定义了__getattr__()，当访问object不存在的属性时会调用该方法
不定义访问不存在的属性时会报 AttributeError
eg:
class Cat(object):
def __init__(self):
self.name = "jn"
def __getattr__(self, item):
return "tm"
cat = Cat()
print(cat.name)
print(getattr(cat, 'name'))
print("*" * 20)
print(cat.age)
print(getattr(cat, 'age'))