提示SongBird instance has no attribute 'sound'

_metaclass_ = type
class Bird:
    def _init_(self):
        self.hungry = True
    def eat(self):
        if self.hungry:
            print 'Aaaaa.....'
            self.hungry = False
        else:
            print 'No, think'

class SongBird(Bird):
    def _init_(self):
        super(SongBird, self)._init_()
        self.sound = 'Squawk!'
    def sing(self):
        print self.sound

sb = SongBird( )
sb.sing( )

刚开始学python，这是书上的代码。检查过很多遍，就是通不过。