如何在python中使用列表迭代列表？

Question

我有这样一个列表: -

lst = [[1, 2, 3, 4, 5, 6], [4, 5, 6], [7], [8, 9]]

如果我运行这些,我得到这样的输出.我不知道这些是如何工作的.

>>>[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]

任何人都可以解释一下这些输出是如何产生的.这两次迭代之间有什么不同？

Answer 1

在第一次LC结束时i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你正在迭代这个i:

>>> [j for j in i for i in lis]
[8, 8, 8, 8, 9, 9, 9, 9]

两个LC都(大致)相当于:

>>> 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

这已在py3.x中修复:

特别是循环控制变量不再泄漏到周围的范围内.

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