重点是复用
要感受它。

-- coding:utf-8 --
from weakref import WeakKeyDictionary
class Grade(object):

def init(self):

self._values = WeakKeyDictionary()
def __get__(self,instance,instance_type):
    if instance is None:
        return None
    return self._values.get(instance,0)

def __set__(self,value):
    if not ( 0<=value<=100):
        raise ValueError("Grade must be in 0 .. 100")
    self._values[instance] = value
class Student(object):

cn = Grade()

en = Grade()
p1 = Student()

p1.cn = 90

p1.en = 101