Implement Dependency Injection

Works with container.py and container_factory.py files.

Implement Dependency Injection

Purpose

Guides proper integration of services into the dependency injection Container using the dependency-injector library. Ensures correct provider selection, prevents circular dependencies, and enables testability through override patterns.

When to Use

Use this skill when:

• Creating new service - Adding a service that needs dependencies
• Adding dependency to Container - Wiring components for injection
• Debugging circular dependency errors - Resolving dependency cycles
• Wiring components for injection - Setting up proper dependency flow
• Setting up test overrides - Mocking dependencies in tests

Trigger phrases:

• "Add X to the dependency container"
• "Wire Y service with dependencies"
• "Fix circular dependency error"
• "Override dependency for testing"

Quick Start

Adding a simple service to Container:

# In container.py
my_service = providers.Singleton(
    MyService,
    settings=settings,
    dependency1=some_dependency,
)

Table of Contents

Core Sections

• Purpose - Single-sentence skill objective
• Quick Start - Immediate, copy-paste ready example
• Instructions - Complete implementation guide
  • Step 1: Determine Provider Type - Singleton vs Factory vs Dependency
  • Step 2: Add Service to Container - Container.py integration
  • Step 3: Handle Special Cases - Factory functions, config extraction, circular dependencies
  • Step 4: Override for Testing - Test fixture patterns
  • Step 5: Initialize in Factory - MCP server initialization
• Examples - Concrete, working implementations
  • Example 1: Add Repository (Singleton)
  • Example 2: Add Command Handler (Factory)
  • Example 3: Override for Testing
• Requirements - Dependencies, files, quality checks
• Common Pitfalls - Troubleshooting guide

Supporting Resources

• references/provider-reference.md - Complete dependency-injector API reference
• references/circular-dependency-guide.md - Circular dependency resolution strategies
• templates/service-template.py - Service class template with DI

Utility Scripts

• Detect Circular Dependencies - Detect circular dependencies in DI container
• Generate Provider - Auto-generate provider code for container
• Validate DI Patterns - Validate dependency injection patterns in codebase

Instructions

Step 1: Determine Provider Type

Choose the appropriate provider based on lifecycle requirements:

Use providers.Singleton:

• Stateful services (repositories, caches)
• Expensive initialization (database connections, embedding models)
• Shared state needed across requests
• Most application/infrastructure services

Use providers.Factory:

• Request handlers (command/query handlers)
• Stateless operations
• New instance needed per invocation
• Short-lived objects

Use providers.Dependency:

• External dependencies provided at runtime
• Configuration objects (Settings)
• Infrastructure connections (neo4j_driver)
• Services from outside the container

Step 2: Add Service to Container

Location: /Users/dawiddutoit/projects/play/project-watch-mcp/src/project_watch_mcp/interfaces/mcp/container.py

Add provider in the appropriate layer section:

class Container(containers.DeclarativeContainer):
    # Configuration
    settings = providers.Dependency(instance_of=Settings)

    # Infrastructure Layer - Repositories
    my_repository = providers.Singleton(
        MyRepository,
        driver=neo4j_driver,
        settings=settings,
    )

    # Application Layer - Services
    my_service = providers.Singleton(
        MyService,
        settings=settings,
        repository=my_repository,
    )

    # Application Layer - Command Handlers
    my_handler = providers.Factory(
        MyHandler,
        service=my_service,
    )

Ordering matters: Define dependencies before dependents to avoid circular references.

Step 3: Handle Special Cases

Factory Functions (for complex initialization):

def create_extractor_registry(python_ext):
    """Factory function to create and configure extractor registry."""
    registry = ExtractorRegistry()
    registry.register(python_ext)
    return registry

extractor_registry = providers.Singleton(
    create_extractor_registry,
    python_ext=python_extractor,
)

Extracting Config Values:

smart_chunking_service = providers.Singleton(
    SmartChunkingService,
    boundary_adjustment_lines=providers.Factory(
        lambda s: s.chunking.boundary_adjustment_lines,
        s=settings,
    ),
    settings=settings,
)

Avoiding Circular Dependencies:

• Order providers: dependencies before dependents
• Use lazy evaluation with providers.Factory
• Consider extracting shared dependencies to separate provider
• See references/circular-dependency-guide.md for resolution strategies

Step 4: Override for Testing

In test files (conftest.py or test methods):

@pytest.fixture
def container(real_settings, mock_driver):
    """Create container with test overrides."""
    container = Container()

    # Override dependencies
    container.settings.override(real_settings)
    container.neo4j_driver.override(mock_driver)

    return container

def test_service(container):
    """Test service from container."""
    service = container.my_service()
    assert service.settings is not None

Step 5: Initialize in Factory

For MCP server initialization:

Update /Users/dawiddutoit/projects/play/project-watch-mcp/src/project_watch_mcp/interfaces/mcp/container_factory.py:

def initialize_container_and_services(
    repository_monitor: RepositoryMonitor,
    neo4j_rag: Neo4jRAG,
    settings: Settings,
) -> Container:
    """Initialize container with runtime dependencies."""
    container = Container()

    # Override external dependencies
    container.settings.override(settings)
    container.neo4j_driver.override(neo4j_rag.neo4j_database.driver)
    container.repository_monitor.override(repository_monitor)

    # Initialize services with side effects
    _ = container.my_service()

    return container

Examples

Example 1: Add Repository (Singleton)

# In container.py - Infrastructure Layer section

file_metadata_repository = providers.Singleton(
    FileMetadataRepository,
    driver=neo4j_driver,
    settings=settings,
)

Why Singleton: Repositories are stateful, manage connections, expensive to create.

Example 2: Add Command Handler (Factory)

# In container.py - Application Layer section

process_file_handler = providers.Factory(
    ProcessFileHandler,
    repository=file_metadata_repository,
    chunking_service=chunking_service,
    settings=settings,
)

Why Factory: Handlers are invoked per request, should be stateless.

Example 3: Override for Testing

# In tests/integration/mymodule/conftest.py

@pytest.fixture
def container(real_settings):
    container = Container()
    container.settings.override(real_settings)

    # Override with mock driver
    mock_driver = AsyncMock()
    container.neo4j_driver.override(mock_driver)

    return container

def test_handler_uses_repository(container):
    handler = container.process_file_handler()
    # handler now uses mocked driver through repository

Requirements

Dependencies

uv pip install dependency-injector

Project Files

• Container definition: src/project_watch_mcp/interfaces/mcp/container.py
• Factory initialization: src/project_watch_mcp/interfaces/mcp/container_factory.py
• Test examples: tests/integration/container/test_settings_injection.py

Quality Checks

After adding to container:

# Type checking (ensures proper wiring)
uv run pyright src/project_watch_mcp/interfaces/mcp/container.py

# Run tests
uv run pytest tests/integration/container/ -v

Common Pitfalls

1. Circular Dependency Error

Symptom: Error while resolving dependencies

Solution: Reorder providers or use factory functions. See references/circular-dependency-guide.md.

2. Wrong Provider Type

Symptom: State leaking between tests or requests

Solution: Use Factory for handlers, Singleton for services/repositories.

3. Missing Override in Tests

Symptom: Tests fail with "Dependency not set"

Solution: Override all Dependency providers in test fixtures.

4. Optional Dependencies

Symptom: def __init__(self, settings: Settings | None = None)

Solution: NEVER make Settings/Config optional. Use providers.Dependency(instance_of=Settings) and override in tests.

See Also

• references/provider-reference.md - Complete dependency-injector API reference
• references/circular-dependency-guide.md - Circular dependency resolution strategies
• templates/service-template.py - Service class template with DI
• scripts/detect_circular_deps.py - Detect circular dependencies utility
• scripts/generate_provider.py - Generate provider code utility
• scripts/validate_di_patterns.py - Validate DI patterns utility