The Rule of Two¶

What is it?¶

When you need functionality that already exists:

First time: Write it.
Second time: Copy and paste it (if the abstraction isn't obvious).
Third time: Refactor into a shared abstraction.

Why?¶

Premature abstraction creates complex, wrong abstractions. It is often better to have a little duplication than a wrong abstraction.

Seeing three concrete examples usually reveals the correct abstraction. Two points make a line, but three points define a curve (or a pattern). Waiting for the third instance ensures you are solving a general problem, not just coincidentally similar ones.

Corollary¶

If you see someone else has written it twice already and you're about to write a third, strongly consider building the abstraction, informed by the originals.