Sorry. To explain my problem a bit better, I was binding Tab to the behaviour I desired, but I kept getting the undesired behaviour anyway. The bindtags line simply says "instead of checking bindings in the regular order (0,1,2,3), check them in the order that gives my binding precednece (1,0,2,3). (as the comment says, "so widget events are processed before class events")
Of course, it's possible that the other reply to your post (bind prior to MainLoop) will be all you need. I was just tossing this out there in case that didn't solve your problem.