Tkinter, repaint?, keep size?

[skanemupp at yahoo.se]

so my calculator is almost done for u that have read my previous
posts.
i have some minor problems i have to fix though.

*one is i need to repaint once i have performed a calculation so that
the old results are not left on the screen.
cant find a method for that.

*another is now when i write the expression to be evaluated it resizes
the window as the text grows.
i want the windowsize and all the buttonplacements  stay constant, how
do i achieve this?