python中增加动物年龄的调用方法

Question

我被指派创建一只名为'Molly'age的狗1。我调用方法grow来增加年龄2并打印它。我知道如何让莫莉跑，但我对如何增加她的年龄感到困惑，它一直在显示None。任何帮助都会起作用，但最好能解释一下。提前感谢您的帮助。class Dog():    """ a class representing a dog """    def __init__(self, name, age):        """ Initialize name and age attributes. """        self.name = name        self.age = age    def run(self):        """ Simulate a dog running. """        print(self.name.title() + " is running ...")    def grow(self, year):        """ Update age by adding year. """        self.age += yearmolly = Dog("Molly", 1)print(molly.name + " is " + str(molly.grow(2)))molly.run()这个的输出是Molly is NoneMolly is running ...