As we started exploring the Sugar learning environment in class, I popped open the Physics activity, and got to drawing as many shapes, as fast as possible. It was lovely seeing little rectangles and dots flying down the screen and bouncing into each other.

But, of course, I was curious what it’d do in edge cases. What happens if you try to draw a line by folding two edges together? Draw one line of a polygon, and then immediately close it with another line.

The Physics activity froze. Woot. 10 year old me would be so proud.

Now I had something relevant to class to fix, and in Python. This would be a good fit.

A debugging environment

I’ve done enough print debugging to know.. it sucks. Especially with threaded applications. So I wanted to launch Physics from a terminal (rather than through Sugar’s interface) and run the troublesome code through pdb.

I looked through a bit of Sugar’s wiki, and the source code for Physics. However, I wasn’t sure how to launch an activity on my own. I couldn’t simply run physics.py in a Python interpreter. The wiki had a lot of information, but some of it was outdated, or not applicable to my Sugar environment, at least. Fedora’s SoaS remix has activites at /usr/share/sugar/activites, whereas the wiki said they lived in /home/olpc/Activities.

Sugar has their own code to “Sugarize” a Python application. This lets it integrate with Sugar’s UI. It looked cool, from what I could see in the code! You tell it where buttons should go, and what they do. But at this time, it was getting in my way.

I asked on freenode’s #sugar channel for debugging help. But it seems… unlively.

After some digging, I found the sugar-activity command. I pointed that at Physics (in /usr/share/sugar/activities/Physics.activity), and I was good to go!

The bug

The bug didn’t take too long to track down, once I could step through the code. The tools_poly.is_line() function was not checking for identical start and end vertices, when there were not enough total vertices to make a shape. A couple lines to check that was all it took: then the correct drawing function handled that input without a hitch.

My fix is pretty small: https://github.com/sugarlabs/physics/pull/39

Wrapping up

Throughout this process, the hardest part was getting started, for sure. The code was clean, but I had no idea how to start debugging, where I should look, or what I should be looking for in a launcher. The sugar-activity application worked really well for me, but only found it accidentally.

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