Practice Designing code for less bugs
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.gitignore Add pointer to solution for 2-6 players May 9, 2018

BugsZero Kata

Bugs are optional, they sneak into our code much thanks the design choices we do, or fail to do. Let's try refactoring some existing code to make it more bug-repellent!

You'll practice reading code, looking for parts where it is likely that developers would create a bug if they extended the code. Whenever you've found such a weakness in the design your challenge is to strengthen the design in order to make that kind of bug very unlikely, or even impossible!

First Identify a bug or a weakness in the design. Then Explain what feature could cause a bug. Or how the design contributed to its introduction. This is important, simply saying another solution is better avoids thinking of why it is actually better. Finally Explain or refactor the code to show the new design.

Prefer small changes in the design, for instance before applying a design pattern try simpler things, like maybe ExtractConstant or replace conditional logic with a map/dictionnary.

Where do I start?


Make sure you know of a few ways to design away bugs, by looking at a few examples

Have a look at the long list of common bug-patterns, mostly unnecessary weaknesses in design. Beware, those are a bit terse, most patterns need to be explained.


We've found some weaknesses for you. Pick your choice in


If you'e already accustomed this thinking this way, have a look at


After doing your version, have a look at the various solutions in the form of merge-requests. It is easy to see what the change was using the diff-view. They are meant to be (on of) the best trade-offs for the given problem. There's always trade-offs, but if you find a better or simply good alternative, please fork and merge-request and we'll reference it.

Next steps

Go use this on some other code base, like your project!

Give us some suggestions in the issues.

Resources on #BugsZero

Some slides specifically on code design

The general part 15 min intro or go for the full presentation

Blog posts

Bug teams


Contributions of all forms are welcome. Submit another language by fork/merge-request. If you're missing something or hava a question please create an issue.