Injection¶

This guide covers the different ways to inject dependencies using dijay.

Automatic Constructor Injection¶

The container inspects __init__ type hints and resolves dependencies automatically:

from dijay import injectable

@injectable()
class UserService:
    def __init__(self, repo: UserRepository, email: EmailService):
        self.repo = repo
        self.email = email

When UserService is resolved, the container will:

1. Look up UserRepository and EmailService in the registry.
2. Resolve them (recursively resolving their dependencies).
3. Pass them to UserService.__init__.

Token Injection with Inject¶

When the type hint alone isn't enough (e.g., injecting a str value), use Annotated with Inject:

from typing import Annotated
from dijay import Inject, injectable

@injectable()
class DatabaseConnection:
    def __init__(
        self,
        url: Annotated[str, Inject("DB_URL")],
        pool_size: Annotated[int, Inject("DB_POOL_SIZE")],
    ):
        self.url = url
        self.pool_size = pool_size

The Inject("DB_URL") marker tells the container to resolve the "DB_URL" token instead of trying to resolve str.

Optional Dependencies¶

Dependencies with Optional (or X | None) type hints are treated as optional. If the token is not registered, None is injected instead of raising an error:

@injectable()
class NotificationService:
    def __init__(self, sms: SmsGateway | None = None):
        self.sms = sms

Request-Scoped Injection¶

When using REQUEST scope, pass an id parameter to resolve to group dependencies by request:

instance = await container.resolve(RequestContext, id="request-123")

All REQUEST-scoped dependencies resolved with the same id will share instances within that request.

Circular Dependency Detection¶

dijay monitors the resolution stack. If a cycle is detected (e.g., A → B → A), a RuntimeError is raised immediately:

@injectable()
class A:
    def __init__(self, b: B): ...

@injectable()
class B:
    def __init__(self, a: A): ...

# RuntimeError: Circular dependency: <class 'A'>