我有一个这样的列表:-
lst = [[1, 2, 3, 4, 5, 6], [4, 5, 6], [7], [8, 9]]
如果我运行这些，我会得到这样的输出。我不知道它们是如何工作的。
>>>[j for i in lst for j in i]
[1, 2, 3, 4, 5, 6, 4, 5, 6, 7, 8, 9]
>>>[j for j in i for i in lst]
[8, 8, 8, 8, 9, 9, 9, 9]
谁能解释一下它们是如何给出这样的输出的。这两个迭代之间有什么不同？
请您参考如下方法:
在第一个 LC 的末尾 i 被分配给 [8,9]:
>>> lis = [[1, 2, 3, 4, 5, 6], [4, 5, 6], [7], [8, 9]]
>>> [j for i in lis for j in i]
[1, 2, 3, 4, 5, 6, 4, 5, 6, 7, 8, 9]
>>> i
[8, 9]
现在在第二个 LC 中，您正在迭代此 i:
>>> [j for j in i for i in lis]
[8, 8, 8, 8, 9, 9, 9, 9]
两个 LC 都(大致)等同于:
>>> lis = [[1, 2, 3, 4, 5, 6], [4, 5, 6], [7], [8, 9]]
>>> for i in lis:
... for j in i:
... print j,
...
1 2 3 4 5 6 4 5 6 7 8 9
>>> i
[8, 9]
>>> for j in i:
... for i in lis:
... print j,
...
8 8 8 8 9 9 9 9
This has been fixed in py3.x :
in particular the loop control variables are no longer leaked into the surrounding scope.
演示(py 3.x):
>>> lis = [[1, 2, 3, 4, 5, 6], [4, 5, 6], [7], [8, 9]]
>>> [j for i in lis for j in i]
[1, 2, 3, 4, 5, 6, 4, 5, 6, 7, 8, 9]
>>> i
Traceback (most recent call last):
NameError: name 'i' is not defined
>>> j
Traceback (most recent call last):
NameError: name 'j' is not defined
