python 如何压平嵌套

如何把嵌套的python list转成一个一维的python list?
a = [[1,2,3], [5, 2, 8], [7,8,9]], 想把a变成[1,2,3, 5, 2, 8, 7,8,9]

import itertoolsa = [[1,2,3],[4,5,6], [7], [8,9]]
out = list(itertools.chain.from_iterable(a))
或者：
from compiler.ast import flattenflatten(a)