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

Execution Steps

1. RED: write the smallest failing test for the next behavior; run it, confirm it fails for the right reason.
2. GREEN: write the minimal code to pass; run, confirm green.
3. REFACTOR: clean code and tests while green.
4. 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.