I'm not sure on what principles you'd make a principled justification. My most common use of negative indices is for slicing the end of the list, in which context the interpretation similar to len(list) makes sense. E.g., list[:-2] does what you'd expect (the same as list[2:], except from the other end).

> implicitly taking indices mod len(list)?

Isn't this doing that (plus some bounds checking)? -1 mod 4 evaluates to 3.

> implicitly flooring indices?

Not sure I understand what this means.