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5. The last line is mandatory Tk programs. It loops through the code and invokes callback responses.
You left out the "in" before Tk. Also since it is so important to a new GUI programmer to understand that they are dealing with a hidden eventloop, not just some linear code, I think you should emphasize the importance of it more. Some concepts to add:
The MainLoop is tied to the Mainwindow. Kill the loop, kill the window.
Never use sleep in a gui program because it will put the loop to sleep and make the whole window unresponsive.
Other typos: but you knew knew that , Frames are very simple widgets. They are used as a type of border should be "Frames are very simple container widgets; they act as a container for other widgets";
Besides that, you switched to the grid packing manager, from pack, and don't explain it.
Uh.. my eyes are strained already. :-) The last thing I will mention, is you make no mention of the Tk "widget" program, which shows all the widgets in action.