pytest — Advanced Implementation Playbook

§1 — Production Configuration

# pytest.ini
[pytest]
testpaths = tests
python_files = test_*.py
python_classes = Test*
python_functions = test_*
addopts = -v --strict-markers --tb=short -q
markers =
    slow: marks tests as slow (deselect with '-m "not slow"')
    integration: marks integration tests
    smoke: marks smoke tests
    api: marks API tests
filterwarnings =
    error
    ignore::DeprecationWarning

# pyproject.toml (alternative)
[tool.pytest.ini_options]
testpaths = ["tests"]
addopts = "-v --strict-markers --tb=short"
markers = [
    "slow: marks tests as slow",
    "integration: integration tests",
    "smoke: smoke tests",
]

[tool.coverage.run]
source = ["src"]
omit = ["tests/*", "*/migrations/*"]

[tool.coverage.report]
fail_under = 80
show_missing = true

§2 — Fixtures (Scoping, Factories, Teardown)

# conftest.py — shared fixtures
import pytest
from sqlalchemy import create_engine
from sqlalchemy.orm import Session

# Session-scoped: created once per test session
@pytest.fixture(scope="session")
def engine():
    engine = create_engine("sqlite:///test.db")
    Base.metadata.create_all(engine)
    yield engine
    Base.metadata.drop_all(engine)

# Function-scoped: created per test (default), auto-cleanup
@pytest.fixture
def db_session(engine):
    connection = engine.connect()
    transaction = connection.begin()
    session = Session(bind=connection)
    yield session
    session.close()
    transaction.rollback()
    connection.close()

# Factory fixture — create multiple instances
@pytest.fixture
def user_factory(db_session):
    created = []
    def _create_user(name="Test User", email=None, role="viewer"):
        email = email or f"{name.lower().replace(' ', '.')}@test.com"
        user = User(name=name, email=email, role=role)
        db_session.add(user)
        db_session.commit()
        created.append(user)
        return user
    yield _create_user
    for user in created:
        db_session.delete(user)
    db_session.commit()

# Autouse fixture — runs for every test in module
@pytest.fixture(autouse=True)
def reset_cache():
    cache.clear()
    yield
    cache.clear()

# tmp_path for file operations (built-in)
def test_writes_output(tmp_path):
    output_file = tmp_path / "result.json"
    generate_report(output_file)
    assert output_file.exists()
    data = json.loads(output_file.read_text())
    assert data["status"] == "complete"

§3 — Parameterized Tests

import pytest

# Basic parametrize
@pytest.mark.parametrize("input,expected", [
    ("hello", "HELLO"),
    ("world", "WORLD"),
    ("", ""),
    ("123", "123"),
])
def test_uppercase(input, expected):
    assert input.upper() == expected

# Multiple parameters with IDs
@pytest.mark.parametrize("email,valid", [
    ("user@test.com", True),
    ("invalid", False),
    ("", False),
    ("user@.com", False),
], ids=["valid_email", "no_at_sign", "empty", "missing_domain"])
def test_validate_email(email, valid):
    assert validate_email(email) == valid

# Combine parametrize (cartesian product)
@pytest.mark.parametrize("browser", ["chrome", "firefox", "edge"])
@pytest.mark.parametrize("resolution", ["1920x1080", "1366x768", "375x667"])
def test_responsive_layout(browser, resolution):
    assert render_page(browser, resolution).is_valid()

# Indirect parametrize (pass to fixture)
@pytest.fixture
def user(request):
    return create_user(role=request.param)

@pytest.mark.parametrize("user", ["admin", "editor", "viewer"], indirect=True)
def test_permissions(user):
    assert user.can_view()

§4 — Mocking with pytest-mock

# pip install pytest-mock

def test_send_email(mocker):
    mock_smtp = mocker.patch("myapp.email.smtplib.SMTP")
    send_email("test@example.com", "Hello", "Body")
    mock_smtp.return_value.sendmail.assert_called_once()

def test_api_call(mocker):
    mock_get = mocker.patch("myapp.api.requests.get")
    mock_get.return_value.json.return_value = {"id": 1, "name": "Alice"}
    mock_get.return_value.status_code = 200
    user = get_user(1)
    assert user["name"] == "Alice"
    mock_get.assert_called_once_with("https://api.example.com/users/1")

def test_database_error(mocker):
    mocker.patch("myapp.db.session.commit", side_effect=IntegrityError("duplicate"))
    with pytest.raises(DuplicateError):
        create_user("Alice", "alice@test.com")

# Spy — track calls without replacing
def test_logging(mocker):
    spy = mocker.spy(logger, "info")
    process_order(order)
    spy.assert_called_with("Order processed: %s", order.id)

# Mock environment variables
def test_config(monkeypatch):
    monkeypatch.setenv("API_KEY", "test-key-123")
    monkeypatch.setenv("DEBUG", "true")
    config = load_config()
    assert config.api_key == "test-key-123"
    assert config.debug is True

§5 — Async Testing

# pip install pytest-asyncio

import pytest

@pytest.mark.asyncio
async def test_async_fetch():
    result = await fetch_data("https://api.example.com/data")
    assert result["status"] == "ok"

@pytest.mark.asyncio
async def test_async_exception():
    with pytest.raises(ConnectionError):
        await fetch_data("https://invalid.example.com")

# Async fixtures
@pytest.fixture
async def async_client():
    async with AsyncClient(app=app, base_url="http://test") as client:
        yield client

@pytest.mark.asyncio
async def test_api_endpoint(async_client):
    response = await async_client.get("/api/users")
    assert response.status_code == 200
    assert len(response.json()) > 0

§6 — Testing Exceptions & Warnings

# Exception testing
def test_division_by_zero():
    with pytest.raises(ZeroDivisionError):
        1 / 0

def test_error_message():
    with pytest.raises(ValueError, match=r".*invalid email.*"):
        validate_email("not-an-email")

def test_raises_with_info():
    with pytest.raises(PermissionError) as exc_info:
        delete_file("/protected/file.txt")
    assert "permission denied" in str(exc_info.value).lower()
    assert exc_info.value.errno == 13

# Warning testing
def test_deprecation_warning():
    with pytest.warns(DeprecationWarning, match="use new_func"):
        old_func()

§7 — Markers & Custom Plugins

# Custom marker usage
@pytest.mark.slow
def test_full_data_processing():
    result = process_large_dataset()
    assert result.row_count > 1_000_000

@pytest.mark.integration
def test_database_connection():
    assert db.is_connected()

# Run by marker: pytest -m "not slow"
# Run by marker: pytest -m "smoke and not integration"

# Custom plugin — conftest.py
def pytest_collection_modifyitems(config, items):
    """Auto-mark tests in integration/ directory"""
    for item in items:
        if "integration" in str(item.fspath):
            item.add_marker(pytest.mark.integration)

# Custom report header
def pytest_report_header(config):
    return f"Environment: {os.getenv('ENV', 'local')}"

§8 — Class-Based Test Organization

class TestUserService:
    @pytest.fixture(autouse=True)
    def setup(self, db_session, user_factory):
        self.db = db_session
        self.create_user = user_factory
        self.service = UserService(db_session)

    def test_create_user(self):
        user = self.service.create("Alice", "alice@test.com")
        assert user.id is not None
        assert user.name == "Alice"

    def test_find_by_email(self):
        self.create_user(name="Bob", email="bob@test.com")
        user = self.service.find_by_email("bob@test.com")
        assert user.name == "Bob"

    def test_delete_nonexistent(self):
        with pytest.raises(NotFoundError):
            self.service.delete(999)

    class TestPermissions:
        """Nested class for permission-related tests"""
        def test_admin_can_delete(self, user_factory):
            admin = user_factory(role="admin")
            assert admin.can_delete()

        def test_viewer_cannot_delete(self, user_factory):
            viewer = user_factory(role="viewer")
            assert not viewer.can_delete()

§9 — CI/CD Integration

# GitHub Actions
name: Python Tests
on: [push, pull_request]
jobs:
  test:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        python-version: ['3.10', '3.11', '3.12']
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-python@v5
        with: { python-version: '${{ matrix.python-version }}' }
      - name: Install deps
        run: pip install -r requirements-test.txt
      - name: Run tests
        run: pytest --cov=src --cov-report=xml --junitxml=results.xml -v
      - name: Upload coverage
        uses: codecov/codecov-action@v4
        with: { files: coverage.xml }
      - name: Upload results
        uses: actions/upload-artifact@v4
        if: always()
        with: { name: test-results-${{ matrix.python-version }}, path: results.xml }

§10 — Debugging Quick-Reference

§11 — Best Practices Checklist

• ✅ Use fixtures over setup/teardown methods
• ✅ Use conftest.py for shared fixtures (auto-discovered)
• ✅ Use tmp_path for file operations (built-in, auto-cleanup)
• ✅ Use monkeypatch for env vars and attribute patching
• ✅ Use pytest-mock (mocker fixture) over unittest.mock
• ✅ Use @pytest.mark.parametrize for data-driven tests
• ✅ Register all custom markers in pytest.ini
• ✅ Use --strict-markers to catch typos in marker names
• ✅ Use pytest-cov for coverage with --cov-fail-under=80
• ✅ Use pytest-xdist for parallel execution: pytest -n auto
• ✅ Use --tb=short for concise tracebacks in CI
• ✅ Structure: tests/unit/, tests/integration/, conftest.py
• ✅ Name files test_*.py and functions test_*
• ✅ Use factory fixtures for creating test objects
• ✅ Use pytest.raises(match=...) for precise error checking