Strangling Your Code and Growing Your Test Harness: The 9 Phases of Building Automated Tests Into Legacy Code

I'm in the early throes of building tests into my Cartwheel project. Cartwheel was one of the two projects that inspired my Web testing project, twill, so naturally I'm happy to finally be putting twill to good use in my own projects. Naturally the transition from building tools for building tools, to actually using the tools to build a tool, is a bit painful: I can't be general any more, now I have to be specific.

The process I'm going through right now is appropriately referred to as Strangling Legacy Code, in this case by Growing A Test Harness (both great articles). Leveraging the power of nose directory hierarchies, I'm slowly growing my setup/teardown code to cover more and more functional testing scenarios, which in turn exposes more scenarios to test. While it's an endless-seeming process, I think I'm at the inflection point where I've now automated more than half of the testing tasks for the Cartwheel Web server. Note that I'm by no means testing even 50 percent of the functionality, but what remains is relatively specific and accessible to testing by very small increments in my testing code. Given my general time constraints, I'm going to switch my focus to testing newly written code and writing automated tests for reported bugs; down the road I'll probably use coverage analysis to figure out what large masses of untestedness lie hidden in my codebase.

Looking back and prognosticating forward, I'd divvy the process up into N steps:

  1. Shock. (How the heck do I start testing this sprawling mass of code??)
  2. Hello, world. (Hey, look at that, I've got a basic import working in my automated tests!)
  3. Fixture code sucks. (Oh, gawd, I've got to automate setting that up, and that, and that...)
  4. Fixtures rock. (Wow, look at what I can test now!)
  5. Over the hump. (Where I am now.)
  6. What lurks beneath? (Using coverage analysis to find large areas of untested code.)
  7. Relaxing. (I've got XX% of my code covered with some kind of automated test! Hooray!)
  8. Reaction. (Hmm, guess I'm not actually testing for that specific bug...)
  9. Goto 7, increment XX.
  10. Asymptotically approach perfection.

Regardless of the steps ahead, it feels good to be at stage 5...


Legacy Comments

Posted by Ben Finney on 2007-04-01 at 20:29.

One of the good things I find about writing unit tests is that I am
encouraged to make all external dependencies parameterisable. In other
words, the code should **not** expect to be able to make loads of
calls directly to the database -- it should instead accept a single
object which encapsulates that functionality, and defer all database-
specific stuff to that object.    That way, when I'm unit-testing that
code, I just pass in an easy-to-set-up mock database object, which
reacts predictably, instead of having to somehow automate the setting
up of external things.    This is the same for whatever externality
you find troublesome in your fixtures. If it's a lot of effort to set
up for the purpose of a test, that's a bad design smell. Your code
needs to be refactored so it isn't so tightly coupled to that external
thing, and can be more easily unit tested. You'll find that each unit
of code incidentally becomes much simpler in its interface as a

Posted by Titus Brown on 2007-04-02 at 12:37.

Well, yes and no.  I'm explicitly doing **functional** testing, not
unit testing; I find functional testing more useful for Web sites.  In
particular, the complexity in this application has more to do with the
information communicated via the database (by several different
processes) than it does with the code sitting between the database and
the Web site.    Were I doing strict unit testing, I would indeed
decouple things and mock the database more, although I also think that
encapsulating a large data model in an easily testable object is ...
difficult and dangerous to do on the cheap.  So I'd have to really
expect a big benefit from it before I'd start down that road.    As it
is, now that I **have** the fixtures, life is quite good ;)    cheers,

Posted by Ben Finney on 2007-04-02 at 21:02.

Yes, you're right that my comment was in the context of unit tests.
Higher-level, whole-application testing has different tradeoffs.

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