A recent blog post by Steve Klabnik concluded with a statement about tossing unit tests if you have end to end tests covering the code in question.
Don't be afraid to change the tests! As soon as you've verified that you've transcribed the code correctly, don't be afraid to just nuke things and start again. Especially if you have integration level tests that confirm that your features actually work, your unit tests are expendable. If they're not useful, kill them!
A few people on Twitter found this odd, and I'd have included myself in this statement a while back.
— Shaun Finglas (@F1nglas) January 21, 2012
Kent Beck's TDD screencasts changed my view on deleting unit tests however. During the later videos, he actually deleted some tests. Pretty much all TDD resources don't really mention this. One of the key points beginners learn is that if you break any tests, you've introduced a regression. This is not always the case. If you follow the rule of never deleting ANY tests you encounter you are going to be stuck with someone else's implementation forever. Likewise unit tests are there to drive design, not enforce how something works. I remember discussing deleting unit tests with my work colleagues and finding Kent's videos pretty shocking at the time. I mean deleting unit tests!?
The more I do TDD, the less this statement becomes so jarring. For example.
Consider a test for the above behavior, such as we get the result back in a particular state. Pretend the logic is rather simple, and it does not warrant a separate object. Any other developer should be free to come along and change the internals of this method. As long as we get a result back in the correct state, the test should be valid. The test should not care that we are using strings, lists or whatever internally.
Occasionally I find tests like this hard to pass. In other words, I feel like the logic is correct yet the test fails. Maybe I'm using a new language feature, or a language feature that seems to be not working as I expected. If so I'll break out a new unit test that tests the implementation. Such tests are often refereed to as learning tests. Here with a smaller focus I often become aware of what I'm doing wrong. Following Kent Becks example, I ditch the test after and move on.
I feel this sums up my feelings nicely.
— Avdi Grimm (@avdi) January 21, 2012
I and others are not saying bin every unit test you have that is covered by end to end tests. Unit tests are great, you can run hundreds in a matter of seconds. They have their place as part of the development process, but do not find yourself working against them. However I am saying you should delete any test which relies on implementation details. I am saying bin any test which does not make sense. I am also saying bin tests as part of a refactoring session as long as you have test coverage higher up. If you don't have test coverage such as acceptance tests, you cannot be sure you have not broke anything after the refactor.