设计模式系列Python实现（一）-单例模式(Singleton)

#-*- encoding=utf-8 -*-PRint '----------------------方法1--------------------------'#方法1,实现__new__方法#并在将一个类的实例绑定到类变量_instance上,#如果cls._instance为None说明该类还没有实例化过,实例化该类,并返回#如果cls._instance不为None,直接返回cls._instanceclass Singleton(object):    def __new__(cls, *args, **kw):        if not hasattr(cls, '_instance'):            orig = super(Singleton, cls)            cls._instance = orig.__new__(cls, *args, **kw)        return cls._instanceclass MyClass(Singleton):    a = 1one = MyClass()two = MyClass()two.a = 3print one.a#3#one和two完全相同,可以用id(), ==, is检测print id(one)#29097904print id(two)#29097904print one == two#Trueprint one is two#Trueprint '----------------------方法2--------------------------'#方法2,共享属性;所谓单例就是所有引用(实例、对象)拥有相同的状态(属性)和行为(方法)#同一个类的所有实例天然拥有相同的行为(方法),#只需要保证同一个类的所有实例具有相同的状态(属性)即可#所有实例共享属性的最简单最直接的方法就是__dict__属性指向(引用)同一个字典(dict)#可参看:http://code.activestate.com/recipes/66531/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 = 1one = MyClass2()two = MyClass2()#one和two是两个不同的对象,id, ==, is对比结果可看出two.a = 3print one.a#3print id(one)#28873680print id(two)#28873712print one == two#Falseprint one is two#False#但是one和two具有相同的（同一个__dict__属性）,见:print id(one.__dict__)#30104000print id(two.__dict__)#30104000print '----------------------方法3--------------------------'#方法3:本质上是方法1的升级（或者说高级）版#使用__metaclass__（元类）的高级python用法class Singleton2(type):    def __init__(cls, name, bases, dict):        super(Singleton2, cls).__init__(name, bases, dict)        cls._instance = None    def __call__(cls, *args, **kw):        if cls._instance is None:            cls._instance = super(Singleton2, cls).__call__(*args, **kw)        return cls._instanceclass MyClass3(object):    __metaclass__ = Singleton2one = MyClass3()two = MyClass3()two.a = 3print one.a#3print id(one)#31495472print id(two)#31495472print one == two#Trueprint one is two#Trueprint '----------------------方法4--------------------------'#方法4:也是方法1的升级（高级）版本,#使用装饰器(decorator),#这是一种更pythonic,更elegant的方法,#单例类本身根本不知道自己是单例的,因为他本身(自己的代码)并不是单例的def singleton(cls, *args, **kw):    instances = {}    def _singleton():        if cls not in instances:            instances[cls] = cls(*args, **kw)        return instances[cls]    return _singleton@singletonclass MyClass4(object):    a = 1    def __init__(self, x=0):        self.x = xone = MyClass4()two = MyClass4()two.a = 3print one.a#3print id(one)#29660784print id(two)#29660784print one == two#Trueprint one is two#Trueone.x = 1print one.x#1print two.x#1