tkinter function outout to text widget

[Johan Lans]

Hi
I'm totally new on python and I'm doing an assignement where I'm doing
a class that manipulates a text. The program is also supposed to have
a GUI, for which I have used tkinter.
So far I have entry widgets for file names and buttons, its all
working like I want it to.
What is missing is a way to output the changes to the text. I was
thinking that a text-widget would be suitable. Is there a reasonably
easy way to do this?
I tried inserting a string to the textwidget and letting the class
method change this string, but the inserted string isn't updated in
the text-widget.
Would be very happy for any hints.