Python Rocks! and other rants
Weblog of Kent S Johnson

#65 2005-08-21 11:28:00

Unit testing is an enabling technology

I'm reading Pragmatic Unit Testing by Andrew Hunt and David Thomas. I'm a big fan of unit testing and I think one of the biggest benefits is often overlooked.

The direct benefits of unit testing are clear and substantial: better tested, more reliable and often better designed code. The indirect benefit that is often neglected is unit tests as an enabling technology.

Have you ever looked at a pile of code and thought,

  • This is a mess, I should rewrite it
  • There's a lot of dead code here I should rip out
  • This would be easier to understand if I broke it up into smaller pieces
  • There's a lot of duplication here I should factor out
  • If I refactored this a little it would be a lot easier to add this new feature

How often is that thought followed by action, and how often by the thought, "I can't do that, there's too much risk of breaking something"?

This is where unit tests shine long after they are written. If the code in question has extensive tests, you can change it with confidence instead of fear, knowing that any breakage will be quickly discovered. Unit tests provide a safety net. There is a qualitative shift from coding in fear to coding with confidence.

Think of it - no longer do you have to live with messy code, dead code, monster methods, duplicated code, designs that don't reflect your needs! How sweet!

I recently finished making some modifications to some file-generation code. When I started there were three large modules that created three large files. The modules and the files were similar but they shared little code; the first module had been copied and modified to make the second two. There were no tests for any of it. My job was to add some options to all three generated files.

The first thing I did was set up a test framework. The generated files are XML so I created some reference files and used XmlUnit to compare the generated files with the references.

Next I started factoring out duplicated code into a common base class. I focused on the parts I had to change, with a goal of only having to make a change in one place, in the base class, rather than duplicating the change three times.

The result? The three modules are dramatically smaller. Most of the option testing is centralized in the base class. The subclasses contain a top-level driver function and many small callbacks that customize the output to their particular requirements. There is probably less code in the four modules I ended up with than there was in the original three because of all the duplication I removed.

Now that's a reason to write unit tests!

Categories: Agile

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