[wlug] Simple program debug <Python>

Hello All, I have written a small program along the lines of the spam salad example. I have managed to get to the point of if the user inputs an invalid numeral then they are prompted to re enter a number between 1 and 3. I now want to do the same thing if any alpha character or more is entered. at the moment if a alpha is inputted the program crashes with a script error. What do I need do to refer the user to the invalid input prompt?? Regards John