#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._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



	





	#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  
 
@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