#单例模式class Dog(object):        #定义一个类变量        #None相当于c++中的NULL，修饰基本类型和类类型均可        instance = None        #重写__new__方法        def __new__(cls):                if None == cls.instance:                        #调用父类__new__方法创建对象                        cls.instance = object.__new__(cls)                        return cls.instance                else:                        return cls.instanced1 = Dog()print(id(d1))d2 = Dog()print(id(d2))#结果显示两个对象的地址相同

#单例模式--特殊赋值class Cat(object):        instance = None        #python要求__new__方法的参数必须和__init__方法的参数一致，虽然在__new__方法中只有cls这个形参有用        def __new__(cls,name):                if None == cls.instance:                        cls.instance = object.__new__(cls)                        return cls.instance                else:                        return cls.instance        def __init__(self,name):                self.name = namec1 = Cat("tom")print(id(c1))print(c1.name)#打印tomc2 = Cat("jack")print(id(c2))print(c2.name)#打印jack#这是因为__init__方法并没有做instance的判断，所以d2的调用相当于重新给name赋值了

#单例模式--完成版class Cat(object):        instance = None        initFlag = True        def __new__(cls,name):                if None == cls.instance:                        cls.instance = object.__new__(cls)                        return cls.instance                else:                        return cls.instance        def __init__(self,name):                #保证__init__只执行一次                #注意类变量的调用方式                if Cat.initFlag:                        self.name = name                        Cat.initFlag = Falsec1 = Cat("tom")print(id(c1))print(c1.name)#打印tomc2 = Cat("jack")print(id(c2))print(c2.name)#打印tom