architecture-patterns

# domain/entities/user.py
from dataclasses import dataclass
from datetime import datetime
 
@dataclass
class User:
    """Core user entity - no framework dependencies."""
    id: str
    email: str
    name: str
    created_at: datetime
    is_active: bool = True
 
    def deactivate(self):
        """Business rule: deactivating user."""
        self.is_active = False
 
    def can_place_order(self) -> bool:
        """Business rule: active users can order."""
        return self.is_active
 
# domain/interfaces/user_repository.py
from abc import ABC, abstractmethod
from typing import Optional
 
class IUserRepository(ABC):
    """Port: defines contract, no implementation."""
 
    @abstractmethod
    async def find_by_id(self, user_id: str) -> Optional[User]:
        pass
 
    @abstractmethod
    async def save(self, user: User) -> User:
        pass
 
# use_cases/create_user.py
from dataclasses import dataclass
import uuid
from datetime import datetime
 
@dataclass
class CreateUserRequest:
    email: str
    name: str
 
class CreateUserUseCase:
    """Use case: orchestrates business logic."""
 
    def __init__(self, user_repository: IUserRepository):
        self.user_repository = user_repository
 
    async def execute(self, request: CreateUserRequest) -> CreateUserResponse:
        # Business validation
        existing = await self.user_repository.find_by_email(request.email)
        if existing:
            return CreateUserResponse(user=None, success=False, error="Email already exists")
 
        # Create entity
        user = User(
            id=str(uuid.uuid4()),
            email=request.email,
            name=request.name,
            created_at=datetime.now(),
            is_active=True
        )
 
        # Persist
        saved_user = await self.user_repository.save(user)
        return CreateUserResponse(user=saved_user, success=True)
 
# adapters/repositories/postgres_user_repository.py
class PostgresUserRepository(IUserRepository):
    """Adapter: PostgreSQL implementation."""
 
    def __init__(self, pool):
        self.pool = pool
 
    async def find_by_id(self, user_id: str) -> Optional[User]:
        async with self.pool.acquire() as conn:
            row = await conn.fetchrow("SELECT * FROM users WHERE id = $1", user_id)
            return self._to_entity(row) if row else None
 
    async def save(self, user: User) -> User:
        async with self.pool.acquire() as conn:
            await conn.execute(
                """INSERT INTO users (id, email, name, created_at, is_active)
                VALUES ($1, $2, $3, $4, $5)
                ON CONFLICT (id) DO UPDATE SET email = $2, name = $3, is_active = $5""",
                user.id, user.email, user.name, user.created_at, user.is_active
            )
            return user

# Core domain (hexagon center)
class OrderService:
    """Domain service - no infrastructure dependencies."""
 
    def __init__(
        self,
        order_repository: OrderRepositoryPort,
        payment_gateway: PaymentGatewayPort,
        notification_service: NotificationPort
    ):
        self.orders = order_repository
        self.payments = payment_gateway
        self.notifications = notification_service
 
    async def place_order(self, order: Order) -> OrderResult:
        # Business logic
        if not order.is_valid():
            return OrderResult(success=False, error="Invalid order")
 
        # Use ports (interfaces)
        payment = await self.payments.charge(amount=order.total, customer=order.customer_id)
 
        if not payment.success:
            return OrderResult(success=False, error="Payment failed")
 
        order.mark_as_paid()
        saved_order = await self.orders.save(order)
 
        await self.notifications.send(
            to=order.customer_email,
            subject="Order confirmed",
            body=f"Order {order.id} confirmed"
        )
 
        return OrderResult(success=True, order=saved_order)
 
# Adapters
class StripePaymentAdapter(PaymentGatewayPort):
    """Primary adapter: connects to Stripe API."""
    async def charge(self, amount: Money, customer: str) -> PaymentResult:
        # Implementation
 
class MockPaymentAdapter(PaymentGatewayPort):
    """Test adapter: no external dependencies."""
    async def charge(self, amount: Money, customer: str) -> PaymentResult:
        return PaymentResult(success=True, transaction_id="mock-123")

# Value Objects (immutable)
@dataclass(frozen=True)
class Email:
    value: str
 
    def __post_init__(self):
        if "@" not in self.value:
            raise ValueError("Invalid email")
 
@dataclass(frozen=True)
class Money:
    amount: int  # cents
    currency: str
 
    def add(self, other: "Money") -> "Money":
        if self.currency != other.currency:
            raise ValueError("Currency mismatch")
        return Money(self.amount + other.amount, self.currency)
 
# Entities (with identity)
class Order:
    def __init__(self, id: str, customer: Customer):
        self.id = id
        self.customer = customer
        self.items: List[OrderItem] = []
        self.status = OrderStatus.PENDING
        self._events: List[DomainEvent] = []
 
    def add_item(self, product: Product, quantity: int):
        item = OrderItem(product, quantity)
        self.items.append(item)
        self._events.append(ItemAddedEvent(self.id, item))
 
    def submit(self):
        if not self.items:
            raise ValueError("Cannot submit empty order")
        self.status = OrderStatus.SUBMITTED
        self._events.append(OrderSubmittedEvent(self.id))
 
# Aggregates (consistency boundary)
class Customer:
    def __init__(self, id: str, email: Email):
        self.id = id
        self.email = email
        self._addresses: List[Address] = []
 
    def add_address(self, address: Address):
        if len(self._addresses) >= 5:
            raise ValueError("Maximum 5 addresses allowed")
        self._addresses.append(address)