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)

Creating Fakes

Basic Fake Structure

Create a Fake class that implements the same interface as the real dependency:

# 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)

Repository Fake

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(sel...