list append后类型为None

import random

red = [y for y in range (1,34)]

redballs = []
for n in range(1,7):
    ball = red.pop(random.randint(0, (len(red)-1)))
    print ball
    s = redballs.append(ball)
    print s

>>> ================================ RESTART ================================
>>>
6
None
16
None
1
None
12
None
19
None
33
None
>>>
想知道为什么redballs这个list的追加为啥不成功呢？错在哪里呀？实在没看出来
>>> redballs = []
>>> ball = 34
>>> redballs.append(ball)
>>> redballs
[34]
>>> ball = 45
>>> redballs.append(ball)
>>> redballs
[34, 45]
这样却可以。
没明白哪里错了，请高手指点


[解决办法]
没有错，只是你不应该打印s，应该打印redballs看结果