Discussion on TDD at SDC2010

As I mentioned in my last post I chaired a fishbowl discussion at SDC2010 with title "Should a professional developer always use Test Driven Development?". I was delighted that the invited panelists Michael Feathers, Geoff Bache and Andrew Dalke all turned up, along with a few dozen other conference participants. As I predicted, we had a lively and interesting debate.

Michael half-jokingly complained that Bob Martin goes around making these controvertial statements all the time, which Michael then gets to go around defending. Michael has a much more conciliatory attitude than Bob, and his take was that every truly professional developer must have at least given TDD a good try and learnt the technique, even if they then decide not to use it.

Geoff's main point was that we need to widen the definition of TDD to include any process that involves checking in tests at the same time as the code, and not restrict it to just the classic Red-Green-Refactor style with tests in the same language as the code.

Michael was largely receptive to this view, or at least that the soundbite description of "never write any code until you have a failing test" probably was a bit too brief description to encompass the whole of TDD. He did argue though, that the classic TDD style leads to code with good design characteristics of high cohesion, loose coupling, small classes and methods etc, and that he had not found other design techniques which led to better code than TDD. He was not keen to move to a TDD approach without unit tests, and lose these benefits, even if they result in good tests.

Andrew argued that TDD is not sufficient by itself to produce a good suite of tests, and that there are other, better ways to produce these tests. Andrew pointed out that he had examined Fitnesse, a codebase that Bob Martin, (and some others), has created using TDD, and that he found several bugs, including security holes in it. Michael's counterargument was that with TDD, you get as good tests as you are capable of - if you are not skilled/aware of security issues, then you won't test for security holes, whatever process you use to create tests.

Another argument of Andrew's was that he often likes to write tests that he expects to pass, to verify that his code works as expected, for example that he has implemented an algorithm correctly. In the narrow definition of TDD, you are only allowed to write tests you expect to fail. Michael's take was that this was indeed a too narrow definition of TDD. He said that he frequently writes tests as a way of asking questions of his code, and this often leads to tests that pass straight away.

Some of the "audience" also stepped up to the microphones and joined in. Brian Marick pointed out that forcing yourself to write the test first was a very good way of ensuring you do actually write the test, instead of being lazy and just writing more code. The counter to that was along the lines of that there are other processes for arriving at a good test suite, which took different kinds of discipline. Andrew quoted the sqlite project, which boasts 100% branch coverage of their code by their test suite. Publishing your coverage figures and refusing to let them slip is a way of preventing developer laziness too.

Brian Marick wrote an article about coverage and tests over a decade ago, so he summarized it for us, which was interesting, but I think slightly beside the point. I think he was trying to argue that measuring coverage alone is not enough to guarantee you have a good test suite, but I don't think that was what Andrew was trying to claim. Simply doing TDD is not a guarantee that you will end up with a good test suite either.

For me, the interesting outcome of the discussion was pointing out that the alternatives to TDD are not only "cowboy coding" or "test later, ie never", or "bad tests", but that there are other legitimate ways to come up with a good test suite, and professional developers may choose to use them instead of classic TDD. TDD is a discipline which all professional developers should perhaps have in their repertoire though. I think we agreed it is also a teaching aid for learning to write good tests.

Happily, we definitely all agree that creating a good automated test suite alongside code is important. The precise method a professional developer should always use to produce it was not agreed upon though.

Should a professional developer always use TDD?

I'm really looking forward to Scandinavian Developer Conference, and in particular the fishbowl discussion I'll be moderating on the Tuesday at 10:30am. Presenting their views will be Michael Feathers, Andrew Dalke, and Geoff Bache, and the topic under discussion is the same as the title of this post: Should a professional developer always use TDD?

I've been enthusiastic about writing automated tests for my code since 2000 when I discovered eXtreme Programming, and started using JUnit. It's become a habit for me to write tests before code. Occasionly I decide not to, perhaps I am feeling lazy, or think a test would be too difficult to write. I find I usually regret it and end up writing a test afterwards anyway.

One of the things Bob Martin, (a colleague of Michael Feathers), says about TDD in his book "clean code", is that it is a matter of professionalism. Developers should be like doctors. Would you trust a doctor who didn't wash her hands because she didn't belive in it? Well, you shouldn't trust a developer who doesn't use TDD because she doesn't believe in it.

I've known Andrew Dalke since 2002, and we've worked together on and off since then. Recently he wrote this article criticising TDD. Andrew does not believe TDD is necessary for good development work to happen. Is he unprofessional? Far from it.

My experience of working with Andrew tells me that he is an excellent programmer, who produces high quality code and automated tests. However, the process by which he arrives at this code and tests is not TDD. Tests get written during development, but not in advance of the code they test. The tests do not in any way drive the design, in fact, he uses knowledge of the design of the code to inform what tests he writes.

Andrew says in his article "Once I have a good sketch of how the code is going to be, I often continue by filling in the details. At this point unit tests starts to be useful" he likens what he does to an XP spike solution, except that he does not throw away the spike code and start over when he starts adding tests.

The other person I know who has a complex relationship with TDD is my husband Geoff. Several years ago he was labelled a heretic and almost thrown out when he admitted to a room full of XP enthusiasts that he didn't write unit tests at all. Geoff does write tests - a lot of tests in fact - but they are not xUnit tests, and they don't drive the design of his code.

Geoff uses an approach he calls "text-based testing" which involves driving the program from the command line, (or some kind of script), and having his code write a plain text log file of what it is doing. A tool called TextTest picks up the log output and compares it to the saved version from a previous run. Differences are flagged as test failure.

It's a simple idea, but it is actually very effective and easy to use when you get the hang of it. The main advantage over ordinary TDD is that there is little or no code written per test, meaning less code to maintain overall. The fact that the tests are independent of the design of the code makes refactoring easier, and writing tests for legacy code relatively risk-free.

TDD is a bit different with the text-based approach though. Geoff thinks of what he does as TDD, but actually, only half of the test is nailed down in advance of the code - only the part that tells the program which features to exercise. The part that asserts that it did the right thing is simply recorded after the code is written.

So I expect a fascinating and lively discussion to ensue when I get these guys together! Perhaps you'll join us?

(Note: I wrote up the discussion in my next post)