asgi.py

def build_app() -> FastAPI:
    app = FastAPI()
  
    nodepends_provide_dao = lambda: provide_dao(provide_session_factory)
    nodepends_provide_service = lambda: provide_service(dao=nodepends_provide_dao())
  
    app.dependency_overrides[provide_service_stub] = nodepends_provide_service
  
    return app
  

dao.py

class SomeDAO:
    def __init__(self, session_factory: Callable[[], AsyncSession]):
        self.session_factory = session_factory
        
    async def some_method(self) -> Any:
        async with self.session_factory() as session:
            return await session.execute(...)  # Just example

providers.py

def provide_session_factory() -> Callable[[], AsyncSession]:
    engine = create_async_engine(
        "postgresql+asyncpg://scott:tiger@localhost/test",
        echo=True,
    )
    return sessionmaker(
        engine, expire_on_commit=False, class_=AsyncSession
    )


def provide_dao(session_factory: Callable[[], AsyncSession]) -> SomeDao:
    return SomeDao(session_factory=session_factory)
        

def provide_service_stub():
    ...
  
  
def provide_service(dao: SomeDao) -> Service:
    return Service(dao=dao)

service.py

class Service:
    def __init__(self, dao: SomeDAO):
        self.dao = dao
        
    async def some_method(self) -> Any:
        return await self.dao.some_method()  # Just example