校园跑腿系统怎么开发：代取快递、代买代办的任务模型与异常验收

from dataclasses import dataclass, field
from decimal import Decimal, ROUND_HALF_UP
from enum import Enum

class TaskType(Enum):
    EXPRESS = 'express'
    BUY = 'buy'
    ERRAND = 'errand'

class OrderStatus(Enum):
    PENDING_PAYMENT = 'pending_payment'
    PAID = 'paid'
    ACCEPTED = 'accepted'
    PICKED = 'picked'
    DELIVERING = 'delivering'
    DELIVERED = 'delivered'
    COMPLETED = 'completed'
    CANCELLED = 'cancelled'
    REFUNDING = 'refunding'

@dataclass
class FeeBreakdown:
    goods: Decimal = Decimal('0')
    service: Decimal = Decimal('0')
    delivery: Decimal = Decimal('0')
    discount: Decimal = Decimal('0')

    def total(self) -> Decimal:
        value = self.goods + self.service + self.delivery - self.discount
        return value.quantize(Decimal('0.01'), rounding=ROUND_HALF_UP)

@dataclass
class Order:
    order_id: str
    task_type: TaskType
    status: OrderStatus = OrderStatus.PENDING_PAYMENT
    pickup: str = ''
    dropoff: str = ''
    items: list = field(default_factory=list)
    budget: Decimal = Decimal('0')
    fees: FeeBreakdown = field(default_factory=FeeBreakdown)
    events: list = field(default_factory=list)

    def transition(self, to: OrderStatus) -> None:
        allowed = {
            OrderStatus.PENDING_PAYMENT: {OrderStatus.PAID, OrderStatus.CANCELLED},
            OrderStatus.PAID: {OrderStatus.ACCEPTED, OrderStatus.REFUNDING, OrderStatus.CANCELLED},
            OrderStatus.ACCEPTED: {OrderStatus.PICKED, OrderStatus.REFUNDING},
            OrderStatus.PICKED: {OrderStatus.DELIVERING, OrderStatus.REFUNDING},
            OrderStatus.DELIVERING: {OrderStatus.DELIVERED, OrderStatus.REFUNDING},
            OrderStatus.DELIVERED: {OrderStatus.COMPLETED, OrderStatus.REFUNDING},
            OrderStatus.REFUNDING: {OrderStatus.CANCELLED, OrderStatus.COMPLETED},
        }
        if to not in allowed.get(self.status, set()):
            raise ValueError(f'非法状态迁移: {self.status.value} -> {to.value}')
        self.events.append((self.status.value, to.value))
        self.status = to

def quote(order: Order, urgent: bool = False) -> FeeBreakdown:
    if order.task_type is TaskType.EXPRESS:
        delivery = Decimal('2.00') + Decimal('0.50') * max(0, len(order.items) - 1)
        return FeeBreakdown(delivery=delivery)
    if order.task_type is TaskType.BUY:
        if order.budget <= 0 or not order.items:
            raise ValueError('代买必须填写商品清单和预算')
        goods = order.budget
        service = max(Decimal('1.00'), goods * Decimal('0.10')).quantize(Decimal('0.01'))
        return FeeBreakdown(goods=goods, service=service, delivery=Decimal('2.00'))
    if order.task_type is TaskType.ERRAND:
        service = Decimal('5.00') + (Decimal('3.00') if urgent else Decimal('0'))
        return FeeBreakdown(service=service)
    raise ValueError('未知任务类型')

def refund_amount(order: Order, purchased: bool = False) -> Decimal:
    fees = order.fees
    if order.status in {OrderStatus.PENDING_PAYMENT, OrderStatus.CANCELLED}:
        return Decimal('0.00')
    if order.status in {OrderStatus.PAID, OrderStatus.ACCEPTED}:
        return fees.total()
    if order.status is OrderStatus.PICKED:
        goods = fees.goods if purchased else Decimal('0')
        return (goods + fees.service * Decimal('0.50')).quantize(Decimal('0.01'))
    return fees.delivery.quantize(Decimal('0.01'))

def demo():
    order = Order('A001', TaskType.BUY, pickup='北门超市', dropoff='3栋502', items=['牛奶'], budget=Decimal('20.00'))
    order.fees = quote(order)
    order.transition(OrderStatus.PAID)
    order.transition(OrderStatus.ACCEPTED)
    order.transition(OrderStatus.PICKED)
    assert order.fees.total() == Decimal('24.00')
    try:
        order.transition(OrderStatus.PENDING_PAYMENT)
    except ValueError as exc:
        print('异常验收:', exc)
    print('退款示例:', refund_amount(order, purchased=True))

if __name__ == '__main__':
    demo()