Dependency Injection

Dependency Injection in myfy

myfy uses constructor-based dependency injection with three scopes.

Scopes

Provider Declaration

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from myfy.core import provider, SINGLETON, REQUEST, TASK

# SINGLETON: Created once, shared across all requests
@provider(scope=SINGLETON)
def database_pool(settings: DatabaseSettings) -> DatabasePool:
    return DatabasePool(settings.database_url)

# REQUEST: Created per HTTP request, auto-cleaned up
@provider(scope=REQUEST)
def db_session(pool: DatabasePool) -> AsyncSession:
    return pool.get_session()

# TASK: Created per background task execution
@provider(scope=TASK)
def task_logger(ctx: TaskContext) -> Logger:
    return Logger(task_id=ctx.task_id)

Provider Options

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@provider(
    scope=SINGLETON,              # Lifecycle scope
    qualifier="primary",          # Optional qualifier for multiple providers
    name="my_database",           # Optional name for resolution
    reloadable=("log_level",),    # Settings that can hot-reload
)
def database(settings: Settings) -> Database:
    return Database(settings.db_url)

Scope Dependency Rules

1. SINGLETON can depend on: other singletons only
2. REQUEST can depend on: singletons and other request-scoped
3. TASK can depend on: singletons and other task-scoped

SINGLETON cannot depend on REQUEST/TASK - this fails at compile time!

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# WRONG - will fail at startup
@provider(scope=SINGLETON)
def bad_service(session: AsyncSession):  # AsyncSession is REQUEST scope
    return MyService(session)

# CORRECT - use factory pattern
@provider(scope=SINGLETON)
def service_factory(pool: DatabasePool) -> ServiceFactory:
    return ServiceFactory(pool)

@provider(scope=REQUEST)
def service(factory: ServiceFactory, session: AsyncSession) -> MyService:
    return factory.create(session)

Injection in Routes

Parameters are auto-classified in order:

1. Path parameters - from URL template like {user_id}
2. Query parameters - annotated with Query(...) or primitives with defaults
3. Body parameter - Pydantic model or dataclass
4. DI dependencies - everything else (resolved from container)

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from myfy.web import route, Query
from myfy.data import AsyncSession

@route.post("/users/{user_id}/orders")
async def create_order(
    user_id: int,                    # Path param
    limit: int = Query(default=10),  # Query param
    body: OrderCreate,               # Request body (Pydantic model)
    session: AsyncSession,           # DI (REQUEST scope)
    settings: AppSettings,           # DI (SINGLETON)
) -> dict:
    ...

Qualifiers for Multiple Providers

When you have multiple providers of the same type:

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from myfy.core import provider, SINGLETON, Qualifier
from typing import Annotated

@provider(scope=SINGLETON, qualifier="primary")
def primary_db(settings: Settings) -> Database:
    return Database(settings.primary_url)

@provider(scope=SINGLETON, qualifier="replica")
def replica_db(settings: Settings) -> Database:
    return Database(settings.replica_url)

# Inject by qualifier
@route.get("/users")
async def list_users(
    db: Annotated[Database, Qualifier("replica")]
) -> list[dict]:
    return await db.fetch_all()

Common Patterns

Factory Pattern (REQUEST from SINGLETON)

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@provider(scope=SINGLETON)
def email_client(settings: EmailSettings) -> EmailClient:
    return EmailClient(settings.api_key)

@provider(scope=REQUEST)
def email_sender(client: EmailClient, user: User) -> EmailSender:
    return EmailSender(client, from_user=user)

Optional Dependencies

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from typing import Optional

@provider(scope=SINGLETON)
def cache_service(redis: Optional[RedisClient] = None) -> CacheService:
    if redis:
        return RedisCacheService(redis)
    return InMemoryCacheService()

Testing

Override providers in tests using the container's override context:

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from myfy.core import override

async def test_with_mock_db():
    mock_db = MockDatabase()

    with override(Database, mock_db):
        # Inside this block, Database resolves to mock_db
        result = await my_service.do_something()

    assert result == expected

Best Practices

1. Keep providers pure - No side effects in factory functions
2. Use SINGLETON for shared resources - Database pools, HTTP clients, caches
3. Use REQUEST for request-specific state - DB sessions, user context
4. Use TASK for background job state - Task logger, job-specific clients
5. Validate at compile time - All scope violations caught at startup
6. Return type required - Provider functions must have return type annotation