python压平嵌套列表

li=[[1,2],[3,4],[5,6]]
print ([j for i in li for j in i])
#or
from itertools import chain
print (list(chain(*li)))
#or
a=[[1,2],[3,4],[5,6]]
t=[]
[t.extend(i) for i in a]
print (t)
#or
print (sum(li,[]))

def flat(tree):
    res = []
    for i in tree:
        if isinstance(i, list):
            res.extend(flat(i))
        else:
            res.append(i)
    return res

def flatten(seq):
    s=str(seq).replace(\'[\', \'\').replace(\']\', \'\') #当然也可以用正则
    return [eval(x) for x in s.split(\',\') if x.strip()]