新手在学习python时候，会遇到很多的坑，下面来具体说说其中一个。
在使用python编写面向对象的程序时，新手可能遇到TypeError: this constructor takes no arguments这个错误。
例如下面的程序：
class Ball:
def _init_(self,color,size,direction):
self.color=color
self.size=size
self.direction=direction def bounce(self):
if self.direction=="down":
self.direction="up" myBall=Ball("red","small","down")
print "I just created a ball."
print "My ball is",myBall.size
print "My ball is",myBall.color
print "My ball's direction is",myBall.direction
print "Now I'm going to bounce the ball"
myBall.bounce()
print "Now the ball's direction is",myBall.direction
运行会报错：
======================= RESTART: H:\python\bounce1.py ======================= Traceback (most recent call last):
File "H:\python\bounce1.py", line 11, in <module>
myBall=Ball("red","small","down")
TypeError: this constructor takes no arguments
出错原因是，在python中构造函数书写格式是__init__，而不是_init_，即在init两侧都是双下划线，不是单下划线。
修改后如下：
class Ball:
def __init__(self,color,size,direction):
self.color=color
self.size=size
self.direction=direction def bounce(self):
if self.direction=="down":
self.direction="up" myBall=Ball("red","small","down")
print "I just created a ball."
print "My ball is",myBall.size
print "My ball is",myBall.color
print "My ball's direction is",myBall.direction
print "Now I'm going to bounce the ball"
myBall.bounce()
print "Now the ball's direction is",myBall.direction
这是正确的运行结果：
======================= RESTART: H:\python\bounce1.py =======================
I just created a ball.
My ball is small
My ball is red
My ball's direction is down
Now I'm going to bounce the ball Now the ball's direction is up