DRY

DRY — Don’t Repeat Yourself — is a software design principle, coined by Andy Hunt and Dave Thomas in The Pragmatic Programmer (1999), which states that every piece of knowledge should have a single, unambiguous, authoritative representation within a system.

Violations of DRY are sometimes called WET code — "Write Everything Twice" or "We Enjoy Typing".

DRY is widely cited as one of the most fundamental principles in software engineering, and its scope extends beyond code to encompass configuration, documentation, data schemas, and build processes – anywhere that the same piece of knowledge is expressed in more than one place, a change to that knowledge requires multiple coordinated updates, creating opportunities for inconsistency and bugs.

A common misconception is that DRY is simply about avoiding copy-pasted code. Its actual concern is the duplication of knowledge or logic, not superficial textual similarity. Two pieces of code that happen to look alike but represent genuinely different concepts, or which belong to distinctly different parts of a system, should not necessarily be unified under a shared abstraction.

Taking DRY to the extreme can lead to [over-engineering], introducing inappropriate coupling and obscure intent. This tension is sometimes captured in the heuristic that duplication should be tolerated until a pattern is well understood, avoiding premature abstraction.

In testing, deliberate repetition is often preferred for the sake of readability and isolation, and is considered an acceptable exception to the principle.