need for help

[leonardo selmi]

hi guys

i typed the following program:

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

    def _str_(self):
        msg = 'hi, i am a ' + self.size + ' ' + self.color + 'ball!'
        return msg

myball = ball('red', 'small', 'down')
print my ball

BUT I GOT THIS ERROR:

Traceback (most recent call last):
  File "/Users/leonardo/Documents/ball2.py", line 11, in <module>
    myball = ball('red', 'small', 'down')
TypeError: this constructor takes no arguments

can you kindly tell me what is wrong?
thanks a lot!