朋友们 有一个问题，set_gender里面为什么不用return开头直接self. _gender = gender结尾就好

class Student(object):
    def __init__(self, name, gender):
        self.name = name
        self.__gender = gender
    def get_gender(self):
        return self.__gender
    def set_gender(self, gender):
        if self.__gender not in ['male', 'female']:
            raise ValueError('Please enter male or female' )
        else:
            self.__gender = gender