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, s):
        if isinstance(s, str) and (s == 'male' or s == 'female'):
            self.__gender = s
        else:
            raise ValueError('Input error!')

bob = Student('Bob', 'male')
print(bob.get_gender())
#bob.set_gender('trans')
#print(bob.get_gender())