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)

#递归的方法比较容易理解
def expand_list(nested_list):
  for item in nested_list:
    if isinstance(item, (list, tuple)):
      for sub_item in expand_list(item):
        yield sub_item
    else:
      yield item
      
#在stackoverflow看到大牛的列表生成式版本
func = lambda x: [y for l in x for y in func(l)] if type(x) is list else [x]
#生成式的方法还有很多，可以自行摸索

> d=[1,2,3,[4,5],[6,7,[8,9]]]
>from compiler.ast import flatten
>flatten(a)
[1, 2, 3, 4, 5, 6,7,8,9]