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

    def set_gender(self, gender):
        if gender == 'F' or gender == 'M':
            self.__gender = gender
        else:
            raise ValueError('bad gender')

    def get_gender(self):
        return self.__gender

Pengchen = Student('Pengchen', 'M')
Xiao = Student('Xiao', 'F')

print(Pengchen.get_gender())
print(Xiao.get_gender())
Xiao.set_gender('M')
print(Xiao.get_gender())