单实例模式一直是常用的设计模式，对于python的单实例模式，其实其本身就有实现

http://stackoverflow.com/questions/31875/is-there-a-simple-elegant-way-to-define-singletons-in-python/31887#31887

里面说到module，module只会初始化一次，天然的singleton。这是最为python的解决方案。将你所需要的属性和方法，直接暴露在模块中变成模块的全局变量和方法即可。

另外，如果真的要使用代码进行singleton，下面给出几个代码

# Singleton/ClassVariableSingleton.pyclass SingleTone(object):    __instance = None    def __new__(cls, val):        if SingleTone.__instance is None:            SingleTone.__instance = object.__new__(cls)        SingleTone.__instance.val = val        return SingleTone.__instance

# Singleton/SingletonDecorator.pyclass SingletonDecorator:    def __init__(self,klass):        self.klass = klass        self.instance = None    def __call__(self,*args,**kwds):        if self.instance == None:            self.instance = self.klass(*args,**kwds)        return self.instanceclass foo: passfoo = SingletonDecorator(foo)x=foo()y=foo()z=foo()x.val = ‘sausage‘y.val = ‘eggs‘z.val = ‘spam‘print(x.val)print(y.val)print(z.val)print(x is y is z)

# Singleton/SingletonMetaClass.pyclass SingletonMetaClass(type):    def __init__(cls,name,bases,dict):        super(SingletonMetaClass,cls)          .__init__(name,bases,dict)        original_new = cls.__new__        def my_new(cls,*args,**kwds):            if cls.instance == None:                cls.instance =                   original_new(cls,*args,**kwds)            return cls.instance        cls.instance = None        cls.__new__ = staticmethod(my_new)class bar(object):    __metaclass__ = SingletonMetaClass    def __init__(self,val):        self.val = val    def __str__(self):        return `self` + self.valx=bar(‘sausage‘)y=bar(‘eggs‘)z=bar(‘spam‘)print(x)print(y)print(z)print(x is y is z)