I think that's a trick question. If "substring()" has "substring(int start, int end)" semantics, then I prefer the strict version: it must be 0 <= start < len(s), start <= end <= len(s), otherwise it'll throw an exception.

But if the semantics is "substring(int start, int length)", then I prefer the partial forgiveness for the length parameter: if start + length > len(s), then assume length was actually len(s) - start; but if length < 0, still throw an exception.