了解Python中的列表推导 [英] Understanding List Comprehensions in Python

问题描述

在阅读官方教程时，我遇到了这个示例:

When reading the official tutorial, I encountered this example:

>>> 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]

我听不懂，所以做了一些实验:

I couldn't understand this, so I did some experiments:

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

我现在更加困惑！

所以，我的问题是:应该以什么顺序阅读列表理解?

So, my question is: In which order should I read a list comprehension?

我确定我没有在任何地方定义值为9的num变量.

I'm sure I haven't defined a num variable with the value 9 anywhere.

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]
>>> 

推荐答案

列表理解中的循环是从左到右读取的.如果您的列表理解将被编写为普通循环，它将看起来像这样:

The loops in list comprehension are read from left to right. If your list comprehension would be written as ordinary loop it would look something like this:

>>> 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，列表推导中的变量共享外部范围，因此num可在以后使用:

In Python 2 the variables within the list comprehension share the outer scope so num is available to be used later:

>>> 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，行为是不同的:

Note that on Python 3 behavior is different:

>>> 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

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