Test Writing

Test Writing Skill

Produces tests that fail when the code is wrong — not tests that exist for coverage theatre. Covers happy path, edge cases, error paths, and state transitions. Does NOT chase 100% coverage as a goal; chases meaningful coverage that catches real regressions.

When to Use

• Writing tests for new code or features.
• Adding coverage to an untested area before changing it.
• A bug slipped through that tests should have caught.
• Practicing test-driven development.
• The verify-before-done skill flagged missing coverage.
• NOT when the user explicitly asked for a spike or throwaway script.

Prerequisites

• A test framework installed for the stack (pytest, vitest, jest, go test, xUnit, JUnit).
• Ability to run the test suite via the powershell tool.
• The view, edit, and grep tools to read existing patterns.
• A baseline run of the existing suite passing.

How to Run

1. Read existing tests with `view`/`grep` to copy the project's pattern.
2. List scenarios: happy path, edges, errors, state transitions.
3. Write one test per scenario using Arrange-Act-Assert.
4. Run the suite — confirm new tests pass and nothing else broke.
5. If TDD: write the test first, watch it fail for the right reason, then implement.

Quick Reference

Procedure

Step 1: Read the Existing Pattern

Use grep for test_* or *.spec.* files and view one of them. Match the project's naming, location, fixtures, and assertion style. Do not introduce a second testing dialect.

Step 2: Enumerate Scenarios

For the code under test, list:

1. Happy path inputs.
2. Edge cases (empty, null, boundary, max).
3. Error cases (invalid input, dependency failure).
4. State transitions, if stateful.

Step 3: Write Arrange-Act-Assert

def test_calculate_discount_for_premium_user():
    # Arrange
    user = create_user(tier="premium")
    order = create_order(total=100.00)

    # Act
    discount = calculate_discount(user, order)

    # Assert
    assert discount == 20.00

Name tests as documentation: test_login_with_expired_token_returns_401, not test_login.

Step 4: For Bug Fixes — Test First

1. Write a test that reproduces the bug.
2. Run it. Confirm it fails for the right reason.
3. Implement the fix.
4. Run it. Confirm it passes.
5. Run the full suite — no regressions.

Step 5: Run and Record

Run the suite via the powershell tool. Capture pass/fail counts and paste them into the tasks/todo.md Verification Results block (per verify-before-done).

Pitfalls

• DO NOT write tests that cannot fail (assert user is not None on a freshly constructed object).
• DO NOT test implementation details (e.g. mocking that quicksort was called). Test the behavior.
• DO NOT mock the world. Prefer real collaborators where the cost is acceptable.
• DO NOT ignore a flaky test — fix it or delete it. A flaky test erodes trust in the whole suite.
• DO NOT chase 100% coverage as a target. Coverage is a metric, not a mission.

Verification

• Every new test was observed to fail before it passed (TDD) OR a deliberate mutation was introduced to confirm it fails.
• The full suite is green after the change.
• Test names describe scenario + expected behavior.
• Each bug fix has a regression test that would have caught the original bug.