python 列表推导式，双重for循环

my_list = [
    [[1, 2, 3], [4, 5, 6]]
]
flat = [x for sub1 in my_list for sub2 in sub1 for x in sub2]
print(flat)
# [1, 2, 3, 4, 5, 6]

flat2 = [((x for x in sub2) for sub2 in sub1) for sub1 in my_list]
print(list(flat))
# [1, 2, 3, 4, 5, 6]