Thursday, April 26, 2007

TotT: Refactoring Tests in the Red

With a good set of tests in place, refactoring code is much easier, as you can quickly gain a lot of confidence by running the tests again and making sure the code still passes.

As suites of tests grow, it's common to see duplication emerge. Like any code, tests should ideally be kept in a state that's easy to understand and maintain. So, you'll want to refactor your tests, too.

However, refactoring tests can be hard because you don't have tests for the tests.

How do you know that your refactoring of the tests was safe and you didn't accidentally remove one of the assertions?

If you intentionally break the code under test, the failing test can show you that your assertions are still working. For example, if you were refactoring methods in CombineHarvesterTest, you would alter CombineHarvester, making it return the wrong results.

Check that the reason the tests are failing is because the assertions are failing as you'd expect them to. You can then (carefully) refactor the failing tests. If at any step they start passing, it immediately lets you know that the test is broken – undo! When you're done, remember to fix the code under test and make sure the tests pass again.
(revert is your friend, but don't revert the tests!)

Let's repeat that important point:

When you're done...remember to fix the code under test!

Summary

  • Refactor production code with the tests passing. This helps you determine that the production code still does what it is meant to.
  • Refactor test code with the tests failing. This helps you determine that the test code still does what it is meant to.

Remember to download this episode of Testing on the Toilet and post it in your office.

5 comments:

  1. The key insight here is "failure is the test for tests".

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  2. Modifying the implementation to make sure the tests fail is exactly what heckle does in ruby. Using it as a mechanism for refactoring is novel tho... I'll have to poke around with that. Thanks!

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  5. Often there are several ways of breaking the code to make the test fail. How to you choose the "right way"?

    By breaking the code in a single way, you only know that your refactoring of the test code didn't stop testing that single aspect. But it may now let other bugs pass by that where tested before. So do you also - after the refactoring - break the tested code in other ways to make sure you didn't weaken the test code in any other way?

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