方法一 ：使用itertools

import itertools

a = [[1, 2, 3, 4], [4, 4, 5, 6], [7], [7, 8, 9]]
out = list(itertools.chain.from_iterable(a))
print(out)

方法二：使用sum()函数

a = [[1, 2, 3, 4], [4, 4, 5, 6], [7], [7, 8, 9]]
out = sum(a, [])
print(out)

方法三：使用operator、reduce函数

import operator
from functools import reduce

a = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
print(reduce(operator.add, a))