tdd

name	tdd
description	Test-driven development with red-green-refactor loop. Use when user wants to build features or fix bugs using TDD, mentions "red-green-refactor", wants integration tests, or asks for test-first development.

Test-Driven Development

Philosophy

Core principle: Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't.

Good tests exercise real code paths through public APIs. They describe what the system does, not how it does it. A good test reads like a specification — "aircraft rejects non-positive weight" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure.

Bad tests are coupled to implementation. They mock internal collaborators, test private methods, or assert on call order/count. The warning sign: your test breaks when you refactor, but behavior hasn't changed.

For detailed test style conventions, mocking guidance, and examples, see unit-testing-guide.md.

Mocking boundary in this project

Mock only at infrastructure boundaries: external solver binaries (HiGHS, GLPK), file system, HTTP clients, time. Solver adapters wrap external binaries and are acceptable mock targets. Do not mock domain models, value objects, or internal collaborators within the same layer.

Anti-Pattern: Horizontal Slices

DO NOT write all tests first, then all implementation. This is "horizontal slicing" — treating RED as "write all tests" and GREEN as "write all code."

This produces crap tests:

• Tests written in bulk test imagined behavior, not actual behavior
• You end up testing the shape of things (data structures, function signatures) rather than user-facing behavior
• Tests become insensitive to real changes — they pass when behavior breaks, fail when behavior is fine
• You outrun your headlights, committing to test structure before understanding the implementation

Correct approach: Vertical slices via tracer bullets. One test → one implementation → repeat. Each test responds to what you learned from the previous cycle.

WRONG (horizontal):
  RED:   test1, test2, test3, test4, test5
  GREEN: impl1, impl2, impl3, impl4, impl5

RIGHT (vertical):
  RED→GREEN: test1→impl1
  RED→GREEN: test2→impl2
  RED→GREEN: test3→impl3
  ...

Workflow

1. Planning

Before writing any code:

• Identify which architecture layer the new code belongs to (domain / application / infrastructure / controller) — see docs/architecture.md
• Confirm with user what interface changes are needed
• Confirm with user which behaviors to test (prioritize)
• Identify opportunities for deep modules (small interface, deep implementation)
• Design interfaces for testability
• List the behaviors to test (not implementation steps)
• Mark integration tests with @pytest.mark.integration; all other tests run as unit tests by default
• Get user approval on the plan

Ask: "What should the public interface look like? Which behaviors are most important to test?"

You can't test everything. Confirm with the user exactly which behaviors matter most. Focus testing effort on critical paths and complex logic, not every possible edge case.

2. Tracer Bullet

Write ONE test that confirms ONE thing about the system:

RED:   Write test for first behavior → test FAILS
GREEN: Write minimal code to pass → test PASSES

This is your tracer bullet — proves the path works end-to-end.

3. Incremental Loop

For each remaining behavior:

RED:   Write next test → fails
GREEN: Minimal code to pass → passes

Rules:

• One test at a time
• Only enough code to pass current test
• Don't anticipate future tests
• Keep tests focused on observable behavior

4. Refactor

After all tests pass, look for refactor candidates:

• Extract duplication
• Deepen modules (move complexity behind simple interfaces)
• Apply SOLID principles where natural
• Consider what new code reveals about existing code
• Run tests after each refactor step

Never refactor while RED. Get to GREEN first.

Example: One Red-Green Cycle

Suppose we add a rule that CargoRequest.weight_kg must not exceed 50,000 kg.

RED — write the test first (it fails because the validation doesn't exist yet):

# tests/domain/test_cargo_request.py

class TestCargoRequest:

    def test__cargo_request__weight_exceeds_max__raises(self):
        # ARRANGE / ACT / ASSERT
        with pytest.raises(ValueError, match="weight_kg must not exceed 50000"):
            CargoRequest(
                id="CR-1",
                client_name="Acme",
                weight_kg=60000,
                volume_m3=10,
                revenue_usd=500,
            )

Run: pytest tests/domain/test_cargo_request.py -k "weight_exceeds_max" → FAILS (no validation yet).

GREEN — add the minimal validation in the domain model:

# app/domain/cargo_request.py

@dataclass
class CargoRequest:
    ...

    def __post_init__(self) -> None:
        ...
        if self.weight_kg > 50000:
            raise ValueError("weight_kg must not exceed 50000")

Run the same test → PASSES. Move to the next behavior.

Checklist Per Cycle

[ ] Test describes behavior, not implementation
[ ] Test uses public interface only
[ ] Test would survive internal refactor
[ ] Test follows naming: test__[unit]__[scenario]__[outcome]
[ ] Code is minimal for this test
[ ] No speculative features added