How-to: BackgroundTasks

How to run work after the response using FastAPI's BackgroundTasks.

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1. Basic pattern

from fastapi import BackgroundTasks, FastAPI
from fastapi.responses import JSONResponse

app = FastAPI()

def send_notification(message: str) -> None:
    # slow work (sending email, calling an external API, etc.)
    print(f"Sending: {message}")

@app.post("/orders", status_code=201)
def create_order(
    body: CreateOrderBody,
    background_tasks: BackgroundTasks,
) -> JSONResponse:
    order = process_order(body)
    background_tasks.add_task(send_notification, f"Order {order.order_id} created")
    return JSONResponse({"order_id": order.order_id}, status_code=201)

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2. Keeping it separate from the UseCase

To keep the UseCase HTTP-independent, don't pass BackgroundTasks into it. Receive the event in the handler layer and add it to BackgroundTasks there.

# ✅ the UseCase doesn't know about BackgroundTasks
class CreateOrderUseCase:
    def execute(self, body: CreateOrderInput) -> CreateOrderOutput:
        order = Order(...)
        return CreateOrderOutput(order_id=order.order_id, notify_email=body.email)

# use BackgroundTasks in the handler layer
@app.post("/orders", status_code=201)
def create_order(
    body: CreateOrderBody,
    background_tasks: BackgroundTasks,
    use_case: CreateOrderUseCase = Depends(get_use_case),
) -> JSONResponse:
    result = use_case.execute(CreateOrderInput(email=body.email))
    background_tasks.add_task(send_notification, result.notify_email)
    return JSONResponse({"order_id": result.order_id}, status_code=201)

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3. Behavior under TestClient

Under TestClient, BackgroundTasks runs synchronously before the response is returned.

executed: list[str] = []

def track_task(msg: str) -> None:
    executed.append(msg)

# in tests, BackgroundTasks runs synchronously
r = client.post("/orders", json={"email": "alice@example.com"})
assert r.status_code == 201
assert len(executed) == 1  # already executed

In production it runs asynchronously (after the response); note it runs synchronously in tests.

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4. A failure does not cause a 500

If an exception is raised inside a BackgroundTasks task, the response has already been sent, so it does not become a 500. The error is logged.

def risky_task() -> None:
    raise RuntimeError("Background task failed")

# the response returns 201 (the background error is hidden)
background_tasks.add_task(risky_task)

For important work, don't rely on BackgroundTasks — use a job queue (Celery, ARQ, etc.).

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5. Combining with async def

background_tasks.add_task() accepts both sync and async functions.

async def async_notification(email: str) -> None:
    await send_email_async(email)

background_tasks.add_task(async_notification, user.email)