Background tasks in FastAPI

Introduction

Background tasks let your app do work after sending a response. This keeps the app fast and smooth for users.

Send a confirmation email after a user signs up.

Clean up temporary files after a request finishes.

Log user activity without slowing down the response.

Update a database record after responding to a client.

Syntax

FastAPI

from fastapi import BackgroundTasks
from fastapi import FastAPI

app = FastAPI()

async def some_task(param: str):
    # task code here
    pass

@app.post("/endpoint")
async def endpoint(background_tasks: BackgroundTasks):
    background_tasks.add_task(some_task, "value")
    return {"message": "Task started"}

Use BackgroundTasks parameter in your path operation function.

Add tasks with background_tasks.add_task(function, args).

Examples

This example writes a log message after responding to the client.

FastAPI

from fastapi import FastAPI, BackgroundTasks

app = FastAPI()

def write_log(message: str):
    with open("log.txt", "a") as f:
        f.write(message + "\n")

@app.post("/send")
async def send(background_tasks: BackgroundTasks):
    background_tasks.add_task(write_log, "User sent data")
    return {"status": "Log task added"}

This example sends an email in the background after signup.

FastAPI

from fastapi import FastAPI, BackgroundTasks

app = FastAPI()

async def send_email(email: str):
    # pretend to send email
    print(f"Email sent to {email}")

@app.post("/signup")
async def signup(background_tasks: BackgroundTasks):
    background_tasks.add_task(send_email, "[e-mail]")
    return {"message": "Signup complete"}

Sample Program

This FastAPI app has a POST endpoint '/process'. When called, it immediately returns a message. Meanwhile, it writes "Process started" to a file named 'log.txt' in the background.

FastAPI

from fastapi import FastAPI, BackgroundTasks

app = FastAPI()

def write_log(message: str):
    with open("log.txt", "a") as f:
        f.write(message + "\n")

@app.post("/process")
async def process(background_tasks: BackgroundTasks):
    background_tasks.add_task(write_log, "Process started")
    return {"message": "Processing started"}

Important Notes

Background tasks run after the response is sent, so they don't delay the user.

They are good for simple tasks, but not for heavy or long-running jobs.

For complex jobs, consider using a task queue like Celery.

Summary

Background tasks let you run code after sending a response.

Use BackgroundTasks and add_task to add tasks.

Great for sending emails, logging, or cleanup without slowing users.

Practice

1. What is the main purpose of using BackgroundTasks in FastAPI?

easy

A. To handle database connections synchronously

B. To replace the main event loop in FastAPI

C. To block the request until a task finishes

D. To run code after sending the response without delaying the user

Solution

1. Step 1: Understand what BackgroundTasks do

   BackgroundTasks allow running code after the response is sent, so the user is not delayed.

2. Step 2: Compare options with this behavior

   Only To run code after sending the response without delaying the user describes running code after response without delay; others describe blocking or unrelated tasks.

3. Final Answer:

   To run code after sending the response without delaying the user -> Option D

4. Quick Check:

   BackgroundTasks run code after response = B [OK]

Hint: BackgroundTasks run after response to avoid user wait [OK]

Common Mistakes:

• Thinking BackgroundTasks block the response
• Confusing BackgroundTasks with synchronous code
• Assuming BackgroundTasks replace main event loop

2. Which of the following is the correct way to add a background task in a FastAPI endpoint?

easy

A. Import BackgroundTasks but do not use it

B. Call your_function() directly inside the endpoint

C. Use background_tasks.add_task(your_function, arg1, arg2) inside the endpoint

D. Use await your_function() inside the endpoint

Solution

1. Step 1: Recall how to add background tasks in FastAPI

   You add tasks by calling background_tasks.add_task(function, args) inside the endpoint function.

2. Step 2: Check each option

   Use background_tasks.add_task(your_function, arg1, arg2) inside the endpoint correctly uses add_task. Call your_function() directly inside the endpoint calls function immediately, blocking. Use await your_function() inside the endpoint awaits function, blocking. Import BackgroundTasks but do not use it does nothing.

3. Final Answer:

   Use background_tasks.add_task(your_function, arg1, arg2) inside the endpoint -> Option C

4. Quick Check:

   Use add_task to schedule background work = D [OK]

Hint: Use add_task method on BackgroundTasks instance [OK]

Common Mistakes:

• Calling the function directly instead of scheduling
• Awaiting the function which blocks response
• Importing BackgroundTasks but not using it

3. Given this FastAPI code snippet, what will be printed after the response is sent?

from fastapi import FastAPI, BackgroundTasks

app = FastAPI()

def write_log(message: str):
    print(f"Log: {message}")

@app.get("/items/{item_id}")
async def read_item(item_id: int, background_tasks: BackgroundTasks):
    background_tasks.add_task(write_log, f"Item {item_id} accessed")
    return {"item_id": item_id}

medium

A. Log: Item {item_id} accessed is printed after response

B. Log: Item {item_id} accessed is printed before response

C. Nothing is printed because print is not awaited

D. SyntaxError because BackgroundTasks is not imported

Solution

1. Step 1: Understand background task behavior

   The function write_log is scheduled as a background task, so it runs after the response is sent.

2. Step 2: Analyze print timing

   Print inside write_log will execute after response, printing the log message with the item_id value.

3. Final Answer:

   Log: Item {item_id} accessed is printed after response -> Option A

4. Quick Check:

   Background task runs after response = C [OK]

Hint: Background tasks run after response, so print happens later [OK]

Common Mistakes:

• Thinking print runs before response
• Assuming print won't run without await
• Missing import of BackgroundTasks (actually imported here)

4. Identify the error in this FastAPI code using background tasks:

from fastapi import FastAPI, BackgroundTasks

app = FastAPI()

def send_email(email: str):
    print(f"Sending email to {email}")

@app.post("/send")
async def send(background_tasks: BackgroundTasks, email: str):
    background_tasks.add_task(send_email)
    return {"message": "Email scheduled"}

medium

A. Missing argument when adding task to background_tasks

B. BackgroundTasks should not be a parameter

C. send_email should be async

D. Email parameter must be inside the function send_email

Solution

1. Step 1: Check how add_task is called

   The function send_email requires an argument email, but add_task is called without arguments.

2. Step 2: Identify the missing argument

   The call should be background_tasks.add_task(send_email, email) to pass the email parameter.

3. Final Answer:

   Missing argument when adding task to background_tasks -> Option A

4. Quick Check:

   add_task needs function arguments = A [OK]

Hint: Pass all function args in add_task call [OK]

Common Mistakes:

• Forgetting to pass required arguments to add_task
• Thinking BackgroundTasks can't be a parameter
• Assuming send_email must be async

5. You want to log user activity and send a welcome email after a user registers. Which is the best way to do this using FastAPI background tasks?

hard

A. Call logging and email functions directly inside the endpoint synchronously

B. Add two background tasks: one for logging, one for sending email, both added via background_tasks.add_task

C. Use await on both logging and email functions inside the endpoint

D. Run logging in background task but send email synchronously

Solution

1. Step 1: Understand the goal

   We want to run both logging and sending email after response without delaying user.

2. Step 2: Evaluate options

   Add two background tasks: one for logging, one for sending email, both added via background_tasks.add_task schedules both tasks as background tasks, so they run after response. Options B and C block response. Run logging in background task but send email synchronously delays sending email.

3. Final Answer:

   Add two background tasks: one for logging, one for sending email, both added via background_tasks.add_task -> Option B

4. Quick Check:

   Schedule all post-response tasks with add_task = A [OK]

Hint: Add all post-response jobs as background tasks [OK]

Common Mistakes:

• Running tasks synchronously blocking response
• Mixing sync and async incorrectly
• Scheduling only one task in background