Tkinter tab focus traversal causes listbox selection to clear,ie, lose the selected item

[Gerardo ARnaez]

Hi.
I am writing a program to help determine coumadin regimens
to look at the code: http://sourceforge.net/projects/coumadinregimen/

The issue is that I have a variable that I want the use to select if
they don't like the default.
I have made this as a listbox widget

The rest of the gui is just entry widgets

What I noticed is that when I tab to through the entry  widgets, I
lose the listbox selection: the item selected, gets unselected.

Not sure what is happening,hope I have explained it.
Thanks,
G