Python單例模式的4種實現方法 分類： python學習 2015-05-08 16:49 42人閱讀 評論(0) 收藏

來源：http://blog.csdn.net/ghostfromheaven/article/details/7671853

    #-*- encoding=utf-8 -*-  
    print '----------------------方法1--------------------------'  
    #方法1,實現__new__方法  
    #並在將一個類的實例綁定到類變量_instance上,  
    #若是cls._instance爲None說明該類尚未實例化過,實例化該類,並返回  
    #若是cls._instance不爲None,直接返回cls._instance  
    class 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._instance  
      
    class MyClass(Singleton):  
        a = 1  
      
    one = MyClass()  
    two = MyClass()  
      
    two.a = 3  
    print one.a  
    #3  
    #one和two徹底相同,能夠用id(), ==, is檢測  
    print id(one)  
    #29097904  
    print id(two)  
    #29097904  
    print one == two  
    #True  
    print one is two  
    #True  
      
    print '----------------------方法2--------------------------'  
    #方法2,共享屬性;所謂單例就是全部引用(實例、對象)擁有相同的狀態(屬性)和行爲(方法)  
    #同一個類的全部實例自然擁有相同的行爲(方法),  
    #只須要保證同一個類的全部實例具備相同的狀態(屬性)便可  
    #全部實例共享屬性的最簡單最直接的方法就是__dict__屬性指向(引用)同一個字典(dict)  
    #可參看: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 ob  
      
    class MyClass2(Borg):  
        a = 1  
      
    one = MyClass2()  
    two = MyClass2()  
      
    #one和two是兩個不一樣的對象,id, ==, is對比結果可看出  
    two.a = 3  
    print one.a  
    #3  
    print id(one)  
    #28873680  
    print id(two)  
    #28873712  
    print one == two  
    #False  
    print one is two  
    #False  
    #可是one和two具備相同的（同一個__dict__屬性）,見:  
    print id(one.__dict__)  
    #30104000  
    print id(two.__dict__)  
    #30104000  
      
    print '----------------------方法3--------------------------'  
    #方法3:本質上是方法1的升級（或者說高級）版  
    #使用__metaclass__（元類）的高級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._instance  
      
    class MyClass3(object):  
        __metaclass__ = Singleton2  
      
    one = MyClass3()  
    two = MyClass3()  
      
    two.a = 3  
    print one.a  
    #3  
    print id(one)  
    #31495472  
    print id(two)  
    #31495472  
    print one == two  
    #True  
    print one is two  
    #True  
      
    print '----------------------方法4--------------------------'  
    #方法4:也是方法1的升級（高級）版本,  
    #使用裝飾器(decorator),  
    #這是一種更pythonic,更elegant的方法,  
    #單例類自己根本不知道本身是單例的,由於他自己(本身的代碼)並非單例的  
    def singleton(cls, *args, **kw):  
        instances = {}  
        def _singleton():  
            if cls not in instances:  
                instances[cls] = cls(*args, **kw)  
            return instances[cls]  
        return _singleton  
     
    @singleton  
    class MyClass4(object):  
        a = 1  
        def __init__(self, x=0):  
            self.x = x  
      
    one = MyClass4()  
    two = MyClass4()  
      
    two.a = 3  
    print one.a  
    #3  
    print id(one)  
    #29660784  
    print id(two)  
    #29660784  
    print one == two  
    #True  
    print one is two  
    #True  
    one.x = 1  
    print one.x  
    #1  
    print two.x  
    #1