aspy-dependency-injection 0.4.1

Dependency injection

Features

• Use it everywhere: Use dependency injection in web servers, background tasks, console applications, Jupyter notebooks, tests, etc.
• Lifetimes: Singleton (same instance per application), Scoped (same instance per HTTP request scope) and Transient (different instance per resolution).
• FastAPI integration out of the box, and pluggable to any web framework.
• Automatic resolution and disposal: Automatically resolve constructor parameters and manage async and non-async context managers. It's not longer your concern to know how to create or dispose services.
• Clear design inspired by one of the most used and battle-tested DI libraries, adding async-native support, important features and good defaults.
• Centralized configuration: Register all services in one place using a clean syntax, and without decorators.
• ty and Pyright strict compliant.

Installation

uv add aspy-dependency-injection

Quickstart with FastAPI

Inject services into async endpoints using Annotated[..., Inject()].

class EmailService:
    pass


class UserService:
    def __init__(self, email_service: EmailService) -> None:
        self.email_service = email_service
    
    async def create_user(self) -> None:
        pass


app = FastAPI()

@app.post("/users")
async def create_user(user_service: Annotated[UserService, Inject()]) -> None:
    await user_service.create_user()

services = ServiceCollection()
services.add_transient(EmailService)
services.add_transient(UserService)
services.configure_fastapi(app)

Quickstart without FastAPI

You convert the service collection into a service provider:

class EmailService:
    pass


class UserService:
    def __init__(self, email_service: EmailService) -> None:
        self.email_service = email_service
    
    async def create_user(self) -> None:
        pass

    
services = ServiceCollection()
services.add_transient(EmailService)
services.add_transient(UserService)

async def main() -> None:
    async with services.build_service_provider() as service_provider:
        user_service = await service_provider.get_required_service(UserService)
        await user_service.create_user()


if __name__ == "__main__":
    asyncio.run(main())

If you want a scope per operation (e.g., per HTTP request or message from a queue), you can create a scope from the service provider:

async with service_provider.create_scope() as service_scope:
    user_service = await service_scope.get_required_service(UserService)
    await user_service.create_user()

Lifetimes

• Transient: A new instance is created every time the service is requested. Examples: Services without state, workflows, repositories, service clients...
• Singleton: The same instance is used every time the service is requested. Examples: Settings (pydantic-settings), machine learning models, database connection pools, caches.
• Scoped: A new instance is created for each new scope, but the same instance is returned within the same scope. Examples: Database clients, unit of work.

Factories

Sometimes, you need to use a factory function to create a service. For example, you have settings (a connection string, database name, etc.) stored using the package pydantic-settings and you want to provide them to a service DatabaseClient to access a database.

class ApplicationSettings(BaseSettings):
    database_connection_string: str


class DatabaseClient:
    def __init__(self, connection_string: str) -> None:
        pass

In a real DatabaseClient implementation, you must use a sync or async context manager, i.e., you instance it with:

async with DatabaseClient(database_connection_string) as client:
    ...

And, if you want to re-use it, you create a factory function with yield:

async def create_database_client(application_settings: ApplicationSettings) -> AsyncGenerator[DatabaseClient]:
    async with DatabaseClient(application_settings.database_connection_string) as database_client:
        yield database_client

With that factory, you have to provide manually a singleton of ApplicationSettings, and to know if DatabaseClient implements a sync or async context manager, or neither. Apart from that, if you need a singleton or scoped instance of DatabaseClient, it's very complex to manage the disposal of the instance.

Then, why don't just return it? With this package, you just have this:

def inject_database_client(application_settings: ApplicationSettings) -> DatabaseClient:
    return DatabaseClient(
        connection_string=application_settings.database_connection_string
    )

services.add_transient(inject_database_client)

The factories can take as parameters other services registered. In this case, inject_database_client takes ApplicationSettings as a parameter, and the dependency injection mechanism will resolve it automatically.

Testing

TBD

Registration with a base class

You can register a service by specifying both the service type (base class / interface) and the implementation type (concrete class). This is useful when you want to inject services using abstractions.

class NotificationService(ABC):
    async def send_notification(self, recipient: str, message: str) -> None:
        ...


class EmailService(NotificationService):
    @override
    async def send_notification(self, recipient: str, message: str) -> None:
        pass


class UserService:
    def __init__(self, notification_service: NotificationService) -> None:
        self.notification_service = notification_service

    async def create_user(self, email: str) -> None:
        await self.notification_service.send_notification(email, "Welcome to our service!")


services.add_transient(NotificationService, EmailService)