了解列表综合Python中

Question

当读official tutorial，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] 

我不明白这一点，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] 

而且我甚至现在更困惑了！

所以，我的问题是：我应该以哪种顺序阅读列表理解？

---

编辑：我敢肯定，我还没有和值9任何位置定义的变量num。

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

列表理解的循环读取由左到右。如果您的列表理解将被写为普通循环会是这个样子：

>>> 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列表解析份额内的变量外范围，使num可以供以后使用：

>>> 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 

需要注意的是关于Python 3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

不用担心:)。

当您执行列表理解时，num的值为9，所以下一次迭代vec时。你会得到一个9的清单。

看到这个。

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] 

Answer 3

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 4

让我尽量让答案更清零。 而且顺序显然是从左到右，最右边的值将被存储在变量num和elem中。

初始化数据：

vec = [[1,2,3], [4,5,6], [7,8,9]] 
num # Undefined 
elem # Undefined 

步骤-1：线执行后[num for elem in vec for num in elem]

# Now the value of undefined variable will be 
num = 9 # will keep the last value of the loop as per python 2.7 
elem = [7, 8, 9] # Same applies here (as the loop for elem in vec will get executed first followed by for num in elem) 

步骤2：行执行后[num for elem in vec]

# Output: [9, 9, 9] 
# Since num value is 9 and its get repeated 3 times because of vec has 3 elements (three list object in a list, so for loop will run 3 times) 
# Now the variable value would be 
num = 9 # no change 
elem = [7, 8, 9] # The last tuple of variable vec 

步骤3：执行后[num for elem in (vec for num in elem)]

1. In the first/right loop i.e (vec for num in elem) 
    Here the result will be a generator that will have run thrice since length of elem is 3. 
2. Final for loop will iterate over RESULT1 (the result of for loop #1 having length 3) and since the num value is 9. the result will be [9, 9, 9] # num value repeated thrice.