Minimal
stressed for motivation and achievement2005-09-28
Stacking unit tests
There are still some parts of test driven development that I'm not 100% comfortable with. Sometimes, for example, I’ll be writing a unit test for a method in class A that, it turns out, needs to call a method in class B to do its thing. Only problem is that class B’s method doesn’t exist yet. So I stub it and my test fails. Hurrah.
Now, however, I’ve got methods in two classes that aren’t fully implemented. Really, the method in class B should be covered by its own unit test before I start calling it from elsewhere. So I write a failing test for that method too. I now have 2 failing tests at the same time. This is something that, from my understanding of the literature, should be avoided. So, what should I do to avoid it? [more...]
One solution might be to write class B and its methods before I even touch class A. However, if I do that, I’m risking creating an interface that isn’t suitable for use by class A. I’d be trying to predict future use of the class. Instead, shouldn’t I be driving the development of the class B’s interface by developing class A and uncovering its interface requirements?
Of course, if I do develop class A first, we get to the situation described above: multiple failing tests. In fact, it could easily cascade further if class B’s method required a call to a method in class C. Et cetera, ad infinitum. So, what do I actually do?
At the moment, the approach I’m taking is the first one. I have multiple failing tests. I find this preferable as it avoids trying to predict required interfaces—something I’ve already had bad experiences with. However, as soon as I realise I’ve got to write a second unit test before getting the first to pass, I give the first an Ignore attribute in NUnit. The text I put in the Ignore attribute is something like "Added to Ignore stack: 28-Sep-05 11:30". Once I get the second test to pass, I look back for any ignored tests in the NUnit GUI and continue work on the test with the most recent Ignore stack date.
One thing that this approach requires, however, is that you don’t have any other tests ignored in your test suite. Otherwise, it’s too easy to forget the Ignore stack exists. If you’re currently ignoring a set of tests based on them taking a long time to run, think about using the Category attribute instead.
That thought, in turn, opens another. How do you /know/ you need the new method on class B? Is it impossible to write inline in class A? Are you really taking the very simplest route to a passing test?
Imagine you could write all of the necessary code in class A. In this case, the one test is sufficient. Then you decide to refactor, and move part of the code into class B. Would you add a new test at this stage? Even though you have complete test coverage?
: 2:23 pmOr are you saying that I'd only reuse B elsewhere by way of refactoring and that I would therefore already have tests covering the code it was replacing?
(And yes, any other TDD'd code that ends up using B will already be tested.)
Don't think of TDD as a testing technique: it's a /design/ technique. You're using "tests" to design the interface of A (and refactoring to design the interface of B and its relationship with A). Have you looked at BDD?
: 9:18 pmNo, I haven't. But I will now - thanks. Is Dave Astels' stuff a good place to start? Or do you know of better intros/references?
Oh, and thanks for taking the time to check back here again.
: 10:14 pmBut his recent article did start a flurry of threads on extremeprogramming and agile-testing over at Yahoo. They aren't overly enlightening just yet, however...
: 2:21 pm<< Home
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