Test-first mindset

Red · Write a failing test

If this feature were already done, which test would turn green first?

Write one test that describes one behavior. Run it and confirm it fails for the reason you expected, not for an import or environment error.

Green · Minimal implementation

What's the least code that turns this test green?

Write only enough to make the test pass, even if it's ugly or duplicated. Don't add edge cases. Don't refactor. That's the next step.

Green · Run it again

Is the whole suite still green?

Run the whole suite, not just the new test. If something else is red, your change affected it — fix that first.

Refactor · Clean up

While tests stay green, what duplication can you eliminate?

Refactor under green protection: extract functions, introduce abstractions, rename. Run tests after each change. Roll back the moment anything goes red.