在此插入代码
>>> class Student:
    def get_grade(self):
        self.score='54'
        if self.score>=90:
            return 'A'
        elif self.score>=60:
            return 'B'
        else:
            return 'C'


>>> a=Student
>>> a.get_grade()
Traceback (most recent call last):
  File "<pyshell#54>", line 1, in <module>
    a.get_grade()
TypeError: get_grade() missing 1 required positional argument: 'self'