Defining Celery tasks in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import Celery in a Flask app.

Flask

from celery import [1]

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

ACelery

BTask

Capp

DFlask

Attempts:

3 left

💡 Hint

Common Mistakes

Importing 'Task' instead of 'Celery'.

Trying to import 'Flask' from celery.

You import Celery from the celery package to create tasks.

2fill in blank

medium

Complete the code to create a Celery instance with Flask app's config.

Flask

celery = Celery(__name__, broker=[1])

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

Aconfig['BROKER']

Bapp.broker_url

Capp.config['CELERY_BROKER_URL']

D'redis://localhost:6379/0'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string literal instead of app config.

Using a wrong config key.

The broker URL is usually stored in Flask app config under CELERY_BROKER_URL.

3fill in blank

hard

Fix the error in defining a Celery task decorator.

Flask

@celery.[1]

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

Atasks

Bexecute

Crun

Dtask

Attempts:

3 left

💡 Hint

Common Mistakes

Using @celery.tasks which does not exist.

Using @celery.run or @celery.execute which are invalid.

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

4fill in blank

hard

Fill both blanks to define a simple Celery task function.

Flask

@celery.[1]
def [2]():
    return 'Hello from Celery!'

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

Atask

Bhello_task

Crun_task

Dexecute

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid decorator names.

Using function names with spaces or invalid characters.

The decorator is @celery.task and the function name can be hello_task.

5fill in blank

hard

Fill all three blanks to create a Celery task that adds two numbers.

Flask

@celery.[1]
def [2](a, b):
    return a [3] b

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

Atask

Badd_numbers

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction instead of addition.

Using wrong decorator or function names.

The decorator is @celery.task, the function name add_numbers, and the operator is + to add.

Practice

1. What is the main purpose of defining a Celery task in a Flask application using @celery_app.task?

easy

A. To make the Flask app run faster by disabling routes

B. To automatically restart the Flask server

C. To run functions asynchronously in the background

D. To create database models

Solution

1. Step 1: Understand Celery task decorator

   The @celery_app.task decorator marks a function to run as a background task outside the main Flask process.

2. Step 2: Purpose of asynchronous tasks

   This allows long-running or slow tasks to run without blocking the main app, improving responsiveness.

3. Final Answer:

   To run functions asynchronously in the background -> Option C

4. Quick Check:

   Celery tasks = background async functions [OK]

Hint: Celery tasks run outside Flask to avoid blocking [OK]

Common Mistakes:

• Thinking Celery tasks restart the server
• Confusing tasks with database models
• Believing tasks disable routes

2. Which of the following is the correct way to define a Celery task in Flask?

easy

A. @celery_app.task
def my_task(): pass

B. celery_app.run_task(my_task)

C. @app.route('/task')
def my_task(): pass

D. def my_task(): pass

Solution

1. Step 1: Identify the decorator for Celery tasks

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

2. Step 2: Check syntax correctness

   @celery_app.task def my_task(): pass correctly uses @celery_app.task above the function definition.

3. Final Answer:

   @celery_app.task\ndef my_task(): pass -> Option A

4. Quick Check:

   Use @celery_app.task to define tasks [OK]

Hint: Look for @celery_app.task decorator above function [OK]

Common Mistakes:

• Using Flask route decorator instead of Celery task
• Calling run_task instead of using decorator
• Defining function without decorator

3. Given the following code, what will be the output when add.delay(4, 5) is called?

from celery import Celery
celery_app = Celery('tasks')

@celery_app.task
def add(x, y):
    return x + y

result = add.delay(4, 5)
print(type(result))

medium

A. <class 'celery.result.AsyncResult'>

B. <class 'int'>

C. 9

D. None

Solution

1. Step 1: Understand what delay() returns

   The delay() method queues the task and returns an AsyncResult object, not the actual result.

2. Step 2: Check the printed type

   Printing type(result) shows the class of the returned object, which is celery.result.AsyncResult.

3. Final Answer:

   <class 'celery.result.AsyncResult'> -> Option A

4. Quick Check:

   delay() returns AsyncResult object [OK]

Hint: delay() returns AsyncResult, not the task output [OK]

Common Mistakes:

• Expecting delay() to return the task result immediately
• Confusing AsyncResult with the actual return value
• Printing result instead of type(result)

4. Identify the error in the following Celery task definition:

from celery import Celery
celery_app = Celery('tasks')

@celery_app.task()
def multiply(x, y):
    return x * y

result = multiply.delay(3, 7)
print(result.get())

medium

A. Using parentheses in @celery_app.task() decorator causes error

B. Calling delay() instead of apply_async() is wrong

C. Missing Flask app context inside the task

D. No error; code runs correctly

Solution

1. Step 1: Check decorator syntax

   Using parentheses in @celery_app.task() is allowed and works correctly.

2. Step 2: Verify delay() usage and result retrieval

   Calling delay() queues the task, and result.get() waits for and returns the result properly.

3. Final Answer:

   No error; code runs correctly -> Option D

4. Quick Check:

   Parentheses in decorator are optional and valid [OK]

Hint: Parentheses in @celery_app.task() are optional [OK]

Common Mistakes:

• Thinking parentheses cause syntax error
• Confusing delay() with apply_async()
• Assuming Flask context is always needed in tasks

5. You want to define a Celery task in Flask that sends emails but only if the recipient email is not empty. Which of the following task definitions correctly applies this condition inside the task?

hard

A. @celery_app.task
def send_email(email):
if email is not None:
print(f"Sending email to {email}")

B. @celery_app.task
def send_email(email):
if not email:
return
print(f"Sending email to {email}")

C. @celery_app.task
def send_email(email):
if email is None:
print("No email provided")
else:
print(f"Sending email to {email}")

D. @celery_app.task
def send_email(email):
print(f"Sending email to {email}")

Solution

1. Step 1: Understand the condition to skip empty emails

   The task should not send emails if the email is empty or falsy (like empty string).

2. Step 2: Check which option correctly skips empty emails

   @celery_app.task def send_email(email): if not email: return print(f"Sending email to {email}") uses if not email: return to exit early, preventing sending to empty emails.

3. Final Answer:

   if not email: return to skip empty emails -> Option B

4. Quick Check:

   Use if not email: return to skip empty input [OK]

Hint: Return early if email is empty to skip sending [OK]

Common Mistakes:

• Not checking for empty string, only None
• Sending email without any condition
• Using incorrect condition that doesn't skip empty