Yield dependency doesn't allow to add a background task in its closing code

[and-semakin]

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Example

Here's a self-contained, minimal, reproducible, example with my use case:

from fastapi import FastAPI, Request, Depends, BackgroundTasks

app = FastAPI()


async def dep(background: BackgroundTasks):
    background.add_task(print, "before yield")
    yield "something"
    background.add_task(print, "after yield")

@app.get("/")
async def read_root(d: str = Depends(dep)):
    return {"Hello": "World", "dependency": d}

Description

• Request the endpoint;
• The first background task (that is before yield) is added and executed;
• The second background task is silently ignored, there is no error and no warning.

Environment

• OS: [e.g. Linux / Windows / macOS]: macOS
• FastAPI Version [e.g. 0.3.0]: 0.62.0
• Python version: 3.8

Additional context

I know that a task added after a response being sent has no chance to be executed. I guess there should be a warning or error if a user tries to do that, something like: You can't add new background tasks after the yield keyword. Also, probably, this behavior should be documented.

[falkben]

Agreed that it should at least be mentioned in the documentation. Would you be up for adding a note at the end of this section about this behavior and submitting a PR? https://fastapi.tiangolo.com/tutorial/background-tasks/#dependency-injection

[and-semakin]

Thanks for the answer @falkben! Sure, it needs to be documented. I'll send a PR soon.

Another solution could be extending Starlette's BackgroundTask and BackgroundTasks classes with a boolean that indicates if the tasks were executed or not. This way these classes will become one-time single-use objects. It won't be possible to add a new task to an already executed task collection or execute a task one more time. Need to discuss it with Starlette first.

[tiangolo]

Yep, dependencies with yield are closed after background tasks are done. This was to be able to use the same DB session.

But I see that in some cases it would make sense to close them before the response is sent. That's something I've been considering and thinking about how to properly implement.

All that is documented a bit here, wth a pretty diagram and everything: https://fastapi.tiangolo.com/tutorial/dependencies/dependencies-with-yield/#dependencies-with-yield-and-httpexception

Sorry for the long delay! 🙈 I wanted to personally address each issue/PR and they piled up through time, but now I'm checking each one in order.

[github-actions[bot]]

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.