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 self.__score>=80:

            return 'A'

        elif self.__score>=60:

            return 'B'

        return 'C'

s = Student('Bob', 59)

print s.grade

s.score = 60

print s.grade

s.score = 99

print s.grade