tkMessagebox.askyesno always returns False

[peter]

I have a weird problem in some code I am writing.  The user selects a
number of files from a list and then can select an option which will
rename the selected files.  Before the process starts, a yes/no dialog
box pops up just to confirm.

Most of the time this works fine, but occasionally it seem the dialog
box gets into a state where it always returns False, irrespective of
the button clicked.

I must be doing something wrong, but I just can't see it!  Any
suggestions?

<code>

import tkMessageBox as mb
         :
         :
        if action=='Rename':
            yn=mb.askyesno(file_list[0],'Ok to rename %d selected
file(s)' % file_count)
            print 'Response: ',yn # Debug only
            if not yn: exit_flag=1

</code>