Trolling the 100% TDD coverage approach - or not

When I read this blog post about how and why one should achieve 100% code coverage on his code, I was flabbergasted, and when I noticed Bart’s reply, I decided to check:

The subject

Here is the gist of the article:

For me, there is no question 100% coverage is the only worthy objective, do not settle for less.

There is one simple golden rule: to reach and maintain 100% coverage, you do not need to add tests, you have to remove not covered lines!

This is important, so let me restate that: a line of code that is not covered is not maintainable, must be seen as not working and must be removed!


In my personal opinion, this is wrong on so many levels so I decided to ask if he was trolling…. He said he was not, so this was my reply:

Well, IRL working code providing business value beats coverage/good design by miles… Also, there is a lot of added business value in legacy code, so don’t delete it, but refactor it (see fowler’s book).

Another thing people sometimes do is throw away test code, as it is holding them back (in fact, lots of tests are considered legacy), so I wouldn’t approach this black & white.

While I love good, well written code (preferably developed with BDD or similar), that is not always the case. Just deleting it would be …

To quote someone replying on twitter: “rm -rf /"; 100% coverage done, now let’s grab a beer…

This was the tweet I was referring to, BTW:

The reply

Thanks for your feedback. Working software over anything else, yes :-), but working software without adequate automated tests getting legacy by the minute..

Definitely, I did not have legacy code in mind, so I will edit the post to clarify that. Adding test before refactoring is a good approach, but 100 % coverage is not only overkill, but counterproductive.

When tests are thrown away, someone has failed: either the test was not relevant (probably too close to the implementation) or the maintainer was lazy…

My 2 cts

Which might sound reasonable, however, there are some caveats IMO:

My reply

There’s different concepts of testing; TDD usually tests the implementation, not the behavior AFAIK… So when you change your implementation, you need to change your tests. BDD removes some of those issues.

Using TDD for problem exploration is usually a major PITA; Peter Norvig’s Sudoku [1] is a commonly known example…

When you think about the problem first, manage to understand it and keep the solution simple, a few integration tests are all you need…

Test-driven design can help you to create an implementation, not to find a solution. Proper modelling is necessary and way too often people forget that.

Also, if you want to verify whether your code works: 100% test coverage actually proves nothing; take the simple example if forgetting to check for a division by zero somewhere.

If you really want to verify how good your solution matches the problem space, I would suggest you to use property based testing; that will discover way more edge- and side-cases then you could ever imagine….


In conclusion

Don’t get me wrong; I love Behaviour-Driven development, and even use some TDD from time to time, but if the problem is not complicated I just verify whether my solution model covers my problem space, using integration-like tests.

If something really needs to be thoroughly tested, I would suggest to try property-based testing; at least that will show you where the problems in your solution space are.