clean-pytest

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Summary

Write clean, maintainable pytest tests using Fake-based testing, contract testing, and dependency injection patterns. Use when setting up test suites for Python/MCP projects, creating Fakes for external dependencies, writing contract tests, or implementing test patterns with fixtures and parametrization.

SKILL.md

Clean Pytest

Clean, maintainable pytest test patterns using Fake-based testing, contract testing, and dependency injection. Focuses on test isolation, reusability, and clarity through explicit AAA pattern and well-structured fixtures.

When to Use

Setting up test suites for Python/MCP projects
Creating Fake implementations for external dependencies
Writing contract tests for MCP tools/controllers
Implementing test patterns with dependency injection
Testing layered architectures (Controllers → Services → Repositories)
Writing parametrized tests for multiple scenarios

Core Principles

1. Fakes over Mocks

Use Fake classes instead of mocking with unittest.mock. Fakes are in-memory implementations that mimic real dependencies without external calls.

Why Fakes?

More readable and maintainable
Easier to debug
Better test isolation
No monkey-patching magic
Self-documenting behavior

2. Explicit AAA Pattern

Structure every test into three clear phases with comments:

# Arrange
# Set up test data and dependencies

# Act
# Execute the code under test

# Assert
# Verify the result

3. Dependency Injection in Fixtures

Inject dependencies between fixtures to maintain relationships and avoid duplication.

4. Contract Testing

Verify that components register tools/functions correctly and pass expected arguments.

Architecture Pattern

Controller (MCP Tools)
    ↓
Service (Business Logic)
    ↓
Repository (Data Access)
    ↓
Fake (Test Implementation)

# tests/fakes.py
from typing import Any, Dict, List, Optional

class FakeAuth:
    """Fake implementation of AuthProvider for testing."""
    def __init__(self) -> None:
        self.created: List[Dict[str, Any]] = []
        self.deleted: List[str] = []
        self._seq = 0
        self.fail_on_create: bool = False

    def create_user(self, email: str, password: str, display_name: str) -> str:
        if self.fail_on_create:
            raise RuntimeError("create_user failed (fake)")
        self._seq += 1
        uid = f"uid-{self._seq}"
        rec = {"uid": uid, "email": email, "display_name": display_name}
        self.created.append(rec)
        return uid

    def delete_user(self, uid: str) -> None:
        self.deleted.append(uid)

class FakeUsersRepo:
    """Fake implementation of UsersRepository."""
    def __init__(self) -> None:
        self.users: Dict[str, Dict[str, Any]] = {}
        self.fail_on_upsert: bool = False

    def upsert_user_doc(self, uid: str, data: Dict[str, Any]) -> None:
        if self.fail_on_upsert:
            raise RuntimeError("upsert_user_doc failed (fake)")
        self.users[uid] = dict(data)

    def list_users(self, limit: Optional[int] = None) -> List[Dict[str, Any]]:
        items = list(self.users.values())
        if limit and limit > 0:
            items = items[:limit]
        return [dict(it) for it in items]

class FakeAuth:
    def __init__(self) -> None:
        self.fail_on_create: bool = False  # Control failure in tests

    def create_user(self, email: str, password: str, display_name: str) -> str:
        if self.fail_on_create:
            raise RuntimeError("create_user failed (fake)")
        # ... rest of implementation

class FakeSectorsRepo:
    def __init__(self, institutions: FakeInstitutionsRepo | None = None) -> None:
        self.institutions = institutions  # Inject dependency
        self.data: Dict[str, Dict[str, Dict[str, Any]]] = {}

    def institution_exists(self, institution_id: str) -> bool:
        return bool(self.institutions and institution_id in self.institutions.data)

    def upsert_sector(self, institution_id: str, sector_id: str, data: Dict[str, Any]) -> None:
        self.data.setdefault(institution_id, {})[sector_id] = dict(data)

# tests/conftest.py
import pytest
from tests.fakes import FakeAuth, FakeUsersRepo

@pytest.fixture()
def fake_auth():
    """Provide a fresh FakeAuth for each test."""
    return FakeAuth()

@pytest.fixture()
def fake_users_repo():
    """Provide a fresh FakeUsersRepo for each test."""
    return FakeUsersRepo()

@pytest.fixture()
def fake_sectors_repo(fake_institutions_repo):
    """FakeSectorsRepo depends on FakeInstitutionsRepo."""
    return FakeSectorsRepo(institutions=fake_institutions_repo)

@pytest.fixture()
def fake_rooms_repo(fake_sectors_repo):
    """FakeRoomsRepo depends on FakeSectorsRepo."""
    return FakeRoomsRepo(sectors=fake_sectors_repo)

@pytest.fixture()
def user_env(fake_auth, fake_users_repo):
    """Provide service and all dependencies for user operations."""
    from myapp.services.user_service import UserService
    svc = UserService(fake_auth, fake_users_repo)
    return svc, fake_auth, fake_users_repo

@pytest.fixture()
def user_env_seeded(user_env):
    """Environment with pre-seeded data."""
    svc, auth, repo = user_env
    svc.add_user(email="[email protected]", password="secret", name="Test User")
    return svc

@pytest.fixture()
def temp_file():
    """Provide a temporary file and clean up after test."""
    import tempfile
    import os
    fd, path = tempfile.mkstemp()
    os.close(fd)
    yield path
    os.unlink(path)

# tests/test_user_service.py
import pytest
from myapp.services.user_service import UserService

def test_add_user_success(fake_auth, fake_users_repo):
    # Arrange
    svc = UserService(fake_auth, fake_users_repo)
    email = "[email protected]"
    password = "secret"
    name = "Test User"

    # Act
    result = svc.add_user(email=email, password=password, name=name)

    # Assert
    assert result["status"] == "ok"
    assert result["user"]["email"] == email
    assert result["user"]["name"] == name
    assert result["uid"] in fake_users_repo.users

@pytest.mark.parametrize(
    "email,password,name,role",
    [
        ("[email protected]", "secret", "Alice", "admin"),
        ("[email protected]", "p@ss", "Bob", "user"),
    ],
)
def test_add_user_parametrized(user_env, email, password, name, role):
    svc, _auth, _repo = user_env

    # Act
    res = svc.add_user(email=email, password=password, name=name, global_role=role)

    # Assert
    assert res["status"] == "ok"
    assert res["user"]["email"] == email
    assert res["user"]["name"] == name
    assert res["user"]["globalRole"] == role

@pytest.mark.parametrize("email", ["[email protected]", "[email protected]"])
def test_add_user_rollback_on_firestore_failure(fake_auth, fake_users_repo, email):
    # Arrange
    fake_users_repo.fail_on_upsert = True
    svc = UserService(fake_auth, fake_users_repo)

    # Act & Assert
    with pytest.raises(RuntimeError):
        svc.add_user(email=email, password="secret", name="Bob")

    # Assert rollback
    assert fake_auth.deleted, "Expected auth user to be deleted on Firestore failure"

def test_list_users_normalizes_timestamps_to_iso(user_env):
    # Arrange
    svc, _auth, repo = user_env
    from datetime import datetime
    repo.users["u1"] = {
        "id": "u1",
        "email": "[email protected]",
        "name": "X",
        "globalRole": "user",
        "createdAt": datetime(2024, 1, 1),
        "updatedAt": datetime(2024, 1, 2),
    }

    # Act
    res = svc.list_users(limit=10)

    # Assert
    assert res["status"] == "ok"
    assert res["count"] == 1
    user = res["users"][0]
    assert isinstance(user["createdAt"], str)
    assert isinstance(user["updatedAt"], str)

# tests/test_controllers_contract.py
from typing import Any, Callable, Dict

class FakeMCP:
    """Minimal FakeMCP for contract testing."""
    def __init__(self) -> None:
        self.tools: Dict[str, Callable[..., Any]] = {}
        self.meta: Dict[str, Dict[str, Any]] = {}

    def tool(self, name: str, description: str, tags: Optional[set] = None, meta: Optional[dict] = None):
        def decorator(fn: Callable[..., Any]):
            self.tools[name] = fn
            self.meta[name] = {
                "description": description,
                "tags": set(tags or set()),
                "meta": dict(meta or {}),
            }
            return fn
        return decorator


class FakeUserService:
    """Simple fake service that records calls."""
    def __init__(self):
        self.calls = []

    def add_user(self, **kwargs):
        self.calls.append(("add_user", kwargs))
        return {"status": "ok", "op": "add_user", "args": kwargs}


def test_users_controller_contract():
    # Arrange
    from myapp.controllers.users_controller import UsersController
    fake = FakeMCP()
    svc = FakeUserService()
    UsersController(fake, svc)

    # Assert tool registration
    assert "add_user" in fake.tools
    assert "list_users" in fake.tools

    # Act & Assert tool behavior
    res = fake.tools["add_user"](
        email="[email protected]", password="s3cr3t", name="Alice", global_role="admin"
    )
    assert res["status"] == "ok"
    assert res["op"] == "add_user"
    assert res["args"]["email"] == "[email protected]"

@pytest.mark.parametrize(
    "email,password,name,role",
    [
        ("[email protected]", "s3cr3t", "Alice", "admin"),
        ("[email protected]", "p@ssw0rd", "Bob", "user"),
    ],
)
def test_users_add_user_parametrized(_users_env, email, password, name, role):
    # Arrange
    fake, _ = _users_env

    # Act
    res = fake.tools["add_user"](
        email=email, password=password, name=name, global_role=role
    )

    # Assert
    assert res["status"] == "ok"
    assert res["op"] == "add_user"
    assert res["args"]["email"] == email

@pytest.fixture()
def repo_env(fake_institutions_repo, fake_sectors_repo):
    # Seed data
    fake_institutions_repo.upsert("inst1", {"id": "inst1", "name": "Inst One"})
    fake_sectors_repo.upsert_sector(
        "inst1", "er", {"id": "er", "name": "ER", "slug": "er", "isActive": True}
    )
    return fake_sectors_repo

@pytest.mark.parametrize("rooms", [
    ["101"],
    ["201", {"name": "102", "id": "room-102"}],
])
def test_add_and_list_rooms(room_env, rooms):
    svc, _ = room_env

    # Act
    res = svc.add_sector_rooms("inst1", "er", rooms)

    # Assert
    assert res["status"] == "ok"
    assert res["count"] == len(rooms)

    lst = svc.list_sector_rooms("inst1", "er", limit=10)
    assert lst["status"] == "ok"
    assert lst["count"] == len(rooms)

@pytest.mark.parametrize("limit", [1, 3])
def test_list_rooms_limits(room_env_seeded, limit):
    svc = room_env_seeded

    # Act
    lst = svc.list_sector_rooms("inst1", "er", limit=limit)

    # Assert
    assert lst["status"] == "ok"
    assert lst["count"] == min(2, limit)  # 2 items seeded

@pytest.mark.parametrize("room_id,deleted", [
    ("room-102", True),
    ("room-999", False),
])
def test_remove_rooms_parametrized(room_env_seeded, room_id, deleted):
    svc = room_env_seeded

    # Act
    res = svc.remove_sector_room("inst1", "er", room_id)

    # Assert
    assert res["deleted"] is deleted
    if not deleted:
        assert res.get("reason") == "room_not_found"

# tests/test_integration_wiring.py
import os
import pytest

# Gate this integration test on presence of credentials
_ENV_KEYS = (
    "FIREBASE_SERVICE_ACCOUNT",
    "GOOGLE_APPLICATION_CREDENTIALS",
)
_has_env_creds = any(os.getenv(k) for k in _ENV_KEYS)

pytestmark = [
    pytest.mark.integration,
    pytest.mark.skipif(
        not _has_env_creds,
        reason=(
            "Integration test requires Firebase Admin credentials via env "
            "(FIREBASE_SERVICE_ACCOUNT or GOOGLE_APPLICATION_CREDENTIALS)"
        ),
    ),
]

@pytest.mark.integration
def test_build_app_initializes_and_registers_tools():
    # Arrange
    from myapp.wiring import build_app

    # Act
    app = build_app()

    # Assert
    assert hasattr(app, "run")

def test_user_created_in_one_test_not_visible_in_another(fake_auth, fake_users_repo):
    # Arrange
    svc1 = UserService(fake_auth, fake_users_repo)

    # Act
    result1 = svc1.add_user(email="[email protected]", password="secret", name="User1")

    # Assert - second test with fresh fixtures should not see this user
    svc2 = UserService(fake_auth, fake_users_repo)
    users = svc2.list_users()
    assert users["count"] == 1  # Only the user from this test

@patch('firebase_admin.auth.create_user')
def test_add_user(mock_create_user):
    mock_create_user.return_value = Mock(uid="uid-1")
    # ... test code

def test_add_user(fake_auth, fake_users_repo):
    svc = UserService(fake_auth, fake_users_repo)
    # ... test code

def test_add_user(fake_auth, fake_users_repo):
    svc = UserService(fake_auth, fake_users_repo)
    svc.add_user(email="[email protected]", password="secret", name="User")
    assert fake_auth.created == [{"uid": "uid-1", ...}]  # Implementation detail

def test_add_user(fake_auth, fake_users_repo):
    svc = UserService(fake_auth, fake_users_repo)
    result = svc.add_user(email="[email protected]", password="secret", name="User")
    assert result["status"] == "ok"
    assert result["user"]["email"] == "[email protected]"

def test_add_user_success(fake_auth, fake_users_repo):
    # Only tests success case

def test_add_user_success(fake_auth, fake_users_repo):
    # Happy path

def test_add_user_rollback_on_firestore_failure(fake_auth, fake_users_repo):
    # Error path

def test_add_user_handles_duplicate_email(fake_auth, fake_users_repo):
    # Edge case

# Run all tests
pytest

# Run with coverage
pytest --cov=myapp --cov-report=term-missing

# Run specific test file
pytest tests/test_user_service.py

# Run specific test
pytest tests/test_user_service.py::test_add_user_success

# Run parametrized tests with verbose output
pytest -v tests/test_user_service.py::test_add_user_parametrized

# Skip integration tests
pytest -m "not integration"

# Run only integration tests
pytest -m integration

# Stop on first failure
pytest -x

# Show local variables on failure
pytest -l

# Run tests in parallel (with pytest-xdist)
pytest -n auto

clean-pytest

Summary

Clean Pytest

When to Use

Core Principles

1. Fakes over Mocks

2. Explicit AAA Pattern

3. Dependency Injection in Fixtures

4. Contract Testing

Architecture Pattern

Creating Fakes

Basic Fake Structure

Repository Fake

Controlled Failure Fakes

Nested Repository Fakes

Fixtures

Basic Fixture (conftest.py)

Fixture with Dependency Injection

Environment Fixture

Seeded Environment Fixture

Fixture with Cleanup

Service Layer Testing

Basic AAA Pattern Test

Parametrized Tests

Testing Error Scenarios with Fakes

Testing Timestamp Normalization

Contract Testing

MCP Tool Registration Contract

Parametrized Contract Tests

Repository Layer Testing

Testing Repository Operations

Testing Multiple Data Scenarios

Testing Limit Behavior

Testing Not Found Scenarios

Integration Testing

Conditional Integration Tests

Test Isolation

Testing Anti-Patterns to Avoid

Don't Mock What You Don't Own

Don't Test Implementation Details

Don't Skip Error Paths

Running Tests

Best Practices Checklist

Related skills