理解Python中的列表理解

Question

在阅读official tutorial时，我遇到了以下示例：

>>> 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 "<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为普通循环，它看起来像这样：

>>> 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，list comprehension中的变量共享外部作用域，因此num可供以后使用：

^{pr2}$

请注意，在Python 3上的行为是不同的：

>>> 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

Answer 2

list1 = [num for elem in vec for num in elem]

#is equivalent to:

list1 = []
for elem in vec:
    for num in elem:
       list1.append(num)

Answer 3

不用担心：）。在

当您执行list comprehension时，num的值是9，所以下次您迭代{}。您将得到一个9的列表。在

看这个。在

In [1]: vec = [[1,2,3], [4,5,6], [7,8,9]]
In [2]: [num for elem in vec for num in elem]
Out[2]: [1, 2, 3, 4, 5, 6, 7, 8, 9]
In [3]: num
Out[3]: 9
In [4]: [num for elem in vec]
Out[4]: [9, 9, 9]