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JP-Ellis · 2025-10-15T23:51:03Z

This fixes a regression introduced in 0.118 whereby background tasks added after a yield within a dependency don't get executed.

Resolves: #14137

Add test to verify that background tasks added _after_ a `yield` statement are executed correctly. This is a regression introduced in 0.118. Ref: fastapi#14137 Signed-off-by: JP-Ellis <[email protected]>

This fixes a regression introduced in 0.118 whereby background tasks added _after_ a `yield` within a dependency don't get executed. Ref: fastapi#14137 Signed-off-by: JP-Ellis <[email protected]>

ezeparziale · 2025-10-27T14:42:13Z

Any update on this PR?

…sk-after-yield-regression

YuriiMotov · 2025-11-07T21:06:22Z

I pushed some updates:

merged master into PR's branch
updated code to execute all bg tasks after sending response and exiting async stack
updated tests - I think it's enough to test only async endpoint + async dependency. Also, I added asserts to ensure bg tasks executed only once and dependency is finalized at the moment of bg tasks execution

If you disagree with any of those changes - feel free to revert, I will then open alternative PR and we will ask Sebastian to decide.

The main question now is: do we want to allow users to use results of dependencies with yield in background tasks?
According to the docs, results of dependencies with yield are not supposed to be used in background tasks. If it's not a mistake in docs, then we are on the right way

…ield`

JP-Ellis · 2025-11-08T00:58:24Z

Thanks for the update, looks good to me.

To answer you question:

Do we want to allow users to use results of dependencies with yield in background tasks?

No, I don't think so. Starlette does not offer any way to access any results from background tasks, and I don't think FastAPI should deviate from that.

Even if you did somehow allow for the results to be accessed, where would that happen? It is executed after the async stack, so it is not available to any of the dependencies, nor the main handler for the endpoint.

tiangolo · 2025-11-20T18:01:03Z

Thanks for the interest and work here! ☕

I was checking #14137 and replied there. I think your use case can be solved with the new Dependency scopes: https://fastapi.tiangolo.com/tutorial/dependencies/dependencies-with-yield/#early-exit-and-scope

I also need to review and improve all the docs around dependencies. 🤓

On the other hand, I think that there's a chance to improve further the idea of background tasks in a way that could go beyond what Starlette can offer by default, but that's something to explore in the future.

I think this specific use case would be solved by the new dependencies with scopes, so I'll pass on this PR, but thanks for the work and conversation everyone here! 🍰

JP-Ellis · 2025-11-20T20:13:25Z

I think this specific use case would be solved by the new dependencies with scopes, so I'll pass on this PR, but thanks for the work and conversation everyone here!

Unfortunately, in my particular use case, it is not. I have provided a more complete response to this in the linked discussion.