Python教程：用__new__ 创建单例模式

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._instance

a = Singleton()

b = Singleton()

c = Singleton()

print(a,id(a))

print(b,id(b))

print(c,id(c)) 

输出:

 <__main__.Singleton object at 0x000000000287A240> 42443328

<__main__.Singleton object at 0x000000000287A240> 42443328

<__main__.Singleton object at 0x000000000287A240> 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 ob

class MyClass2(Borg):

    a = 1

b1 = MyClass2()

b2 = MyClass2()

b3 = MyClass2()

b1.a='萨菲的'

b2.a='萨菲02'

b1.a='萨菲03'

MyClass2.a = 333

print(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03

167568080 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

@singleton

class MyClass:

    a = 1

b1 = MyClass()

b2 = MyClass()

b3 = MyClass()

print(id(b1),id(b2),id(b3))

print(b1.a,b3.a,b2.a) 

输出:

 42712928 42712928 42712928

1 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.__v

obj = 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 None

false_1: 这是cls.__v <__main__.Foo object at 0x00000000025002E8>

true: 这是cls.__v <__main__.Foo object at 0x00000000025002E8>

true: 这是cls.__v <__main__.Foo object at 0x00000000025002E8>

true: 这是cls.__v <__main__.Foo object at 0x00000000025002E8>

<__main__.Foo object at 0x00000000025002E8> 哈哈 38798056

<__main__.Foo object at 0x00000000025002E8> 哈哈 38798056

<__main__.Foo object at 0x00000000025002E8> 哈哈 38798056

<__main__.Foo object at 0x00000000025002E8> 哈哈 38798056 

单例默认:

 '''

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'''

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

<__main__.Singleton object at 0x000000000288BEF0> 阿斯蒂芬01

这是a 42516208 <__main__.Singleton object at 0x000000000288BEF0>

阿斯蒂芬

<__main__.Singleton object at 0x000000000288BEF0> 阿斯蒂芬02

这是b 42516208 <__main__.Singleton object at 0x000000000288BEF0>