Tdd

Test-Driven Development

Red-green-refactor. Write the test, watch it fail, make it pass, clean up.

Workflow

1. Red — Write a Failing Test

Write the simplest test that describes the desired behavior:

def test_user_can_login_with_valid_credentials():
    result = login("alice", "correct-password")
    assert result.success is True
    assert result.user.name == "alice"

Run it. Confirm it fails for the right reason (missing function, not syntax error).

2. Green — Make It Pass

Write the minimum code to make the test pass. Don't generalize. Don't optimize. Don't clean up. Just make it green.

3. Refactor — Clean Up

Now that tests are green, improve the implementation:

• Remove duplication
• Extract clear names
• Simplify logic

Run tests again after refactoring. Still green? Good. Commit.

Test Quality Rules

• Test behavior, not implementation. Tests should survive refactoring.
• One assertion per concept. Multiple assert calls are fine if they verify one logical thing.
• Descriptive names. test_expired_token_returns_401 not test_auth_3.
• No mocks unless necessary. Prefer real objects. Mock only external services and I/O.
• Arrange-Act-Assert structure. Set up, do the thing, check the result.

When to Use TDD

Good fit:

• Business logic with clear inputs/outputs
• Bug fixes (write the test that catches the bug first)
• API endpoints (expected request → expected response)
• Data transformations and validators

Poor fit:

• Exploratory/prototype code (write tests after)
• UI layout (use visual testing)
• Infrastructure/config (use integration tests)

Framework Detection

Detect the project's test framework from existing files:

# Python
ls pytest.ini pyproject.toml setup.cfg 2>/dev/null | head -1
grep -r "import pytest\|import unittest" tests/ --include="*.py" -l | head -3

# JavaScript/TypeScript
grep -E "jest|vitest|mocha" package.json 2>/dev/null

# Go — built in
ls *_test.go 2>/dev/null

Follow the existing test patterns. Don't introduce a new framework.