方法一：使用列表推導式

>>> vec = [[1,2,3],[4,5,6],[7,8,9]]
>>> get = [num for elem in vec for num in elem]
>>> get

[1, 2, 3, 4, 5, 6, 7, 8, 9]

方法相當於

>>> vec = [[1,2,3],[4,5,6],[7,8,9]]
>>> result = []
>>> for elem in vec:
for num in elem:
result.append(num)
>>> result

[1, 2, 3, 4, 5, 6, 7, 8, 9]

方法二：

使用sum函式

>>> vec = [[1,2,3],[4,5,6],[7,8,9]]
>>> get = sum(vec,[])
>>> get

[1, 2, 3, 4, 5, 6, 7, 8, 9]

或者是使用chain函式

>>> vec = [[1,2,3],[4,5,6],[7,8,9]]
>>> from itertools import chain
>>> list(chain(*vec))

[1, 2, 3, 4, 5, 6, 7, 8, 9]