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Example Code

from fastapi import BackgroundTasks, FastAPI

app = FastAPI()

def notify(email: str, message=""):
    with open("log.txt", mode="a") as email_file:
        content = f"notification for {email}: {message}\n"
        email_file.write(content)
    if "some" in message:
        raise RuntimeError("Sample exception")

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

Description

Starlette documentation mentions an important point about how the background tasks are processed that would be really useful for users to know directly from FastAPI docs instead of going through all the Starlette docs to see this warning.

Important
The tasks are executed in order. In case one of the tasks raises an exception, the following tasks will not get the opportunity to be executed.

Tasks are executed in (FIFO) order and any exception thrown by a single task causes all the following tasks to fail as well. For example the sample code will only write a single line of code, and yes I updated the write mode to append.

Operating System

Windows

Operating System Details

No response

FastAPI Version

0.128.0

Pydantic Version

2.12.5

Python Version

Python 3.12.11

Additional Context

No response