Returning a value from a Tk dialog

[Gordon Airporte]

The dialogs in tkColorChooser, tkFileDialog, etc. return useful values 
from their creation somehow, so I can do stuff like this:

filename = tkFileDialog.askopenfilename( master=self )

I would like to make a Yes/No/Cancel dialog that can be used the same 
way (returning 1/0/-1), but I just cannot figure out how to do it. At 
best I've been able to get some kind of instance id from the object. How 
does this work?