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TDD isn't a panacea

Wednesday, 25 January 2017

On Monday, I attended the Software Cornwall Business Connect Event. This was a day of talks and workshops for people in the local software development community here in Cornwall. One of the talks was on TDD, and how that fitted into the software development process at one of the larger software companies in the area.

There was an interesting question asked by one of the attendees: how do you know that your tests are correct? What if you make a mistake in your tests, and then change the code to make it pass?

The answer that the presenter gave, and the one that is most common in the literature, is that the tests check the code, and the code checks the tests. There was a brief discussion around this point, but I thought that it was worth elaborating here.

Baby steps

One of the key parts of TDD is the idea of "baby steps". You write code incrementally, with small changes, and small, focused tests.

Consequently, when you write a new test, you're testing some small change you're about to make. If you make a mistake, this gives you a reasonable chance of spotting it.

However, that's still relying on you spotting it yourself. You just wrote the test, and it's very easy to read back what you intended to write, rather than what you actually did write. This is where "the code checks the tests" comes in.

Red

The TDD cycle is often dubbed "Red, Green, Refactor". At this point, the relevant part is "Red" — you just wrote a new test, so it should fail. If you run it, and it passes then something is wrong, most likely a mistake in the test. This gives you a second chance to revisit the test, and ensure that it is indeed correct. Maybe you mistyped one of the values; maybe you missed a function call. Either way, this gives you a second chance to verify the test.

OK, so you wrote the test (wrong), the test fails (as expected), so you write the code to make it pass. This gives you the next opportunity to verify the test.

Green

TDD doesn't exempt you from thinking. You don't just write a test and then blindly make it pass. You write a test, often with an idea of the code change you're going to make to make it pass.

So, you've written a failing test, and you make the desired change. You run the tests, and the new test still fails. You haven't got the "Green" outcome you desired or intended. Obviously, the first thing to check here is the code, but it also gives you another chance to check the test. If the code really does do what you intended, then the test must be wrong, and vice versa.

Of course, we don't have just the one test, unless this is the beginning of a project. We have other tests on the code. If the new test is incorrect, and thus inconsistent with the previous tests, then changing the code to make the new test pass may well cause previous tests to fail.

At this point, you've got to be certain that the test is right if you're going to change the code. Alternatively, you made exactly the same mistake in the code as you did in the test, and the test passes anyway. Either way, this is the point at which the faulty test becomes faulty code.

It's not the last chance we have to check the test, though: the final step in the TDD loop is "Refactor".

Refactor

Refactoring is about simplifying your code — removing duplication, applying general algorithms rather than special cases, and so forth. If you've made a mistake in a test, then the implementation may well end up with a special case to handle that particular eventuality; the code will be resistant to simplification because that test is inconsistent with the others. Again, TDD does not exempt you from thinking. If you intended to implement a particular algorithm, and the code does not simplify in a way that is consistent with that algorithm then this is a red flag — something is not right, and needs checking. This gives you yet another chance to inspect the test, and verify that it is indeed correct.

At this point, if you still haven't fixed your test, then you have now baked the mistake into your code. However, all is not lost: "Red, Green, Refactor" is a cycle, so we begin again, with the next test.

More tests

As we add more tests, we get more and more opportunities to verify the correctness of the existing tests. As we adjust the code to make future tests pass, we can spot inconsistencies, and we can observe that changes we make break the earlier test. When we see the broken test, this gives us a further opportunity to verify that the test is indeed correct, and potentially fix it, rather than fixing the code.

Acceptance tests then provide a further level of checking, though these tend to be far courser-grained, so may work fine unless the particular inputs happen to map to those in the incorrect lower level test.

TDD is not a panacea; but it is a useful tool

So, it is possible that a mistake in a test will be matched by incorrect code, and this will make it into production. However, in my experience, this is unlikely if the incorrect test is merely a mistake. Instead, the bugs that make it through tend to fall into two categories:

  • errors of omission — I didn't think to check that scenario, or
  • errors due to misunderstanding — my understanding of the requirements didn't match what was wanted, and the code does exactly what I intended, but not what the client intended.

TDD is not a panacea, and won't fix all your problems, but it is a useful tool, and has the potential to greatly reduce the number of bugs in your code.

Please let me know what you think by adding a comment.

Posted by Anthony Williams
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