代理

代理对象通常用于确保对另一个对象的保护访问，这个内部业务逻辑我们不希望污染安全要求。

假设我们要保证只有特定权限的用户才能访问资源。

代理定义:(确保只有实际可以看到预订的用户才能使用 consumer_service）

from datetime import date
from operator import attrgetter

class Proxy:
    def __init__(self, current_user, reservation_service):
        self.current_user = current_user
        self.reservation_service = reservation_service

    def highest_total_price_reservations(self, date_from, date_to, reservations_count):
        if self.current_user.can_see_reservations:
            return self.reservation_service.highest_total_price_reservations(
                date_from,
                date_to,
                reservations_count
              )
        else:
            return []

#Models and ReservationService:

class Reservation:
    def __init__(self, date, total_price):
        self.date = date
        self.total_price = total_price

class ReservationService:
    def highest_total_price_reservations(self, date_from, date_to, reservations_count):
        # normally it would be read from database/external service
        reservations = [
            Reservation(date(2014, 5, 15), 100),
            Reservation(date(2017, 5, 15), 10),
            Reservation(date(2017, 1, 15), 50)
        ]

        filtered_reservations = [r for r in reservations if (date_from <= r.date <= date_to)]

        sorted_reservations = sorted(filtered_reservations, key=attrgetter('total_price'), reverse=True)

        return sorted_reservations[0:reservations_count]

class User:
    def __init__(self, can_see_reservations, name):
        self.can_see_reservations = can_see_reservations
        self.name = name

#Consumer service:

class StatsService:
    def __init__(self, reservation_service):
        self.reservation_service = reservation_service

    def year_top_100_reservations_average_total_price(self, year):
        reservations = self.reservation_service.highest_total_price_reservations(
            date(year, 1, 1),
            date(year, 12, 31),
            1
        )

        if len(reservations) > 0:
            total = sum(r.total_price for r in reservations)

            return total / len(reservations)
        else:
            return 0

#Test:
def test(user, year):
    reservations_service = Proxy(user, ReservationService())
    stats_service = StatsService(reservations_service)
    average_price = stats_service.year_top_100_reservations_average_total_price(year)
    print("{0} will see: {1}".format(user.name, average_price))

test(User(True, "John the Admin"), 2017)
test(User(False, "Guest"),         2017)

优点

• 当访问限制发生变化时，我们会避免在 ReservationService 中进行任何更改。
• 我们没有将业务相关数据（date_from，date_to，reservations_count）与服务中的域无关概念（用户权限）混合在一起。
• 消费者（StatsService）也没有权限相关的逻辑

CAVEATS

• 代理接口始终与它隐藏的对象完全相同，因此使用代理包含的服务的用户甚至不知道代理存在。