为什么扩展切片不能反转列表？

Question

I'm slicing lists in python and can't explain some results. Both of the following seem natural to me:

>>>[0,1,2,3,4,5][1:4:1]
[1, 2, 3]

>>>[0,1,2,3,4,5]
[::-1] == [5,4,3,2,1,0]

However,

>>>[0,1,2,3,4,5][1:4:-1]
[]

thought I expected it to be [3,2,1]. Why does it produce [ ]? Why does it not reverse the list? What happens first inside python, the step or the slicing?

I also found that

>>>[0,1,2,3,4,5][-3:-6:-1]
[3,2,1]

Answer 1

The third number in the slice is the step count. So, in [0,1,2,3,4,5][1:4:-1] , the slicing starts at 1 and goes DOWN by 1 until it is less than 4 , which is immediately is. Try doing this:

>>> [0,1,2,3,4,5][4:1:-1]
[4, 3, 2]

Answer 2

If you are slicing this then slicing will look like this [start:end:step] . For this one:

>>> [0,1,2,3,4,5][1:4:1]
[1, 2, 3]

It is staring from index 1 to index 3 because it exlcudes the index 4 with 1 step at a time. You are getting an empty list in the second one because you are stepping -1 from 1st index. So this will be the solution.

>>> [0,1,2,3,4,5][4:1:-1]
[4, 3, 2]

This works because you are taking an index from 4 to one with -1 step forward.