#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 =

print one.a

#

print id(one)

#

print id(two)

#

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 =

    def __init__(self, x=):

        self.x = x  

one = MyClass4()

two = MyClass4()  

two.a =

print one.a

#

print id(one)

#

print id(two)

#

print one == two

#True

print one is two

#True

one.x =

print one.x

#

print two.x

#