DRY - Don't Repeat Yourself

Purpose

DRY (Don't Repeat Yourself) is a fundamental software engineering principle: every piece of knowledge should have a single, authoritative representation in your system. This guide provides tool-agnostic patterns for eliminating duplication across SQL, Python, and data platform development.

Overview

Don't Repeat Yourself means extracting reusable logic, calculations, and patterns into centralized, maintainable components. When you find yourself copying code, ask: "Can this be abstracted into a function, module, or shared artifact?"

Why DRY Matters

Single source of truth: Changes propagate automatically everywhere - no risk of inconsistent implementations or desynchronized business rules. Reduced maintenance: Update once instead of hunting through N files. Fewer places for bugs to hide, easier code reviews. Improved testing: Test the shared component once; all consumers inherit the fix automatically. Better collaboration: Team members reuse proven patterns. Knowledge stays centralized and documented for faster onboarding.

When to Break DRY

Don't over-abstract - sometimes duplication is acceptable.

Acceptable Duplication Scenarios

Scenario	Reason to Duplicate
One-time use	If logic appears only once, inline it for clarity
Different contexts	Similar code with different business meaning shouldn't be unified
Performance-critical	Abstraction overhead may hurt performance
Temporary code	Exploratory analysis or POC code can duplicate temporarily
Clear readability	Sometimes explicit duplication is more readable than abstraction

DRY Checklist

Before writing code, ask yourself:

• Have I seen this pattern before in the project?
• Will other parts of the codebase need this logic?
• Does a shared utility/function already exist?
• Would abstraction make this more maintainable?
• Is this the third occurrence (Rule of Three)?
• Will the abstraction be easy to understand?
• Does performance allow for abstraction?