python中的列表理解_理解Python中的列表理解

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

>>> [num for elem in vec for num in elem]

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

我不明白，所以我做了一些实验：

^{pr2}$

我现在更糊涂了！在

所以，我的问题是：我应该按照什么顺序阅读列表理解？在

编辑：我确定我没有在任何地方定义值为9的num变量。在sunqingyaos-MacBook-Air:Documents sunqingyao$ python

Python 2.7.10 (default, Oct 23 2015, 19:19:21)

[GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.0.59.5)] on darwin

Type "help", "copyright", "credits" or "license" for more information.

>>> num

Traceback (most recent call last):

File "", line 1, in

NameError: name 'num' is not defined

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

>>> [num for elem in vec for num in elem]

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

>>> [num for elem in vec]

[9, 9, 9]

>>> [num for elem in (vec for num in elem)]

[9, 9, 9]

>>>