My Test-Driven Design (TDD) Experiment

Test Driven Design

Photo by Helloquence on Unsplash

Is a Test-Driven Design approach effective, even when working on a medium scale personal project? Yes, and here’s why.

I’ve long been an advocate of Unit Testing in software development, having found that even though the work is often tedious, the investment in good tests significantly reduces overall development time. In my experience, the time invested in unit testing pays back by a factor of ten or more.

For my latest as-yet-unreleased project, I decided to try a Test-Driven Design approach. In short, the TDD method requires that you write tests for your code before writing the code. This has some significant benefits from a design/architecture perspective:

  • It forces the developer to think more about the external interface for a package and less about the implementation details.
  • It requires a more precise definition of what the inputs and outputs of a functional block are. This can expose flaws in the overall architecture. In the case of my project, I was working on the principle that the module would take one input file and generate an output. While creating the third test case, I realized that I was duplicating content from one input file to the next, that a far better approach would be to break that information into a second, common file that could be used multiple times.
  • It assures near 100% code coverage for tests. In theory it guarantees 100% code coverage. In practice that’s more difficult. More on that later.
  • Writing a test for everything forces you to to become a user of your own code. This serves to highlight problems early, before they become deeply embedded in code. When designing something, it’s easy to oversimplify. The only easy part of the “that part will be easy” trap is falling into it. In this project my “easy” trap required no less than three refactorings and a partial re-write before it was actually easy for the user. If I was coding first and testing later, backing out of the initial design would have been much more difficult and required throwing out a significant amount of code. Code is time.
  • It reduces useless code. I have a distaste for missing functionality. This means that when I write a method to do something, I’m inclined to generate similar methods that do parallel things at the same time. TDD puts an end to that.
  • It highlights common functionality that may not have been evident in the requirements phase. This makes it easier to spin out that functionality into independent classes early in the implementation.

Software Engineering Purity and Test Structure

Although I’m a strong advocate of automated testing, it turns out I’m far from a purist. Even though there are some clear benefits, I’m not likely to build a suite of mock classes just to isolate a subject class. 100% coverage is always a good thing, but I’ll take 80% coverage, knowing that some abstract class I don’t have an explicit test for got a pretty thorough run through by other tests, metrics be damned.

If tests require some expensive operation to complete, such as accessing a database, then creating a data provider is worth the effort, but doing so purely for the sake of correctness? Not so much.

It’s a little too easy to write code for something obvious, like checking for a null value, without writing a corresponding test. Sticking to Test-Driven Design requires a level of discipline that’s difficult to maintain, particularly when you’re the only developer on a project. Because of this my test suites tend to be a mix of unit tests, integrated tests, and some kind of ugly hybrid between the two, and I’ve decided that I don’t really care. Generating sample output in a bastardized test that ends with assertTrue(true) is still useful. Even though that test always passes, every once in a while the “real world example” test exercises an unexpected pathway and throws an error that would otherwise sneak by. I’ll take that find over purism ten times out of ten.

TDD and Over-engineering

I’m also more relaxed about software engineering principles when it comes to testing. I’m more likely to copy and paste test code than I am to carefully craft a hierarchy of test classes. I may be more relaxed but I’m not lax… as it turns out this project has a bunch of test cases that are common across multiple classes. I initially just copied the first class, with all the cases, which are fairly elaborate. Then I needed to copy them again for a third class. Three is the magic number when you realize you just screwed up. It took a fair bit of effort to go back and decouple the test case generation from the expected results, but there is an immediate payback: tests that exercise more sophisticated features in the latter classes are now automatically passed to their predecessors. If a simpler class can’t handle the new case in a reasonable way, it’s evident immediately.

While I have clearly strayed form a pure Test-Driven Design methodology, starting out with TDD gave my project an obvious lift.

Reducing gold plating and improving the design

On more than one occasion I found that I was inclined to embellish code with things like getters and setters that looked useful but actually had no use case. All the “I should add this” moments were converted to “I need to write a test for this first” and it’s not long before you realize that you don’t need a test because nothing will ever need to use the method in this way. Better yet, it makes you think about how that task would ever be used. The end result of this was twofold.

First, entirely removing functionality that seemed like a good idea early in the design process but in reality was just useless baggage. Second, if the functionality was useful, it was usually in the wrong place. This led to a series of code refactors that extracted that functionality into a conceptually clean solution, either a trait or a stand-alone class, in either case useful in many places. Less code or better code. Both excellent benefits of starting with TDD.

TDD Offers Measurable Progress

When working on a project of significant complexity, particularly when working alone, it’s easy to lose track of where you are. That’s not a problem if you have a client deadline looming in the not too distant future, but when it’s a personal project, and particularly if you’re blessed with a healthy dose of ADD, it’s easy to lose momentum.

For me, loss of momentum is the death of a project. I’ve got a long list of unfinished projects that I thought would take a few weeks when I started, but in fact they needed many months. Nearly all of them died from a momentum deficit.

Test-Driven design, with it’s focus on top level functionality, really helps with that. Even though my current project is perhaps 50% complete, it’s generating useful results. The implementation is partial, but it’s functional, and needless to say it’s tested, well structured, and robust. Instead of substantially complete code that winds up with significant problems when put to actual use, I have working but functionally incomplete code that I expect will be a joy to keep working on. All of these things are giving me enthusiasm for implementing the next level of features.


Even though I have strayed form the TDD methodology as the project progressed, starting with TDD was the best thing I’ve ever done when working on something of significant size.

If I was working in a team, not only would I start with TDD, I’d be far more strict about sticking to it throughout the development cycle. It highlights architectural issues very early in the development process, when it’s far easier to adjust and fix the problem. A dozen minor restructurings when the code base is small is a thousand times easier than rewriting thousands of lines of code after the mistakes have been baked into the project.

It’s hardly an objective measure, but I think this code represents some of the best work I’ve done. My three week project has extended to five months so far, but I’m still excited about it. Best of all it’s still fun. It also has the unexpected benefit of spinning off Configurable, which has proven itself to be very useful in other projects (not to mention that it’s just cool, IMO).