Unit Testing TkInter Apps (Shallow Thoughts)

Unit Testing TkInter Apps

I've been making a lot of tweaks lately to MetaPho, in particular its Python/TkInter based replacement for my C/GTK2 image viewer Pho.

Pho has always had quite a few modes: it can be fullscreen, in a window sized for the current image, or in a fixed-size window; images can be scaled to the window/screen size, or you can zoom in/out, or you can view them at full size (pixel for pixel). It's fairly common that when I fix a bug in one mode, it introduces a new bug in a different mode because of the way the scaling code works.

Ideally, in a complicated program, you guard against problems like that with automated tests. But that's hard to do in a GUI (graphical user interface) app. A window comes up, but how do you make it do different things? How do you check whether it's showing the right thing, or if it's the right size?

I've tried a couple times to find hints on how to unit test Python scripts in either Tk or GTK, but there's not much help available. I think most people just give up and don't test their GUIs — just as I've always given up.

This time, I decided to really dive in and see if I could write a TkInter unit test script for testing all those different TkPho modes. It wasn't easy, but now I have a basic framework that I should be able to use for other GUI apps as well.

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