Python中的单例，工厂模式，观察者模式，策略模式

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#单例 #工厂模式 #观察者模式 #策略模式

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本文深入讲解了四种设计模式：单例模式确保类只有一个实例；工厂模式提供创建对象的接口；观察者模式定义对象间的一对多依赖关系；策略模式封装算法，使它们可以互换。通过具体代码示例，展示了每种模式的实现方式及其应用场景。

单例

class A():
    def __new__(cls, *args, **kwargs):
        if not hasattr(cls,"instance"):
            cls.instance = object.__new__(cls)
        return cls.instance
    def __init__(self,num):
        self.num = num
a = A(5)
b = A(6)
print(a is b)      # True
print(a.num)      # 6
print(b.num)      # 6

工厂模式

class Bmw():
    def say(self):
        print("这是一辆宝马")
class Benchi():
    def say(self):
        print("这是一辆奔驰")
class Car():
    @classmethod
    def abc(cls,name):
        if name == "宝马":
            return Bmw()
        elif name == "奔驰":
            return Benchi()
a = Car.abc("宝马")
b = Car.abc("奔驰")
a.say()      # 这是一辆宝马
b.say()      # 这是一辆奔驰

观察者模式

class Boss():
    def __init__(self):
        self.obList = []
        self.msg = ""
    def attach(self,obj):
        self.obList.append(obj)
    def notify(self):
        for obj in self.obList:
            obj.update()
class Market():
    def __init__(self,name,boss):
        self.name = name
        self.boss = boss
    def update(self):
        print("市场部的{}您好，{}".format(self.name,self.boss.msg))
class Tech():
    def __init__(self,name,boss):
        self.name = name
        self.boss = boss
    def update(self):
        print("技术部的{}您好，{}".format(self.name,self.boss.msg))
boss = Boss()
zs = Market("张三", boss)
ls = Market("李四", boss)
ww = Tech("王五", boss)
boss.attach(zs)
boss.attach(ls)
boss.attach(ww)
boss.msg = "下午开会"
boss.notify()
# 运行结果
# 市场部的张三您好，下午开会
# 市场部的李四您好，下午开会
# 技术部的王五您好，下午开会

策略模式

class CashNormal():
    def getCashMoney(self, money):
        return money
class CashRate():
    def __init__(self, rate):
        self.rate = rate
    def getCashMoney(self, money):
        return money*self.rate
class CashReturn():
    def __init__(self, moneyCondition, moneyReturn):
        self.moneyCondition = moneyCondition
        self.moneyReturn = moneyReturn
    def getCashMoney(self, money):
        if money > self.moneyCondition:
            return money - (money//self.moneyCondition)*self.moneyReturn
        return money
class Oper():
    def __init__(self, cash):
        self.cash = cash
    def getCash(self, money):
        return self.cash.getCashMoney(money)
cashNormal = CashNormal()
cashRate = CashRate(0.8)
cashReturn = CashReturn(400,50)
a = Oper(cashNormal)
b = Oper(cashRate)
c = Oper(cashReturn)
money = int(input("请输入钱数"))
cl = int(input("请输入方案"))
if n == 1:
    print(a.getCash(money))
elif n == 2:
    print(b.getCash(money))
elif n == 3:
    print(c.getCash(money))