零基础搭建校园外卖订餐系统：四端流程、校门分拣与上线验收

from dataclasses import dataclass, field
from enum import Enum

class State(str, Enum):
    CREATED = "CREATED"
    PAID = "PAID"
    ACCEPTED = "ACCEPTED"
    READY = "READY"
    PICKED_UP = "PICKED_UP"
    GATE_RECEIVED = "GATE_RECEIVED"
    SORTED = "SORTED"
    DELIVERING = "DELIVERING"
    DELIVERED = "DELIVERED"
    REFUNDED = "REFUNDED"

TRANSITIONS = {
    (State.CREATED, "PAY"): State.PAID,
    (State.PAID, "ACCEPT"): State.ACCEPTED,
    (State.ACCEPTED, "READY"): State.READY,
    (State.READY, "PICKUP"): State.PICKED_UP,
    (State.PICKED_UP, "GATE_RECEIVE"): State.GATE_RECEIVED,
    (State.GATE_RECEIVED, "SORT_BUILDING"): State.SORTED,
    (State.SORTED, "START_DELIVERY"): State.DELIVERING,
    (State.DELIVERING, "DELIVER"): State.DELIVERED,
}

@dataclass
class Order:
    order_id: str
    paid_amount: int = 0
    refunded_amount: int = 0
    state: State = State.CREATED
    logs: list = field(default_factory=list)

class OrderService:
    def __init__(self):
        self.seen_requests = set()

    def apply(self, order: Order, action: str, request_id: str, amount: int = 0):
        # 同一个请求号重复提交时，返回已有结果，不重复写状态或账务
        if request_id in self.seen_requests:
            return order

        if action == "REFUND":
            if order.state not in (State.PAID, State.ACCEPTED, State.DELIVERED):
                raise ValueError("当前状态不允许退款")
            if order.refunded_amount + amount > order.paid_amount:
                raise ValueError("累计退款不能超过实付金额")
            order.refunded_amount += amount
            if order.refunded_amount == order.paid_amount:
                order.state = State.REFUNDED
            order.logs.append((request_id, action, amount))
            self.seen_requests.add(request_id)
            return order

        next_state = TRANSITIONS.get((order.state, action))
        if next_state is None:
            raise ValueError("非法状态跳转")
        order.state = next_state
        order.logs.append((request_id, action, 0))
        self.seen_requests.add(request_id)
        return order