How to use yield dependencies for resource management in FastAPI

Yield Dependencies for Resource Management

Dependencies that use yield instead of return run setup code before the route handler, inject the yielded value, then run teardown code after the response is sent — even if an exception occurred.

Database session pattern

from fastapi import Depends
from sqlalchemy.orm import Session
from .database import SessionLocal

def get_db():
    db = SessionLocal()   # setup
    try:
        yield db          # injected into route
    finally:
        db.close()        # teardown — always runs

@app.get("/users/")
def list_users(db: Session = Depends(get_db)):
    return db.query(User).all()

The finally block guarantees the session closes regardless of whether the route raises an exception. Without yield, you would need to manage this manually in every route.

Multiple yield dependencies

A route can declare multiple yield dependencies, and each teardown runs in reverse order after the response. This mirrors how nested with statements work in Python.

def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

def get_cache():
    client = RedisClient()
    try:
        yield client
    finally:
        client.disconnect()

@app.get("/data")
def get_data(db=Depends(get_db), cache=Depends(get_cache)):
    return {"ok": True}

FastAPI handles the lifecycle correctly. You never need to call teardown code inside route handlers.