[Tutor] is the the two style of writting the same?

[Steven D'Aprano]

On Tue, Aug 20, 2013 at 05:20:23PM +0800, sikonai sikonai wrote:
> >>> list=[1,2,3,4,5,6,7,8,9]
> >>> list[9:0:-2]
> [9, 7, 5, 3]
> >>> list[10:0:-2]
> [9, 7, 5, 3]
> 
> I want to know whether they have some difference.

Consider:

2+2
2*2
2**2

All three give the same result, 4, and yet they are different. You can 
see that they are different, because if you change the numbers from 2 to 
something else, the results will no longer be the same:

3+3
3*3
3**3

No longer equal! This proves that + * and ** are not the same thing.

Go back to your example:

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

It turns out that L[9:0:-2] happens to be the same as L[10:0:-2], but 
that is only because the list only has 9 items. But if you do this:

L = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]

for example, and then compare L[9:0:-2] and L[10:0:-2], you will see 
they are quite different:

py> L = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
py> L[9:0:-2]
[10, 8, 6, 4, 2]
py> L[10:0:-2]
[11, 9, 7, 5, 3]



-- 
Steven