定义一个宠物类来存储宠物的名字、年龄和体重。添加适当的构造函数、访问器和mutator函数。还要定义一个名为getLifespan的函数，它返回一个值为“未知寿命”的字符串。 接下来，定义一个派生自Pet的Dog类。Dog类应该有一个名为breed的私有成员变量，用于存储狗的品种。为breed变量和适当的构造函数添加mutator和accessor函数。重新定义getLifespan函数，如果狗的体重超过100磅，则返回“大约7年”和“大约”

以下为Python代码实现：

# 宠物类
class Pet:
    def __init__(self, name, age, weight):
        self.__name = name
        self.__age = age
        self.__weight = weight

    def getName(self):
        return self.__name

    def getAge(self):
        return self.__age

    def getWeight(self):
        return self.__weight

    def setName(self, name):
        self.__name = name

    def setAge(self, age):
        self.__age = age

    def setWeight(self, weight):
        self.__weight = weight

    def getLifespan(self):
        return "未知寿命"


# 狗类
class Dog(Pet):
    def __init__(self, name, age, weight, breed):
        super().__init__(name, age, weight)
        self.__breed = breed

    def getBreed(self):
        return self.__breed

    def setBreed(self, breed):
        self.__breed = breed

    def getLifespan(self):
        if self.getWeight() > 100:
            return "大约7年"
        else:
            return "大约" + super().getLifespan()


# 测试代码
dog = Dog("旺财", 3, 120, "藏獒")
print(dog.getName())
print(dog.getAge())
print(dog.getWeight())
print(dog.getBreed())
print(dog.getLifespan())

dog.setWeight(80)
print(dog.getLifespan())

运行结果为：

旺财
3
120
藏獒
大约7年
大约未知寿命