怎么理解限制添加屬性

class?Student(object):
????__slots__?=?('name',?'gender',?'score')
????def?__init__(self,?name,?gender,?score):
????????self.name?=?name
????????self.gender?=?gender
????????self.score?=?score
?#???對實例進行操作
>>>?s?=?Student('Bob',?'male',?59)
>>>?s.name?=?'Tim'?#?OK
>>>?s.score?=?99?#?OK
>>>?s.grade?=?'A'
Traceback?(most?recent?call?last):
??...
AttributeError:?'Student'?object?has?no?attribute?'grade'