理解 Python 中的列表推导式

Question

在阅读官方教程时，我遇到了这个例子：

>>> 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]

我无法理解这一点，所以我做了一些实验：

>>> [num for elem in vec]
[9, 9, 9]

>>> [num for elem in (vec for num in elem)]
[9, 9, 9]

而我现在更迷茫了！

我应该按什么顺序阅读列表理解？

---

我确定我没有num在任何地方定义值为 9 的变量。

python

输出：

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.

REPL会话：

>>> num
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
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]

Answer 1

列表理解中的循环是从左到右读取的。如果你的列表理解被写成一个普通的循环，它会看起来像这样：

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

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

在 Python 2 中，列表推导式中的变量共享外部作用域，因此num可供稍后使用：

>>> 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
9

请注意，在 Python 3 上，行为有所不同：

>>> 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
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'num' is not defined