Python flat a list of list

>>> l=[[1, 2, 3], [4, 5, 6]]
>>> [i for sublist in l for i in sublist]
[1, 2, 3, 4, 5, 6]

http://stackoverflow.com/questions/952914/making-a-flat-list-out-of-list-of-lists-in-python

- 2016-04-12 edit