Why not just show the out-of-range index?

[Russ]

> Rather, they (like I) will encourage to OP to submit a patch that fixes the problem.

Now, that would be rather silly. I would have to familiarize myself
with the code for the Python interpreter, then send a patch to the
maintainers (and hope they notice it in their inboxes), while the
maintainers themselves could probably "fix" the problem in two minutes
flat. No thanks!

My suggestion is trivial to implement and would benefit every Python
programmer (even if only slightly), so I don't think it is too much to
ask for.