请问这条代码怎么改？系统显示第二个def的str有问题，谢谢帮助！

AVAN

# accessing class attributes

class Animal(object):
    def __init__(self, sound):
        self.sound = sound

    def __ str__(self):
        rep = 'Animal\n'
        rep += 'sound: '+self.sound+'\n'
        return rep

    def talk(self):
        print('self.sound, \n')

#main
dog = Animal('woof')
dog.talk()

cat = Animal('meow')
cat.talk()

print ('Cat says: ')
print (cat.sound)

print ('Dog says: ')
print (dog.sound)
        

塔利班

str左边多了个空格

AVAN

Hao de , xie xie !