How can I make use of starlette background feature inside FastApi?

[tiangolo]

How can I make use of starlette background feature inside FastApi?

[euri10]

afaik you have to explicitly use a JSONResponse, but @tiangolo might have a way better view on this.

I was unable to find a way to pass the background kwargs to a FastAPI Response without it.

here's a working example shamelessly adapted from the starlette test cases

def test_multiple_tasks2():

    TASK_COUNTER = 0
    def increment(amount):
        nonlocal TASK_COUNTER
        TASK_COUNTER += amount
    app = FastAPI()

    @app.get("/")
    async def root():
        tasks = BackgroundTasks()
        tasks.add_task(increment, amount=1)
        tasks.add_task(increment, amount=2)
        tasks.add_task(increment, amount=3)
        message = {'status': 'successful'}
        return JSONResponse(message, background=tasks)

    client = TestClient(app)
    response = client.get("/")
    assert response.json() == {'status': 'successful'}
    assert TASK_COUNTER == 1 + 2 + 3

[wshayes]

Yes - this works, and to reinforce the warning above - if you don't return JSONResponse with the background=tasks parameter - it won't initiate the background tasks.

[tiangolo]

Thanks for your help here guys @euri10 and @wshayes !

It is now integrated into FastAPI in a more simple way, similarly to when using the Request object directly, just declare a function parameter of type BackgroundTasks and add tasks to it directly. The rest is handled by FastAPI. And now you don't have to return a JSONResponse directly.

Here are the new docs: https://fastapi.tiangolo.com/tutorial/background-tasks/

In short:

from fastapi import BackgroundTasks, FastAPI

app = FastAPI()


def write_notification(email: str, message=""):
    with open("log.txt", mode="w") as email_file:
        content = f"notification for {email}: {message}"
        email_file.write(content)


@app.post("/send-notification/{email}")
async def send_notification(email: str, background_tasks: BackgroundTasks):
    background_tasks.add_task(write_notification, email, message="some notification")
    return {"message": "Notification sent in the background"}

[tiangolo]

As this is now solved, I'll close this issue. But feel free to add more comments or create new issues.

[github-actions]

Assuming the original need was handled, this will be automatically closed now. But feel free to add more comments or create new issues or PRs.