Dependencies with yield: Sessions and Resources

Dependencies that yield run setup code before the request and teardown after it — ideal for database sessions, transactions and locks. Wrap the yield in try/except/finally to roll back on errors. Treat these resources as request-scoped: the exact moment teardown runs relative to sending the response has changed between FastAPI versions, so never rely on a request's session inside background tasks — open a new session in the task instead.

from collections.abc import AsyncIterator
from sqlalchemy.ext.asyncio import AsyncSession


async def get_db() -> AsyncIterator[AsyncSession]:
    async with SessionLocal() as session:
        try:
            yield session
            await session.commit()
        except Exception:
            await session.rollback()
            raise


DB = Annotated[AsyncSession, Depends(get_db)]

Common pitfalls

• Swallowing exceptions in the teardown (always re-raise).
• Using the request's session inside BackgroundTasks after the response.
• Committing in every repository method instead of once per unit of work.

Key points

• yield = setup + teardown per request
• Rollback on errors and re-raise
• Don't reuse request resources in background tasks

Further reading: FastAPI: Dependencies with yield

Go deeper: FastAPI and Starlette: Middleware, Exceptions and Dependencies in the Request Lifecycle course traces this step of the request, with a production bug and a runnable simulation.