class Student(object):

def __init__(self,name,score):

self.name=name

self.__score=score

@property

def score(self):

return self.__score

@score.setter

def score(self,score):

if score<0 or score>100:

raise ValueError('invalid score')

self.__score = score

@property

def grade(self):

if score>=80:

grade=A

elif score>=60:

grade=B

else:

grade=C

>>> s = Student('Bob',69)

>>> print s.grade

Traceback (most recent call last):

  File "<pyshell#7>", line 1, in <module>

    print s.grade

  File "<pyshell#5>", line 15, in grade

    if score>=80:

NameError: global name 'score' is not defined