The old error of tuple.index(x): x not in tuple seemed very confusing to me. It was also harder to quickly understand what the program was doing wrong. This saves people a second pass through the program under the debugger because they can just see what the invalid value was.

The reason I am explicitly calling repr instead of using %R is that I didn't want to call repr twice if I didn't need to. If people think that is fine the format can just be tuple.index(%R): %R not in tuple instead.