リストの「次元」変換、3次元から2次元、2次元から1次元へ

a=[[[1,2,3],[1,2,3]],[[4,5,6],[4,5,6]]]
print(a)
print(sum(a,[]))
b=sum(a,[])
print(sum(b,[]))

結果

[[[1, 2, 3], [1, 2, 3]], [[4, 5, 6], [4, 5, 6]]]
[[1, 2, 3], [1, 2, 3], [4, 5, 6], [4, 5, 6]]
[1, 2, 3, 1, 2, 3, 4, 5, 6, 4, 5, 6]