python2.x如何列表推导二维数组？

>>> a = [[1, 2, 3], [4, 5, 6]]
>>> b = [x for x in i for i in a]
>>> b
[1, 1, 2, 2, 3, 3]
>>> c = [x for x in i for i in a] 
>>> c
[4, 4, 5, 5, 6, 6]

1. 如何解释上面代码的运行结果？
   一般的得到的结果是[4, 4, 5, 5, 6, 6]，但是偶然会得到[1, 1, 2, 2, 3, 3]，这是为什么呢？

2. 如何把[[1, 2, 3], [4, 5, 6]]用列表推导成[1,2,3,4,5,6]的形式？

---

In [5]: b = [x for i in a for x in i]

In [6]: b
Out[6]: [1, 2, 3, 4, 5, 6]

三维的
In [7]: aa = [[[1], [2], [3]], [[4], [5], [6]]]
In [8]: b = [y for i in aa for x in i for y in x]

In [9]: b
Out[9]: [1, 2, 3, 4, 5, 6]

---

a = [[1, 2, 3], [4, 5, 6]]
all = []
for i in a:
    all += i

没想到更简洁的方法。

---

b = [x for x in i for i in a] 在我的解释器中无法运行.

如何把[[1, 2, 3], [4, 5, 6]]用列表推导成[1,2,3,4,5,6]的形式？

>>> b = [item for sublist in a for item in sublist]
>>> b
[1, 2, 3, 4, 5, 6]

flatten 在stack overflow 上有详细的讨论, 上面的方法被发现是最快的,