10/21/18  Simple? Really? OK, it is posted today under the GUI Demo section but a series of observations about problems, solutions, and lessons will be fodder for a couple of blogs.   After posting (a month ago) the bare bones, 36 line, brief function for amortization –  see:
OTHER MODULES & Stuff ->
…..Spreadsheet formulas: a Non-Module ->
………An easy amortization Schedule Function
I decided to take a few minutes and stuff it into a GUI frame.  There I thought to demo a few other issues with a special thought to showing how easy the ttk Combobox makes selection from a list. That was 5 weeks, and about a 1000 hours of work and research, ago.

The Comboboxes took about 5 minutes. The user error checking, and associated issues, took the 5 weeks. There were a handful of issues that may bear an exploration. Here are a few of note:

What size GUI screen will work with 95% of all computers? How do you adapt it?
What general, orderly format is more or less dictated by an object oriented, GUI process?
How do you assure the user enters a floating point number?
How do you cope with the user using commas in a number?
How do you direct or at least account for the flow of data entry on  static screen?
What complications does mouse movement add to data entry? What strategies apply?
How do you limit the range of a numeric entry? Remembering that min and max are keywords.
When a function returns from solving a problem, what can you do to stop the process and alter the course of activity?
How do you make sure output is in neat, even columns?
How much should you worry about numeric accuracy in Python?

…to be continued.