TDD

Valeriy Kuzmin, Lilt, 2020

What it is

Write a failing test

Fix it

Refactor

Key parts

• Failing test is important
• You must focus on code being testable

Some testing theory

• Identifying dependencies
• Picking equivalence classes
• Multiplying factors
• Focusing on determinism
• Pruning the outcome
• Prioritizing all above

(fake) arguments against

• This takes too much time!
• That is useless!
• I am going to miss something anyway
• But we'd have to support that
• I'll write tests later
• I'll delegate it to a QA engineer

(true) arguments against

• Developers need to be trained
• Legacy code is genuinely hard to test
• Not all code is truly predictable
• Sometimes you have to urgently refactor

So, how to apply?

• Be positive, believe in yourself, work on the habit
• Gradually introduce coverage for existing code when working on bugs
• Write new code in TDD. If you cannot, write X% of new code in TDD
• Integrate into CI (git hooks are not CI!)
• Focus on low-level testing first
• Improve and maintain the system

Recap

Compared to no tests (*) or tests after, tests-first have...

But why? Isn't it just a theory?

• Pro: effect on productivity
• Pro: Empirical evidence
• Pro: evidence summary
• Minor: branch coverage & mutation score
• Contra: questionable coverage
• Contra: fragile tests
• Various: discussion on SO

Thanks!

Questions?