class Student(object):

    def __init__(self, name, score):

        self.name = name

        self.score = score

    @property

    def grade(self):

        try:

            if 100 >= self.score > 80:

                return 'A'

            if  80 <= self.score < 60:

                return 'C'

            if  0 <= self.score <= 60:

                return 'B'

            else:

                raise ValueError('invalid score')

        except  Exception as e:

            pass

s = Student('Bob', 59)

print s.grade

s.score = 60

print s.grade

s.score = 99

print s.grade

s.score = 1000

print s.score

print s.grade