What does “DAMP not DRY” mean when talking about unit tests?

It's a balance, not a contradiction

DAMP and DRY are not contradictory, rather they balance two different aspects of a code's maintainability. Maintainable code (code that is easy to change) is the ultimate goal here.

DAMP (Descriptive And Meaningful Phrases) promotes the readability of the code.

To maintain code, you first need to understand the code. To understand it, you have to read it. Consider for a moment how much time you spend reading code. It's a lot. DAMP increases maintainability by reducing the time necessary to read and understand the code.

DRY (Don't repeat yourself) promotes the orthogonality of the code.

Removing duplication ensures that every concept in the system has a single authoritative representation in the code. A change to a single business concept results in a single change to the code. DRY increases maintainability by isolating change (risk) to only those parts of the system that must change.

So, why is duplication more acceptable in tests?

Tests often contain inherent duplication because they are testing the same thing over and over again, only with slightly different input values or setup code. However, unlike production code, this duplication is usually isolated only to the scenarios within a single test fixture/file. Because of this, the duplication is minimal and obvious, which means it poses less risk to the project than other types of duplication.

Furthermore, removing this kind of duplication reduces the readability of the tests. The details that were previously duplicated in each test are now hidden away in some new method or class. To get the full picture of the test, you now have to mentally put all these pieces back together.

Therefore, since test code duplication often carries less risk, and promotes readability, its easy to see how it is considered acceptable.

As a principle, favor DRY in production code, favor DAMP in test code. While both are equally important, with a little wisdom you can tip the balance in your favor.

DAMP - Descriptive And Meaningful Phrases.

"DAMP not DRY" values readability over code re-use. The idea of DAMP not DRY in test cases is that tests should be easy to understand, even if that means test cases sometimes have repeated code.

See also Is duplicated code more tolerable in unit tests? for some discussion on the merits of this viewpoint.

It may have been coined by Jay Fields, in relation to Domain Specific Languages.

"DRY" is "Don't repeat yourself"

This is a term which is used to tell people to write code that is reusable, so that you don't end up writing similar code over and over again.

"DAMP" is "Descriptive And Meaningful Phrases".

This term is intended to tell you to write code which can easily be understood by someone who is looking at it. If you are following this principle, you will have long and descriptive variable and function names, etc.