Calling tasks asynchronously in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Celery class for asynchronous tasks.

Flask

from celery import [1]

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

Aapp

BTask

CAsyncResult

DCelery

Attempts:

3 left

💡 Hint

Common Mistakes

Importing 'Task' instead of 'Celery'.

Using 'app' which is not a class to import.

Confusing 'AsyncResult' with the Celery class.

You need to import Celery to create a Celery app for asynchronous tasks.

2fill in blank

medium

Complete the code to create a Celery app with the name 'myapp'.

Flask

celery_app = [1]('myapp')

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

ACelery

BTask

CAsyncResult

DFlask

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Task' which is for defining tasks, not creating the app.

Using 'Flask' which is unrelated to Celery app creation.

Using 'AsyncResult' which is for checking task status.

Use the Celery class to create a Celery app instance.

3fill in blank

hard

Fix the error in the task decorator to define an asynchronous task.

Flask

@celery_app.[1]

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

Arun

Bexecute

Ctask

Dasync

Attempts:

3 left

💡 Hint

Common Mistakes

Using '@celery_app.run' which does not exist.

Using '@celery_app.async' which is a reserved keyword.

Using '@celery_app.execute' which is not a decorator.

The correct decorator to define a Celery task is @celery_app.task.

4fill in blank

hard

Fill both blanks to call the asynchronous task named 'add' with arguments 4 and 6.

Flask

result = add.[1]([2], 6)

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

Adelay

Bapply_async

C4

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'apply_async' without correct argument format.

Passing 5 instead of 4 as the first argument.

Calling the task directly without delay or apply_async.

Use delay to call the task asynchronously and pass the first argument as 4.

5fill in blank

hard

Fill all three blanks to check if the asynchronous task result is ready and get the result.

Flask

if result.[1]():
    value = result.[2]()
else:
    value = [3]

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

Aready

Bget

CNone

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'status' instead of 'ready' to check completion.

Using 'ready' instead of 'get' to retrieve the result.

Assigning a wrong default value instead of None.

Use ready() to check if the task finished, get() to get the result, and None if not ready.

Practice

1. What is the main benefit of calling tasks asynchronously in a Flask app?

easy

A. It forces users to refresh the page manually.

B. It makes the app run slower by adding extra steps.

C. It disables other parts of the app until the task finishes.

D. It allows the app to handle long tasks without making users wait.

Solution

1. Step 1: Understand asynchronous task purpose

   Asynchronous tasks let the app do long jobs in the background.

2. Step 2: Connect to user experience

   This means users don't have to wait for the task to finish before using the app.

3. Final Answer:

   It allows the app to handle long tasks without making users wait. -> Option D

4. Quick Check:

   Async tasks improve user wait time = C [OK]

Hint: Async tasks run in background, so users don't wait [OK]

Common Mistakes:

• Thinking async tasks slow down the app
• Believing users must refresh manually
• Assuming async disables other app parts

2. Which of the following is the correct way to start a background thread in Flask?

easy

A. start_thread(task)

B. thread.start(task())

C. threading.Thread(target=task).start()

D. Thread.run(task)

Solution

1. Step 1: Recall Python threading syntax

   To start a thread, create Thread with target function, then call start().

2. Step 2: Match options to syntax

   threading.Thread(target=task).start() matches correct syntax: threading.Thread(target=task).start()

3. Final Answer:

   threading.Thread(target=task).start() -> Option C

4. Quick Check:

   Correct thread start syntax = A [OK]

Hint: Use threading.Thread(target=func).start() to run async [OK]

Common Mistakes:

• Calling start() on the function instead of the thread
• Using undefined functions like start_thread
• Confusing run() with start() method

3. Given this Flask code snippet, what will be printed?

from threading import Thread
from flask import Flask
app = Flask(__name__)
def background_task():
    print('Task done')
@app.route('/')
def index():
    Thread(target=background_task).start()
    return 'Hello'

medium

A. Hello printed immediately, then 'Task done' printed later

B. Only 'Hello' is printed, 'Task done' never prints

C. Both 'Hello' and 'Task done' print at the same time

D. SyntaxError because Thread is used incorrectly

Solution

1. Step 1: Understand thread behavior in Flask route

   The route starts a thread to run background_task, then returns 'Hello' immediately.

2. Step 2: Predict output order

   'Hello' is sent to user right away; 'Task done' prints later when thread runs.

3. Final Answer:

   Hello printed immediately, then 'Task done' printed later -> Option A

4. Quick Check:

   Thread runs task after response = A [OK]

Hint: Thread runs task after response, so print order differs [OK]

Common Mistakes:

• Thinking 'Task done' prints before 'Hello'
• Assuming thread blocks response
• Believing thread syntax is wrong here

4. What is wrong with this Flask code that tries to run a background task?

from threading import Thread
from flask import Flask
app = Flask(__name__)
def task():
    print(app.name)
@app.route('/')
def index():
    Thread(target=task).start()
    return 'Done'

medium

A. Route function must return JSON, not string.

B. The background thread lacks Flask app context, causing errors.

C. Flask app cannot print its name inside a thread.

D. Thread target function must be called with parentheses.

Solution

1. Step 1: Identify Flask app context issue

   Background threads don't have Flask app context by default, so accessing app.name can fail.

2. Step 2: Understand context handling

   To fix, use app.app_context() inside the thread to provide context.

3. Final Answer:

   The background thread lacks Flask app context, causing errors. -> Option B

4. Quick Check:

   Missing app context in thread = D [OK]

Hint: Background threads need app context to access Flask variables [OK]

Common Mistakes:

• Calling target function instead of passing it
• Thinking Flask disallows printing app.name
• Believing route must return JSON always

5. You want to run a long task asynchronously in Flask and update a database after it finishes. Which approach correctly ensures Flask app context is available in the background task?

hard

A. Use with app.app_context(): inside the background task function.

B. Call the background task directly inside the route without threading.

C. Start the thread without any context handling; Flask shares context automatically.

D. Use app.run() inside the background task to get context.

Solution

1. Step 1: Recall Flask app context rules

   Background tasks need explicit app context to access Flask features like database.

2. Step 2: Identify correct context usage

   Using with app.app_context(): inside the task ensures context is active during task execution.

3. Step 3: Eliminate incorrect options

   Calling task directly blocks request; Flask does not share context automatically; app.run() starts server, not context.

4. Final Answer:

   Use with app.app_context(): inside the background task function. -> Option A

5. Quick Check:

   Use app.app_context() in thread = B [OK]

Hint: Wrap background code with app.app_context() to access Flask features [OK]

Common Mistakes:

• Running task directly blocking the app
• Assuming Flask shares context automatically
• Misusing app.run() inside tasks