#初级版

class Foo(object):

    instance=None

    def __init__(self):

        pass

    @classmethod         # 类方法  类方法只能访问类变量，不能访问实例变量

    def get_instance(self):

        if Foo.instance:

            return Foo.instance

        else:

            Foo.instance = Foo()

            return Foo.instance

obj1=Foo.get_instance()

obj2=Foo.get_instance()

print(id(obj1),id(obj2))

#高级版

class Foo(object):

    instance =None

    def __init__(self):

        pass

    def __new__(cls, *args, **kwargs):

        if Foo.instance:

            return Foo.instance

        else:

            Foo.instance = object.__new__(cls,*args,**kwargs)

            return Foo.instance

obj1=Foo()

obj2=Foo()

print(id(obj1),id(obj2))