python的单例模式....singleton

class singleton(type):
    def __new__(cls, name, bases, dict):
        old_init = dict.get('__init__')
        old_new = super(cls.__class__, cls).__new__
        def _wrap_init(self, *args, **kwargs):
            if hasattr(self.__class__,'_onlyou_'):
                return
            setattr(self.__class__,'_onlyou_', self)
            old_init(self,*args,**kwargs)
        def _wrap_new(cls, *args, **kwargs):
            if hasattr(cls,'_onlyou_'):
                return getattr(cls, '_onlyou_')
            return old_new(cls, *args, **kwargs)
        dict['__init__'] = _wrap_init
        dict['__new__'] = _wrap_new
        return type.__new__(cls, name, bases, dict)




class A(object):
    __metaclass__ = singleton
    def __init__(self, a,b):
        self.a=a
        self.b=b


a=A(1,2) #first
b=A()
c=A(1,2,3,4,5)


id(a) == id(b) == id(c)