覆盖类方法。打印输出包括NONE

Question

在类PetData下插入时，使用派生的print_all类方法输出问题。我的输出在ID之前包括NONE：4444。为什么？

class AnimalData:
    def __init__(self):
        self.full_name = ''
        self.age_years = 0

    def set_name(self, given_name):
        self.full_name = given_name

    def set_age(self, num_years):
        self.age_years = num_years

    # Other parts omitted

    def print_all(self):
        print('Name:',  self.full_name)
        print('Age:', self.age_years)


class PetData(AnimalData):
    def __init__(self):
        AnimalData.__init__(self)
        self.id_num = 0

    def set_id(self, pet_id):
        self.id_num = pet_id

    # FIXME: Add print_all() member method

    ''' Your solution goes here '''

    def print_all(self):
        print(AnimalData.print_all(self), 'ID:', self.id_num)

user_pet = PetData()
user_pet.set_name('Fluffy')
user_pet.set_age(5)
user_pet.set_id(4444)
user_pet.print_all()

输出：

姓名：Fluffy年龄：5无ID：4444

Answer 1

弄清楚了：

''' Your solution goes here '''

def print_all(self):
    AnimalData.print_all(self)
    print('ID:', self.id_num)