I think we are in agreement :)

Regarding the warning message, I was referring only to the case that the syntax highlighter fails completely and raises an exception. I'm not even sure if that can happen, but in case it does an informative message would be better than just not highlighting, and much better than IDLE dying. I'm bringing this up in this context because IMO if we allow highlighting of arbitrary files, we should at least ensure that this doesn't crash IDLE if the highlighting does fail.