class Student(object):
    def __init__(self, name, score):
        self.name = name
        self.score = score

    def get_grade(self):
        if self.score < 0 or self.score > 100:
            raise ValueError        elif self.score >= 80:
            return 'A'        elif self.score >= 60:
            return 'B'        else:
            return 'C'

运行结果：

....
----------------------------------------------------------------------
Ran 4 tests in 0.000s

OK