Background tasks in FastAPI - Mini Project: Build & Apply

Background Tasks with FastAPI

📖 Scenario: You are building a simple web API that accepts user requests to send welcome emails. Sending emails can take time, so you want to handle this task in the background without making the user wait.

🎯 Goal: Create a FastAPI app that accepts a POST request with a user's email. The app should start a background task to simulate sending a welcome email, then immediately respond to the user.

📋 What You'll Learn

Create a FastAPI app instance named app

Define a POST endpoint at /send-email/ that accepts a JSON body with an email field

Use FastAPI's BackgroundTasks to run a function called send_welcome_email in the background

The send_welcome_email function should accept an email parameter and simulate sending an email by printing a message

The endpoint should return a JSON response immediately confirming the email is being sent

💡 Why This Matters

🌍 Real World

Background tasks are useful in web apps to handle slow operations like sending emails, processing files, or calling external APIs without making users wait.

💼 Career

Understanding background tasks is important for backend developers to build responsive and scalable APIs.

1

Create FastAPI app and email data model

Import FastAPI and BaseModel from fastapi and pydantic respectively. Create a FastAPI app instance called app. Define a Pydantic model called EmailRequest with one field email of type str.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

# Your code here

Hint

Use app = FastAPI() to create the app. Define EmailRequest as a class inheriting from BaseModel with an email string field.

2

Add background task function

Define a function called send_welcome_email that takes one parameter email. Inside the function, add a print statement that says Sending welcome email to {email} using an f-string.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class EmailRequest(BaseModel):
    email: str

# Define send_welcome_email function here
# Your code here

Hint

Define a simple function that prints the welcome email message using the email parameter.

3

Create POST endpoint with background task

Import BackgroundTasks from fastapi. Create a POST endpoint at /send-email/ using @app.post. The endpoint function should be called send_email and accept two parameters: request of type EmailRequest and background_tasks of type BackgroundTasks. Inside the function, add the background task send_welcome_email with the email from request.email. Return a dictionary with the key message and value "Email is being sent to {request.email}".

FastAPI

from fastapi import FastAPI, BackgroundTasks
from pydantic import BaseModel

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

app = FastAPI()

class EmailRequest(BaseModel):
    email: str

# Create POST endpoint /send-email/ here
# Your code here

Hint

Use background_tasks.add_task() to schedule the email sending function. Return a confirmation message immediately.

4

Add type hints and finalize code

Ensure the send_welcome_email function has a return type hint of None. Also, add a type hint request: EmailRequest and background_tasks: BackgroundTasks in the send_email function signature. Confirm the full code includes all imports, the app instance, the data model, the background task function, and the POST endpoint.

FastAPI

from fastapi import FastAPI, BackgroundTasks
from pydantic import BaseModel

app = FastAPI()

class EmailRequest(BaseModel):
    email: str

def send_welcome_email(email: str) -> None:
    print(f"Sending welcome email to {email}")

@app.post("/send-email/")
async def send_email(request: EmailRequest, background_tasks: BackgroundTasks) -> dict:
    background_tasks.add_task(send_welcome_email, request.email)
    return {"message": f"Email is being sent to {request.email}"}

# Finalize code with type hints

Hint

Add -> None to the background task function and -> dict to the endpoint function for clarity.

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