Do Not Repeat Yourself (DRY)¶

What is it?¶

Every piece of knowledge should have a single, unambiguous, authoritative representation within a system.

Guideline¶

Knowledge, not just text: This is about business logic and rules, not just identical lines of code. Two loops might look the same but represent different concepts; that is coincidental, not duplication.
Single Source of Truth: If you change a business rule, dependency or configuration, you should only have to update it in one place.
Consistency: Duplication of logic leads to inconsistency. If you have to remember to update A whenever you update B, you have a DRY violation.

Note: Contrast with The Rule of Two. DRY is the goal; The Rule of Two is a pragmatic heuristic to find the right abstraction to achieve DRY without over-engineering.