Chapter 8: Dependency Injection and Dependencies

ASP.NET Core has a full DI container with singleton, scoped, and transient lifetimes. FastAPI has dependency functions. They are less formal, but request-scoped composition is excellent.

from collections.abc import Generator
from typing import Annotated

from fastapi import Depends
from sqlalchemy.orm import Session


def get_db() -> Generator[Session, None, None]:
    with Session(engine) as session:
        yield session


DbSession = Annotated[Session, Depends(get_db)]


@router.get("/{recipe_id}")
def get_recipe(recipe_id: int, db: DbSession):
    return recipes_service.get_recipe(db, recipe_id)

Dependency functions can depend on other dependencies. This gives a graph similar to DI, but it is resolved by FastAPI for each request.

Use dependencies for:

• Database sessions.
• Current user.
• Authorization checks.
• Settings access.
• Parsed request context.
• Feature flags.
• Unit-of-work or service objects.

Do not hide all construction behind dependencies. Plain Python constructors are good. Dependency injection is a tool, not the whole architecture.