a question about s[i:j] when i is negative

[mensanator at aol.com]

dracula571 wrote:
> s[i:j] slice of s from i to j         (3), (4)
>
> (3)
> If i or j is negative, the index is relative to the end of the string:
> len(s) + i or len(s) + j is substituted. But note that -0 is still 0.
>
>
> (4)
> The slice of s from i to j is defined as the sequence of items with
> index k such that i <= k < j. If i or j is greater than len(s), use
> len(s). If i is omitted or None, use 0. If j is omitted or None, use
> len(s). If i is greater than or equal to j, the slice is empty.
>
> i can't follow (3) very well.
> for example:k = [1,2,3,4,5]
> k[-1:2]=[]
> k[-5:2]=[1,2]
> but k[-6:2] = [1,2]
> why k[-6:2] is [1,2]not [].

Because 6 is greater then len(k), so when you ask
for i=-6, you get i=-5 instead. And relative -5 is actually
absolute 0 which is less than 2, so you do not get an
empty slice.

> i do follow (3),to make i positive by
> plusing len(k) twice.But the result is not what i expect.why,or did i
> misunderstand the rule of (3).

See (4).