Defining Celery tasks in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Celery task call?

Given this Celery task defined in a Flask app, what will result.get() return?

Flask

from celery import Celery

app = Celery('tasks', broker='redis://localhost:6379/0')

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

result = add.delay(4, 5)

A Returns AsyncResult object

B 9

C Raises AttributeError

D None

💡 Hint

Remember that delay() queues the task and get() waits for the result.

The add task adds two numbers. Calling add.delay(4, 5) queues the task. Using result.get() waits for the task to finish and returns the sum, which is 9.

📝 Syntax

intermediate

Which option correctly defines a Celery task in Flask?

Choose the correct way to define a Celery task function in a Flask app.

A

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

B

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

C

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

D

@app.task
def multiply(x, y):
    print(x * y)

💡 Hint

Decorator syntax must be directly above the function definition without parentheses if no arguments.

Option A correctly uses @app.task decorator above the function. Option B places decorator after function which is invalid. Option C uses parentheses which is allowed and valid in Celery 5+. Option D returns None because it prints instead of returning.

❓ state_output

advanced

What is the value of result.status immediately after calling add.delay(2, 3)?

Consider this Celery task call:

result = add.delay(2, 3)

What is the value of result.status right after this line?

Flask

from celery import Celery

app = Celery('tasks', broker='redis://localhost:6379/0')

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

result = add.delay(2, 3)

A "STARTED"

B "SUCCESS"

C "FAILURE"

D "PENDING"

💡 Hint

Think about the task lifecycle immediately after queuing.

When a task is queued with delay(), its initial status is PENDING until a worker picks it up. So immediately after calling delay(), result.status is "PENDING".

🔧 Debug

advanced

Why does this Celery task raise an error?

Given this task definition, why does calling multiply.delay(3, 4) raise an error?

Flask

from celery import Celery

app = Celery('tasks', broker='redis://localhost:6379/0')

def multiply(x, y):
    return x * y

app.task(multiply)

A The task decorator is not applied correctly; it should be @app.task above the function

B The broker URL is invalid causing connection failure

C The function multiply is missing a return statement

D The task name is missing causing registration failure

💡 Hint

Check how the task decorator is used in Celery.

Option A is correct because app.task(multiply) returns a task but does not replace the original function reference. The function is not decorated properly. The correct way is to use @app.task decorator above the function definition.

🧠 Conceptual

expert

Which option best explains how Celery tasks are discovered in a Flask app?

In a Flask app using Celery, how does Celery find and register tasks defined in different modules?

A Tasks must be explicitly imported in the Celery app module to be registered

B Celery uses a configuration setting to list all task names manually

C Celery auto-discovers tasks by scanning all Python files in the project directory

D Tasks are registered only if decorated with @app.task and placed in the main app file

💡 Hint

Think about Python import behavior and how decorators work.

Celery does not auto-discover tasks by scanning files. Tasks must be imported so their decorators run and register them with Celery. Without importing, Celery won't know about tasks in other modules.