test-driven-development

name	test-driven-development
description	New behavior or a bug with a repro. Test first, then the fix.
category	software-development

Test-driven development

Write a failing test that pins the behavior, then the smallest code that makes it pass.

When to use

• New behavior the user asked for
• A bug you can express as input -> expected output
• You are about to change logic that already has a test hook

When not to use

• Pure visual / HTML mockups (use sketch or web-design)
• One-off scripts with no test runner

Procedure

1. Find how this project runs tests (grep for test, ./gradlew test, pytest). Check: one command.
2. Write the failing test with write_file or edit_file. Check: run it, it fails for the right reason.
3. Write the production change. Check: that test now passes.
4. Refactor only after green. Check: tests still pass.
5. If the suite is huge, run the new test plus the nearest related ones.

Pitfalls

• A test that never failed proves nothing.
• Do not mock the unit under test.
• Android: prefer JVM unit tests under app/src/test when the logic does not need a device.

Verification

The new test failed once, then passed, and you kept it.