http://www.zhihu.com/question/37904398?sort=created&page=2
>>> a = [[1,2],[3,4],[5,6,7]]
>>> print sum(a,[])
[1, 2, 3, 4, 5, 6, 7]
>>> l = [1,2,3]
>>> l[1:1] = 'a'
>>> l
[1, 'a', 2, 3]
>>> l[1:1]
[]
>>> l[3:3]="test"
>>> l
[1, 'a', 2, 't', 'e', 's', 't', 3]