Python嵌套列表转一维（压平嵌套列表）

a=[[1,2],[3,4],[5,6]]
print [j for i in li for j in i]
#or
from itertools import chain
print list(chain(*a))
#or
import itertools
a = [[1,2,3],[4,5,6], [7], [8,9]]
out = list(itertools.chain.from_iterable(a))
#or
a=[[1,2],[3,4],[5,6]]
t=[]
[t.extend(i) for i in a]
print t
#or
a=[[1,2],[3,4],[5,6]]
print sum(a,[])
#or
reduce(lambda x, y: x+ y, a)