python __set__ __get__ __delete__

class Attr(object):
    def __init__(self,attrname,attrtype):
        self.attrname=attrname
        self.attrtype=attrtype
    def __get__(self,instance,value):
        return instance.__dict__[self.attrname]
    def __set__(self,instance,value):
        if not isinstance(value,self.attrtype):
             raise TypeError("%s type error"%self.attrname)
        instance.__dict__[self.attrname]=value
    def __delete__(self,instance):
        del instance.__dict__[self.attrname]

class Person(object):
    name=Attr("name",str)
    age=Attr("age",int)
p=Person()
p.age=""