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Dependency injection

Nexy resolves constructor dependencies for controllers and services. A class opts in with @Injectable().

Inject a service

from nexy.decorators import Injectable


@Injectable()
class UserService:
    def get_all(self) -> list[dict]:
        return [{"id": 1, "name": "Ada"}]

from nexy.decorators import Controller
from .user_service import UserService


@Controller(prefix="/users")
class UserController:
    def __init__(self, svc: UserService) -> None:
        self.svc = svc

    def get(self) -> list[dict]:
        return self.svc.get_all()

Register the controller in a module. Listing the service in providers additionally resolves it eagerly at startup:

from nexy.decorators import Module
from .user_controller import UserController
from .user_service import UserService


@Module()
class UsersModule:
    controllers = [UserController]
    providers = [UserService]

How resolution works

• Each __init__ parameter is resolved from its type annotation. The annotation must be an @Injectable() class, or the parameter must have a default value; otherwise resolution raises ValueError.
• Controllers are instantiated with Container.resolve() at startup.
• A dependency cycle raises RecursionError naming the class.
• Services can inject other services, to any depth.

Scopes

from nexy.decorators import Injectable, Scope


@Injectable(scope=Scope.REQUEST)
class CurrentUser: ...

The container

from nexy.decorators import Container

service = Container.resolve(UserService)

Constructing an @Injectable() class yourself raises TypeError: the container owns its lifecycle. Use Container.resolve(...) or receive the class through another constructor.

Container.clear_request_scope() drops the current request's instances.

Route parameters

FastAPI Depends works as usual for handler parameters, next to constructor injection:

from fastapi import Depends


def get(self, limit: int = Depends(pagination)) -> list[dict]: ...

Limits

The container is process-wide. Providers are not isolated per module, exports only decides which providers an importing module resolves eagerly, and there is no public API to swap a provider in tests. See Nexy and NestJS for the full comparison.

Next steps

• Guards and metadata
• Testing

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