tdd

Test-Driven Development (TDD)

The RED-GREEN-REFACTOR Cycle

TDD is a discipline: you do not write implementation code until a failing test demands it.

1. RED — Write a test for the next small behavior. Run it. It must fail (confirming the test actually exercises real code).
2. GREEN — Write the minimum code required to make the test pass. Nothing more.
3. REFACTOR — Clean up the code (and tests) without changing behavior. Run the full suite to confirm nothing broke.

Repeat for every behavior.

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Rules

• Never write implementation without a failing test. If you reach for a file before writing a test, stop.
• One behavior per test. Each test should verify exactly one thing. If the test name requires "and", split it.
• Run the full suite after GREEN. A passing test that breaks another test is not GREEN.
• GREEN means minimum code. Hardcoding a return value to pass a test is valid — the next test will force generalization.
• REFACTOR is not optional. Skipping it accumulates design debt that compounds quickly.
• Bug found = write a reproducing test first. Make the bug visible as a failing test, then fix it. This prevents regressions.

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Test Design

Naming

Name tests by the behavior they verify, not by the method they call.

• Bad: test_calculate()
• Good: test_total_includes_tax_for_taxable_items()

Structure: Arrange-Act-Assert

Every test follows three phases:

// Arrange — set up inputs and dependencies
// Act     — call the unit under test
// Assert  — verify the outcome

Keep each phase visually distinct (blank lines or comments).

What to test

• Test the public interface, not internal implementation details. Tests that reach into private methods break during refactoring.
• One assertion per test (or one logical concept). Multiple assertions obscure which behavior failed.
• Use realistic inputs. Edge cases are important, but start with a normal, representative case.

Test scope

• Unit tests: isolate a single function or class. Mock collaborators.
• Integration tests: verify that two or more real components work together. Use sparingly in TDD cycles — they are slow.
• E2E tests: written after features are complete, not during TDD loops.

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When to Break the Rules

TDD is a discipline, not a religion. Two situations where you may skip the cycle:

1. Exploratory spikes — When you genuinely do not know if an approach is feasible, write throwaway code to find out. Delete it afterward and TDD the real implementation.
2. Simple glue code — Configuration files, framework boilerplate, and one-line adapters with no logic have little value under test. Use judgment.

In all other cases, follow RED-GREEN-REFACTOR.