How to mock the domain center in an onion architecture in Python

from abc import ABC, abstractmethod
from dataclasses import dataclass
from typing import Dict, List, Optional


@dataclass
class Order:
    id: int
    customer: str
    items: List[str]
    total: float


class OrderRepository(ABC):
    @abstractmethod
    def find_by_id(self, order_id: int) -> Optional[Order]:
        pass

    @abstractmethod
    def save(self, order: Order) -> None:
        pass


class InMemoryOrderRepository(OrderRepository):
    def __init__(self) -> None:
        self._orders: Dict[int, Order] = {}

    def find_by_id(self, order_id: int) -> Optional[Order]:
        return self._orders.get(order_id)

    def save(self, order: Order) -> None:
        self._orders[order.id] = order


class OrderService:
    def __init__(self, repo: OrderRepository) -> None:
        self._repo = repo

    def get_order_total(self, order_id: int) -> float:
        order = self._repo.find_by_id(order_id)
        if order is None:
            raise ValueError(f"Order {order_id} not found")
        # Domain logic: apply 10% discount for large orders
        total = order.total
        if len(order.items) >= 3:
            total *= 0.9
        return round(total, 2)


if __name__ == "__main__":
    repo = InMemoryOrderRepository()
    repo.save(Order(id=1, customer="Alice", items=["book", "pen", "notebook"], total=50.00))
    repo.save(Order(id=2, customer="Bob", items=["laptop"], total=1200.00))

    service = OrderService(repo)
    print(f"Order 1 total after discount: ${service.get_order_total(1)}")
    print(f"Order 2 total after discount: ${service.get_order_total(2)}")