TypeError: this constructor takes no arguments

源码：

class Ball:
    def _init_(self,size,color,direction):
        self.color = color
        self.size = size
        self.direction = direction

    def _str_(self):
        msg = "Hi,I'm a " + self.size+ " " + self.color + " ball!"
        return msg

myBall = Ball("red","small","down")
print myBall

运行:

 

问题：

init 和 str两边的下划线，英文应该是两个下划线。