了解Python中的列表推导

Question

在阅读官方教程时 ，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]

我听不懂，所以做了一些实验：

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

而且我现在更加困惑！

因此，我的问题是：应该以什么顺序阅读列表理解？

---

编辑：我确定我没有在任何地方定义数值为9的num变量。

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

列表理解中的循环从左到右读取。 如果您的列表理解将被写为普通循环，它将看起来像这样：

>>> 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，列表推导中的变量共享外部范围，因此num可在以后使用：

>>> 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

请注意，在Python 3上，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

别担心 ：）。

当您执行列表推导时， num值为9因此下次您遍历vec 。 您将得到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

让我尝试使答案更加明确。 而且顺序显然是从左到右，并且最右边的值将存储在变量中，即num和elem。

初始数据：

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

步骤1：在执行第[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：执行第[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[num for elem in (vec for num in elem)]后执行[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.