Python Testing Skill

Purpose

Pytest-first testing emphasizing fakes over mocks and behavior-driven assertions. This skill applies to all Python testing work—always follow its guidance.

Core Philosophy

Bias toward business logic tests over fakes (Layer 4). Fakes track mutations without coupling to implementation. See references/test-doubles.md for details.

Required Tools

pytest, pytest-cov, pytest-asyncio, pytest-mock, hypothesis

Directory Structure

project/
├── src/mypackage/
└── tests/
    ├── conftest.py
    ├── unit/
    │   ├── fakes/          # Layer 1: Fake tests
    │   └── services/       # Layer 4: Business logic
    ├── integration/        # Layer 2: Sanity tests
    └── e2e/                # Layer 5: Real systems

See references/test-layers.md for layer distribution and decision tree.

Quick Patterns

Prefer Fakes Over Mocks

YOU MUST use fakes for business logic tests. Never use mocks when testing behavior—mocks couple tests to implementation and break on every refactor.

# CORRECT: Fake tests behavior (use this approach)
def test_user_creation():
    fake_db = FakeDatabaseAdapter()
    service = UserService(database=fake_db)
    user = service.create_user("alice@example.com")
    assert user.id == 1
    assert "INSERT" in fake_db.executed_queries[0]

# WRONG: Mock couples to implementation—this pattern causes test breakage every time you refactor
def test_user_creation(mocker):
    mock_db = mocker.patch("myapp.service.database")
    # Breaks on refactor

Factory Fixtures

@pytest.fixture
def make_user():
    def _make(name="test", **kwargs):
        return User(name=name, email=f"{name}@example.com", **kwargs)
    return _make

Capture Side Effects

@pytest.fixture
def capture_emails(monkeypatch):
    sent = []
    monkeypatch.setattr("myapp.email.send", lambda **kw: sent.append(kw))
    return sent

YOU MUST

• Test behavior, not implementation—tests that verify implementation details are technical debt
• Use fakes for business logic (mocks without fakes = brittle tests that fail on refactoring)
• Name tests descriptively: test_<what>_<condition>_<expected>
• Use tmp_path for all file operations in tests—never hardcode paths

NEVER

• Use subprocess in unit tests—always use CliRunner from click.testing
• Hardcode paths in tests
• Test private methods directly (test public API behavior instead)
• Use time.sleep() in unit tests (use monkeypatch or freezegun)

References (Load on Demand)

• references/test-doubles.md - Fakes vs mocks, when to use each
• references/anti-patterns.md - Common mistakes
• references/test-layers.md - Five-layer strategy, distribution
• references/fixture-patterns.md - Factory fixtures, scope, teardown
• references/agentic-testing.md - AI-assisted test writing