Python Testing & TDD
Test-first development with pytest. Combines the Red-Green-Refactor workflow with quality test patterns.
Activation Contract
Apply when the user writes tests, builds a feature test-first, asks about pytest/fixtures/mocks/coverage, or wants the full TDD cycle. Default to TDD: test before implementation.
Hard Rules
- Red → Green → Refactor, in that order. Write a failing test first; write the minimal code to pass; then improve with tests green.
- One scenario per test. Name it
test_<subject>_<condition>_<expected>. Follow Arrange-Act-Assert.
- Tests must be isolated and order-independent: no shared mutable module state.
- Mock only external boundaries (APIs, email, payments, time, uuid, random). Use a real test database; never mock internal repositories/services or the code under test.
- Enable branch coverage (
--cov-branch); cover both sides of every conditional, not just lines.
- Use
parametrize only for the same check over many inputs: separate tests for distinct scenarios.
Decision Gates
| Situation |
Action |
| New behavior requested |
Write failing test first (Red) |
| External dependency in unit |
Mock it (AsyncMock/Mock(spec=...)) |
| DB involved |
Real test DB + transaction rollback fixture |
| Same assertion, many inputs |
@pytest.mark.parametrize |
| Distinct scenarios |
Separate named tests |
| Async code |
asyncio_mode = "auto", async fixtures |
Execution Steps
- RED: write the smallest failing test for the next behavior; run it, confirm it fails for the right reason.
- GREEN: write the minimal code to pass; run, confirm green.
- REFACTOR: clean code and tests while green.
- Repeat per behavior; track phases explicitly for non-trivial features.
async def test_create_order_with_two_items_sums_total(service):
# Arrange
items = [{"price": 100, "qty": 2}, {"price": 50, "qty": 1}]
# Act
order = await service.create(items=items)
# Assert
assert order.total == 250
Output Contract
Return tests (and, in TDD, the minimal implementation) with clear behavior-naming, isolation via fixtures, and external-only mocking. State which phase (Red/Green/Refactor) each change belongs to.
References
- Pair with
python-clean-code and python-design-principles during Refactor.
1---2name: python-testing-tdd3description: Trigger: write tests, pytest, TDD, red-green-refactor, fixtures, mocks, parametrize, coverage, test first. Drive Python features test-first with quality pytest patterns.4license: Apache-2.05---67# Python Testing & TDD89Test-first development with pytest. Combines the Red-Green-Refactor workflow with quality test patterns.1011## Activation Contract1213Apply when the user writes tests, builds a feature test-first, asks about pytest/fixtures/mocks/coverage, or wants the full TDD cycle. Default to TDD: test before implementation.1415## Hard Rules1617- Red → Green → Refactor, in that order. Write a failing test first; write the minimal code to pass; then improve with tests green.18- One scenario per test. Name it `test_<subject>_<condition>_<expected>`. Follow Arrange-Act-Assert.19- Tests must be isolated and order-independent: no shared mutable module state.20- Mock only external boundaries (APIs, email, payments, time, uuid, random). Use a real test database; never mock internal repositories/services or the code under test.21- Enable branch coverage (`--cov-branch`); cover both sides of every conditional, not just lines.22- Use `parametrize` only for the same check over many inputs: separate tests for distinct scenarios.2324## Decision Gates2526| Situation | Action |27|-----------|--------|28| New behavior requested | Write failing test first (Red) |29| External dependency in unit | Mock it (`AsyncMock`/`Mock(spec=...)`) |30| DB involved | Real test DB + transaction rollback fixture |31| Same assertion, many inputs | `@pytest.mark.parametrize` |32| Distinct scenarios | Separate named tests |33| Async code | `asyncio_mode = "auto"`, async fixtures |3435## Execution Steps36371. RED: write the smallest failing test for the next behavior; run it, confirm it fails for the right reason.382. GREEN: write the minimal code to pass; run, confirm green.393. REFACTOR: clean code and tests while green.404. Repeat per behavior; track phases explicitly for non-trivial features.4142```python43async def test_create_order_with_two_items_sums_total(service):44 # Arrange45 items = [{"price": 100, "qty": 2}, {"price": 50, "qty": 1}]46 # Act47 order = await service.create(items=items)48 # Assert49 assert order.total == 25050```5152## Output Contract5354Return tests (and, in TDD, the minimal implementation) with clear behavior-naming, isolation via fixtures, and external-only mocking. State which phase (Red/Green/Refactor) each change belongs to.5556## References5758- Pair with `python-clean-code` and `python-design-principles` during Refactor.
JoseVelazcoH