1.用 __new__方法

class Singleton(object):    def __new__(cls):        if not hasattr(cls,'_instance'):            cls._instance=super(Singleton,cls).__new__(cls)#            cls.__instance = object.__new__(cls) #这样也可以        return cls._instancea = Singleton()b = Singleton()c = Singleton()print(a,id(a))print(b,id(b))print(c,id(c))

输出:

 42443328 42443328 42443328

2.共享属性方法

共享属性:创建实例时把所有实例的__dict__指向同一个字典,这样它们具有相同的属性和方法.

class Borg(object):    _state = {}    def __new__(cls, *args, **kw):        ob = super(Borg, cls).__new__(cls, *args, **kw)        ob.__dict__ = cls._state        return obclass MyClass2(Borg):    a = 1b1 = MyClass2()b2 = MyClass2()b3 = MyClass2()b1.a='萨菲的'b2.a='萨菲02'b1.a='萨菲03'MyClass2.a = 333print(id(b1),id(b2),id(b3))print(b1.a,b3.a,b2.a)print(id(b1.a),id(b2.a),id(b3.a))

输出:

42902024 42902080 42901688萨菲03 萨菲03 萨菲03167568080 167568080 167568080

装饰器版本

def singleton(cls, *args, **kw):    instances = {}    def getinstance():        if cls not in instances:            instances[cls] = cls(*args, **kw)        return instances[cls]    return getinstance@singletonclass MyClass:    a = 1b1 = MyClass()b2 = MyClass()b3 = MyClass()print(id(b1),id(b2),id(b3))print(b1.a,b3.a,b2.a)

输出:

42712928 42712928 427129281 1 1

类方法版

class Foo():    __v = None    @classmethod    def get_instance(cls):        if cls.__v:            print("true: 这是cls.__v",cls.__v)            return cls.__v        else:            print("false: 这是cls.__v", cls.__v)            cls.__v = Foo()            print("false_1: 这是cls.__v", cls.__v)            return cls.__vobj = Foo.get_instance()obj1 = Foo.get_instance()obj2 = Foo.get_instance()obj3 = Foo.get_instance()print(obj,'哈哈',id(obj))print(obj1,'哈哈',id(obj1))print(obj2,'哈哈',id(obj2))print(obj3,'哈哈',id(obj3))

输出:

false: 这是cls.__v Nonefalse_1: 这是cls.__v true: 这是cls.__v true: 这是cls.__v true: 这是cls.__v  哈哈 38798056 哈哈 38798056 哈哈 38798056 哈哈 38798056

单例默认:

'''学习中遇到问题没人解答？小编创建了一个Python学习交流群：711312441寻找有志同道合的小伙伴，互帮互助,群里还有不错的视频学习教程和PDF电子书！'''class Singleton(object):    def __new__(cls,*args,**kwargs):        if not hasattr(cls,'_instance'):            print('你好啊',)            cls._instance = super(Singleton,cls).__new__(cls)            print(cls._instance, '阿斯蒂芬01')            return cls._instance        print( '阿斯蒂芬')        print(cls._instance, '阿斯蒂芬02')        return cls._instance        a = Singleton()print('这是a',id(a),a)b = Singleton()print('这是b',id(b),b)

输出:

你好啊 阿斯蒂芬01这是a 42516208 阿斯蒂芬 阿斯蒂芬02这是b 42516208