Potential footgun when using custom Response(background=<Task()>) in conjunction with injected BackgroundTasks - the custom response overwrites the tasks

[netanel-haber]

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Example Code

from fastapi import FastAPI, BackgroundTasks
from starlette.responses import Response, BackgroundTask
import uvicorn

app = FastAPI()

@app.get("/")
async def endpoint(tasks: BackgroundTasks):
    tasks.add_task(lambda: print("This won't be printed"))
    return Response(content="Custom response", background=BackgroundTask(lambda: print("Only this will be printed")))

uvicorn.run(app)

Description

Hey! Thanks for the best framework in Python and generally ❤️

Basically, passing a background task to a custom response overwrites any other tasks added to the injected BackgroundTasks.

I'm not sure this is considered a bug, but I think it's confusing and can at least be clarified in the Return a Response Directly docs, for example. Other solutions can be actually changing this behavior (I guess that's a breaking change), or printing a warning/throwing [throwing is also a breaking change] if both mechanisms are used at once. But I can't imagine a user desiring this behavior. To be clear, this tripped us up when we couldn't figure out why some of our tasks were being swallowed.

I'd be happy to work on either solution, obviously adding a docs pr would be the easiest first step.

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.97.0

Pydantic Version

1.10.7

Python Version

3.9

Additional Context

No response

[Kludex]

• I've created Potential footgun when using custom Response(background=<Task()>) in conjunction with injected BackgroundTasks - the custom response overwrites the tasks #11215 from this discussion.

Thanks 👍