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Characterization: Beyond Pure Unit Testing


By Andrew Binstock
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March 15, 2007 —  (Page 1 of 2)
If asked to explain, practitioners of unit testing generally recommend the practice because it enables them to be sure their code does what they expect it to. It is a mechanism that gives some reasonable belief that the base being built is solid—that is, it works correctly given a variety of inputs and under varying conditions.

For me—a strong believer in unit testing—the practice has other advantages that are less discussed but certainly acknowledged by developers.

One is that I can reasonably extrapolate when I will be done with a project. The aspect that historically made guessing completion milestones so difficult was that the coding cycle generally opened onto an extensive debugging cycle whose extent and duration were unknown. This was so because developers preferred doing the minimum amount of testing necessary to move forward with feature creation. So, when the package was pushed to QA, simple tests with out-of-range values frequently revealed design flaws and weaknesses that required rewriting large sections of code. Unit testing removes this problem. If I write good unit tests, this kind of refactoring almost completely disappears. So, the estimation I am left with is guessing how long it will take to write the original code and the accompanying unit tests. In general terms, I can estimate this fairly well. And so unit testing gives me the key ability to map out work in a sensible way and proceed according to a known, reasonable schedule.

Another benefit of unit testing is my central theme. By writing lots of unit tests, I create hundreds, even thousands, of seismographs that are embedded in my code. If new code disturbs some functionality or undoes an invariant of my previous code, one of those seismographs will generate a fault message in the form of a failing unit test. This is invariably a valuable insight (and one, I might add, that is possible only if all unit tests are run regularly.)

The concept of using unit tests as seismographs that monitor existing code has been given new life recently in work by Michael Feathers that first appeared in his excellent book “Working Effectively with Legacy Code” (Addison-Wesley, 2004). In it, Feathers discusses “characterization tests.” These are unit tests written explicitly for the purpose of capturing the current functionality of code. The tests are not intended to prove the quality of code, but merely to record how it operates. (Remember the book’s theme is dealing with legacy code, which Feathers defines as existing code with no unit tests.) The benefit is that if you’re refactoring legacy code, you can tell if you’ve disrupted it when any of these characterization tests fail. When you think about




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