Breaking Changes
Using resources from dependencies with `yield` in background tasks is no longer supported.
This change is what supports the new features, read below. 🤓
Dependencies with `yield`, `HTTPException` and Background Tasks
Dependencies with `yield` now can raise `HTTPException` and other exceptions after `yield`. 🎉
Read the new docs here: [Dependencies with `yield` and `HTTPException`](https://fastapi.tiangolo.com/tutorial/dependencies/dependencies-with-yield/#dependencies-with-yield-and-httpexception).
Python
from fastapi import Depends, FastAPI, HTTPException
from typing_extensions import Annotated
app = FastAPI()
data = {
"plumbus": {"description": "Freshly pickled plumbus", "owner": "Morty"},
"portal-gun": {"description": "Gun to create portals", "owner": "Rick"},
}
class OwnerError(Exception):
pass
def get_username():
try:
yield "Rick"
except OwnerError as e:
raise HTTPException(status_code=400, detail=f"Onwer error: {e}")
app.get("/items/{item_id}")
def get_item(item_id: str, username: Annotated[str, Depends(get_username)]):
if item_id not in data:
raise HTTPException(status_code=404, detail="Item not found")
item = data[item_id]
if item["owner"] != username:
raise OwnerError(username)
return item
---
Before FastAPI 0.106.0, raising exceptions after `yield` was not possible, the exit code in dependencies with `yield` was executed *after* the response was sent, so [Exception Handlers](https://fastapi.tiangolo.com/tutorial/handling-errors/#install-custom-exception-handlers) would have already run.
This was designed this way mainly to allow using the same objects "yielded" by dependencies inside of background tasks, because the exit code would be executed after the background tasks were finished.
Nevertheless, as this would mean waiting for the response to travel through the network while unnecessarily holding a resource in a dependency with yield (for example a database connection), this was changed in FastAPI 0.106.0.
Additionally, a background task is normally an independent set of logic that should be handled separately, with its own resources (e.g. its own database connection).
If you used to rely on this behavior, now you should create the resources for background tasks inside the background task itself, and use internally only data that doesn't depend on the resources of dependencies with `yield`.
For example, instead of using the same database session, you would create a new database session inside of the background task, and you would obtain the objects from the database using this new session. And then instead of passing the object from the database as a parameter to the background task function, you would pass the ID of that object and then obtain the object again inside the background task function.
The sequence of execution before FastAPI 0.106.0 was like the diagram in the [Release Notes for FastAPI 0.106.0](https://fastapi.tiangolo.com/release-notes/#01060).
The new execution flow can be found in the docs: [Execution of dependencies with `yield`](https://fastapi.tiangolo.com/tutorial/dependencies/dependencies-with-yield/#execution-of-dependencies-with-yield).