Why does my negative slicing in Python not work?

Question

I am new to Python and have read several tutorials on slicing however the examples I run in idle don't seem to return what I expect it to. For example I have assigned the follow list to the variable a

a=[0,1,2,3,4,5,6,7,8,9]

Now I understand slicing to be as [number I want to include:number up to and don't want to include:step]

Hence if I do a[1], I would expect 1. If I do a[1:3], it would be 1,2

Now if I do a[-1], I get 9 BUT if I do a[-1:-5], I get nothing. All I see is []. why is that? I would expect to see 9,8,7,6

I am running Python 2.7 on Windows 7 Professional

Answer 1

In this case, you would need to add in the step argument in order to get what you want:

In [1]: a=[0,1,2,3,4,5,6,7,8,9]

In [2]: a[-1:-5:-1]
Out[2]: [9, 8, 7, 6]

And if you want to save a bit more space, you can omit the first argument:

In [3]: a[:-5:-1]
Out[3]: [9, 8, 7, 6]

The way that Python handles the 'negative' slices is that it adds then len of the object to the negative number. So when you say In a[-1:-5], it is basically saying a[(-1+10):(-5+10)], which equals a[9:5], and since start/end refers to all characters between the two (moving 'forward' through the list), it doesn't return anything (hence your blank list). You can see this by doing something like:

In [5]: a[-5:9]
Out[5]: [5, 6, 7, 8]

In [6]: a[5:9]
Out[6]: [5, 6, 7, 8]

You get the same result with the negative and positive indices, since -5 + 10 = 5.

Providing the -1 step argument tells it to start at the first element but move backwards from the start position.