In article <mailman.3009.1351516065.27098.python-l...@python.org>, Andrew Robinson <andr...@r3dsolutions.com> wrote:
> Show me an example where someone would write a slice with a negative and > a positive index (both in the same slice); > and have that slice grab a contiguous slice in the *middle* of the list > with orientation of lower index to greater index. It's possible in bioinformatics. Many organisms have circular chromosomes. It's a single DNA molecule spliced into a loop. There's an "origin", but it's more a convenience thing for people to assign some particular base-pair to be location 0. From the organism's point of view, the origin isn't anything special (but there *is* a fixed orientation). It's entirely plausible for somebody to want to extract the sub-sequence from 100 bp (base-pairs) before the origin to 100 bp after the origin. If you were storing the sequence in Python string (or list), the most convenient way to express this would be seq[-100:100]. Likewise, if you wanted the *other* fragment, you would write seq[100:-100]. There is a minor confounding factor here in that biologists number sequences starting with 1, not 0. At least that was the way when I was doing this stuff mumble years ago. I don't know what the current convention is. -- http://mail.python.org/mailman/listinfo/python-list