How to add background tasks when request fails and HTTPException is raised in FastAPI?

by

The way to do this is to override the HTTPException error handler, and since there is no BackgroundTasks object in the exception_handler, you can add a background task to a response in the way that is described in Starlette documentation (FastAPI is actually Starlette underneath).

Example

from fastapi import BackgroundTasks, FastAPI, HTTPException, Request
from fastapi.responses import PlainTextResponse
from starlette.exceptions import HTTPException as StarletteHTTPException
from starlette.background import BackgroundTask

app = FastAPI()

def write_notification(message):
    with open('log.txt', 'a') as f:
        f.write(f'{message}'+'\n')

@app.exception_handler(StarletteHTTPException)
async def http_exception_handler(request, exc):
    task = BackgroundTask(write_notification, message=exc.detail)
    return PlainTextResponse(str(exc.detail), status_code=exc.status_code, background=task)
 
@app.get("/{msg}")
def send_notification(msg: str, background_tasks: BackgroundTasks):
    if "hello" in msg:
        raise HTTPException(status_code=500, detail="Something went wrong")

    background_tasks.add_task(write_notification, message="Success")
    return {"message": "Request has been successfully submitted."}

If you need to add multiple background tasks to a response, then use:

from fastapi import BackgroundTasks

@app.exception_handler(StarletteHTTPException)
async def http_exception_handler(request, exc):
    tasks = BackgroundTasks()
    tasks.add_task(write_notification, message=exc.detail)
    tasks.add_task(some_other_function, message="some other message")
    return PlainTextResponse(str(exc.detail), status_code=exc.status_code, background=tasks)

A variation of the above approach is the following (suggested here):

from starlette.background import BackgroundTask

@app.exception_handler(StarletteHTTPException)
async def http_exception_handler(request, exc):
    response = PlainTextResponse(str(exc.detail), status_code=exc.status_code)
    response.background = BackgroundTask(write_notification, message=exc.detail)
    # to add multiple background tasks use:
    # response.background = tasks  # create `tasks` as shown in the code above
    return response

Here are some references that you might also helpful: (1) this answer demonstrates how to add custom exception handlers and (2) this answer shows a custom logging system for the incoming requests and outgoing responses.

More Related Contents:

  1. How to share variables between HTTP requests in FastAPI?
  2. How to Upload a large File (≥3GB) to FastAPI backend?
  3. How to log raw HTTP request/response in Python FastAPI?
  4. How to load a different file than index.html in FastAPI root path?
  5. How do I get my FastAPI application’s console log in JSON format with a different structure and different fields?
  6. How to create a FastAPI endpoint that can accept either Form or JSON body?
  7. How to download a large file using FastAPI?
  8. How to navigate through FastAPI routes by clicking on HTML button in Jinja2 Templates?
  9. How to return a custom 404 Not Found page using FastAPI?
  10. How to pass URL as a path parameter to a FastAPI route?
  11. How to initialise a global object or variable and reuse it in every FastAPI endpoint?
  12. fastapi (starlette) RedirectResponse redirect to post instead get method
  13. Using FastAPI in a sync way, how can I get the raw body of a POST request?
  14. How do I disable log messages from the Requests library?
  15. How to add both file and JSON body in a FastAPI POST request?
  16. How can I color Python logging output?
  17. How to write to a file, using the logging Python module?
  18. Log output of multiprocessing.Process
  19. How do I log a Python error with debug information?
  20. How to list all existing loggers using python.logging module
  21. How to Upload File using FastAPI?
  22. How to upload File in FastAPI, then to Amazon S3 and finally process it?
  23. What is the proper way to make downstream Https requests inside of Uvicorn/FastAPI?
  24. How to display uploaded image in HTML page using FastAPI & Jinja2?
  25. How to post JSON data to FastAPI and retrieve the JSON data inside the endpoint?
  26. python logging to database
  27. How do I send list of dictionary as Body parameter in FastAPI?
  28. How to make a large file accessible to external APIs?
  29. How do I send list of dictionary as Body parameter together with Files in FastAPI?
  30. Redirect to login page if user not logged in using FastAPI-Login package

Leave a Comment