Event-driven architecture in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define an event handler function in FastAPI.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.[1]("/items/")
async def create_item():
    return {"message": "Item created"}

Drag options to blanks, or click blank then click option'

Aget

Bpost

Cput

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using @app.get instead of @app.post for creating resources.

Confusing HTTP methods for event handling.

The @app.post decorator defines a POST request handler, which is commonly used to create resources in event-driven APIs.

2fill in blank

medium

Complete the code to publish an event message asynchronously.

FastAPI

import asyncio

async def publish_event(event_data):
    await [1](event_data)

async def send_message(data):
    print(f"Sending: {data}")

Drag options to blanks, or click blank then click option'

Asend_message

Bdispatch

Cemit

Dpublish

Attempts:

3 left

💡 Hint

Common Mistakes

Using undefined functions like publish or emit without definition.

Not awaiting the asynchronous function.

The function send_message is awaited to simulate sending an event asynchronously.

3fill in blank

hard

Fix the error in the event listener registration code.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@[1]("/event")
async def event_listener():
    return {"status": "Event received"}

Drag options to blanks, or click blank then click option'

Aapp.get

Bapp.listen

Capp.event

Dapp.on

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent decorators like app.listen or app.on.

Confusing event-driven frameworks with FastAPI syntax.

FastAPI uses @app.get or other HTTP method decorators to define endpoints. app.listen or app.on are not valid decorators.

4fill in blank

hard

Fill both blanks to create a background task that handles events.

FastAPI

from fastapi import FastAPI, BackgroundTasks

app = FastAPI()

def process_event(data):
    print(f"Processing {data}")

@app.post("/event")
async def handle_event(data: dict, [1]: BackgroundTasks):
    [2].add_task(process_event, data)
    return {"message": "Event received"}

Drag options to blanks, or click blank then click option'

Abackground_tasks

Btasks

Cbackground

Dbg_tasks

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for the parameter and method call.

Not using BackgroundTasks for background processing.

The parameter name background_tasks is used to receive the BackgroundTasks instance, and then background_tasks.add_task schedules the event processing.

5fill in blank

hard

Fill all three blanks to implement a simple event bus with subscription and publishing.

FastAPI

class EventBus:
    def __init__(self):
        self.subscribers = {}

    def subscribe(self, event_type, handler):
        if event_type not in self.subscribers:
            self.subscribers[[1]] = []
        self.subscribers[event_type].append(handler)

    async def publish(self, event_type, data):
        for handler in self.subscribers.get(event_type, [2]):
            await handler([3])

Drag options to blanks, or click blank then click option'

Aevent_type

B[]

Cdata

Dhandler

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong keys for the subscribers dictionary.

Not providing a default empty list in get method.

Passing wrong argument to handlers.

We use event_type as the key to store handlers, default to empty list [] if no subscribers, and pass data to each handler when publishing.

Practice

1. What is the main benefit of using event-driven architecture in FastAPI?

easy

A. It allows the app to react to actions immediately without waiting.

B. It makes the app run only on a single thread.

C. It requires no code to handle user requests.

D. It disables background tasks for faster response.

Solution

1. Step 1: Understand event-driven architecture

   Event-driven architecture lets the app respond to events as they happen, improving responsiveness.

2. Step 2: Connect to FastAPI's use of BackgroundTasks

   FastAPI uses BackgroundTasks to handle events without blocking the main user request, enabling immediate reactions.

3. Final Answer:

   It allows the app to react to actions immediately without waiting. -> Option A

4. Quick Check:

   Event-driven = Immediate reaction [OK]

Hint: Event-driven means reacting instantly to actions [OK]

Common Mistakes:

• Thinking event-driven means single-threaded only
• Believing no code is needed for requests
• Assuming background tasks are disabled

2. Which of the following is the correct way to import BackgroundTasks in FastAPI?

easy

A. import BackgroundTasks from fastapi

B. from fastapi.background import Tasks

C. from fastapi import BackgroundTasks

D. import fastapi.BackgroundTasks

Solution

1. Step 1: Recall FastAPI import syntax

   FastAPI uses standard Python import syntax: from module import class/function.

2. Step 2: Identify correct module and class

   The BackgroundTasks class is imported from fastapi directly, so the correct syntax is 'from fastapi import BackgroundTasks'.

3. Final Answer:

   from fastapi import BackgroundTasks -> Option C

4. Quick Check:

   Correct import syntax = from fastapi import BackgroundTasks [OK]

Hint: Use 'from fastapi import BackgroundTasks' to import correctly [OK]

Common Mistakes:

• Using wrong import order or syntax
• Trying to import from a non-existent submodule
• Using dot notation incorrectly in import

3. What will be the output when this FastAPI endpoint is called?

from fastapi import FastAPI, BackgroundTasks
app = FastAPI()

results = []

def write_log(message: str):
    results.append(message)

@app.post('/send')
async def send_message(background_tasks: BackgroundTasks):
    background_tasks.add_task(write_log, 'Message sent')
    return {'status': 'ok'}

# After calling POST /send once, what is the value of results?

medium

A. ['Message sent']

B. [] (empty list)

C. ['Message sent', 'Message sent']

D. Error: background_tasks not defined

Solution

1. Step 1: Understand BackgroundTasks behavior

   BackgroundTasks run after the response is sent, but in testing or immediate inspection, the task may not have run yet.

2. Step 2: Check when results list updates

   The results list is updated inside the background task, which runs asynchronously after returning the response, so immediately after the call, results is still empty.

3. Final Answer:

   [] (empty list) -> Option B

4. Quick Check:

   Background task runs after response = empty results [OK]

Hint: BackgroundTasks run after response, so immediate results stay empty [OK]

Common Mistakes:

• Assuming background task runs before return
• Expecting results updated synchronously
• Confusing background_tasks parameter usage

4. Identify the error in this FastAPI code using BackgroundTasks:

from fastapi import FastAPI, BackgroundTasks
app = FastAPI()

@app.post('/notify')
async def notify():
    background_tasks.add_task(print, 'Notify event')
    return {'message': 'Notification sent'}

medium

A. BackgroundTasks must be imported from fastapi.background module

B. print function cannot be used in background tasks

C. Missing await keyword before add_task call

D. background_tasks parameter missing in function definition

Solution

1. Step 1: Check function parameters for BackgroundTasks

   The function uses background_tasks but does not declare it as a parameter, so it is undefined inside the function.

2. Step 2: Confirm correct usage of BackgroundTasks

   BackgroundTasks must be accepted as a parameter in the endpoint function to be used, e.g., 'async def notify(background_tasks: BackgroundTasks)'.

3. Final Answer:

   background_tasks parameter missing in function definition -> Option D

4. Quick Check:

   Missing parameter = background_tasks parameter missing in function definition [OK]

Hint: Always add BackgroundTasks as a function parameter to use it [OK]

Common Mistakes:

• Forgetting to add background_tasks parameter
• Thinking print can't be used in background tasks
• Adding await to add_task (it's not a coroutine)

5. You want to send an email notification after a user registers without delaying the response. Which FastAPI pattern best fits this event-driven need?

hard

A. Use BackgroundTasks to send the email after returning the response

B. Send the email synchronously inside the endpoint before returning

C. Use a global variable to store emails and send later manually

D. Call the email function inside a separate thread without FastAPI support

Solution

1. Step 1: Understand the goal of non-blocking email sending

   We want the user to get a quick response while the email sends in the background.

2. Step 2: Match FastAPI's event-driven feature

   BackgroundTasks lets you schedule tasks to run after the response, perfect for sending emails without delay.

3. Final Answer:

   Use BackgroundTasks to send the email after returning the response -> Option A

4. Quick Check:

   BackgroundTasks = non-blocking post-response tasks [OK]

Hint: Use BackgroundTasks for post-response actions like emails [OK]

Common Mistakes:

• Sending email synchronously causing delay
• Using global variables for task management
• Manually managing threads without FastAPI support