是否必须在init方法中定义类中使用的每个变量？

Question

class student:
    def __init__(self, first, last, marks):
        self.first = first
        self.last = last
        self.marks = marks

    def calgrade(self, grade):
        self.grade = grade
        if(self.marks < 50):
            self.grade = 'C'
        elif(self.marks < 75):
            self.grade = 'B'
        else:
            self.grade = 'A'


s1 = student('Ayush', 'Juyal', 90)
print(s1.first)
print(s1.grade)

  

错误：AttributeError：学生实例没有属性“等级”