访问限制

把下面的Student对象的gender字段对外隐藏起来，用get_gender()和set_gender()代替，并检查参数有效性：

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

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,sex):
        if sex=='male' or sex=='female':
            self.__gender=sex
        else:
            raise ValueError('Wrong gender')
            
bart = Student('Bart Simpson', 'male')
print(bart.get_gender())