItem], customer: Customer):
        self.id = id
        self.items = items
        self.customer = customer
    
    def calculate_total(self) -> Decimal:
        return sum(item.subtotal for item in self.items)
    
    def can_be_cancelled(self) -> bool:
        return self.status in [OrderStatus.PENDING, OrderStatus.PROCESSING]
    
    def apply_discount(self, discount: Discount) -> None:
        if not self.can_apply_discount():
            raise BusinessRuleViolation("Cannot apply discount to this order")
        self.discount = discount

2️⃣ Use Cases (Application Layer)

Application-specific business rules:

class CreateOrderUseCase:
    def __init__(
        self,
        order_repository: OrderRepository,  # Interface, not implementation
        payment_gateway: PaymentGateway,    # Interface
        notification_service: NotificationService,
    ):
        self.orders = order_repository
        self.payments = payment_gateway
        self.notifications = notification_service
    
    def execute(self, request: CreateOrderRequest) -> CreateOrderResponse:
        # Validate request
        if not request.items:
            raise ValidationError("Order must have at least one item")
        
        # Create entity
        order = Order.create(request.customer_id, request.items)
        
        # Apply business rules
        if request.coupon_code:
            coupon = self.orders.find_coupon(request.coupon_code)
            order.apply_discount(coupon.to_discount())
        
        # Save and trigger side effects
        self.orders.save(order)
        self.notifications.notify_order_created(order)
        
        return CreateOrderResponse(order_id=order.id, total=order.calculate_total())

3️⃣ Interface Adapters

Controllers, presenters, gateways — translate between use cases and external world:

class OrderController:
    def __init__(self, use_case: CreateOrderUseCase):
        self.use_case = use_case
    
    def create(self, http_request: HttpRequest) -> HttpResponse:
        try:
            request = CreateOrderRequest(
                customer_id=http_request.user.id,
                items=[OrderItem(**item) for item in http_request.body["items"]],
                coupon_code=http_request.body.get("coupon_code"),
            )
            response = self.use_case.execute(request)
            return HttpResponse.created({"order_id": response.order_id, "total": str(response.total)})
        except ValidationError as e:
            return HttpResponse.bad_request({"error": str(e)})

4️⃣ Frameworks & Drivers (Outermost)

Database implementations, web frameworks, external APIs — all pluggable.

The Dependency Rule

Dependencies only point INWARD:

Frameworks → Interface Adapters → Use Cases → Entities

• Business logic never imports from outer layers
• Outer layers depend on inner layers via interfaces

Outputs

1. Layer structure for your project
2. Entity and use case design
3. Interface definitions for dependencies
4. Adapter implementations
5. Dependency injection setup