Task results and status in Django - Step-by-Step Execution

Concept Flow - Task results and status

Start Task

↓

Task Running

↓

Task Completes?

No→Wait or Retry

Yes↓

Store Result & Status

↓

Check Status & Result

↓

Display or Use Result

This flow shows how a task starts, runs, completes, stores its result and status, and then how the status and result are checked or displayed.

Execution Sample

Django

from django_q.tasks import async_task

# Start a task
async_task('math.copysign', 1, -1)

# Later check status and result
from django_q.models import Task
result = Task.objects.last().result
status = Task.objects.last().success

This code starts an asynchronous task and later retrieves its result and success status from the database.

Execution Table

Step	Action	Task Status	Result Stored	Notes
1	Call async_task to start task	Pending	None	Task is queued to run asynchronously
2	Task starts running	Running	None	Task is executing the function
3	Task completes successfully	Success	-1.0	Result of math.copysign(1, -1) stored
4	Retrieve last task from DB	Success	-1.0	Access task status and result
5	Use result and status	Success	-1.0	Result -1.0 and success True used in app
6	No new tasks	N/A	N/A	Execution stops here

💡 No more tasks to process, final status and result stored

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	Final
task_status	None	Pending	Success	Success	Success
task_result	None	None	-1.0	-1.0	-1.0

Key Moments - 3 Insights

Why is the task status 'Pending' right after calling async_task?

Because the task is only queued to run asynchronously and has not started execution yet, as shown in step 1 of the execution_table.

How do we know the task completed successfully?

At step 3, the status changes to 'Success' and the result is stored, indicating the task finished without errors.

Why do we retrieve the task from the database to get the result?

Because async tasks run outside the main code flow, their results and status are saved in the database for later retrieval, as shown in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the task status immediately after calling async_task?

A Running

B Pending

C Success

D Failed

💡 Hint

Check step 1 in the execution_table where the task is just queued.

At which step does the task result get stored?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look for when the result value changes from None to -1.0 in the variable_tracker and execution_table.

If the task failed, what would you expect the 'task_status' to be at step 3?

A Failed

B Running

C Pending

D Success

💡 Hint

Refer to the meaning of status values in the execution_table and key_moments.

Concept Snapshot

Task results and status in Django Q:
- async_task() queues a task (status: Pending)
- Task runs asynchronously (status: Running)
- On completion, result and status saved (Success or Failed)
- Retrieve task from DB to check status and result
- Use status to handle success or failure in your app

Full Transcript

In Django Q, when you start a task with async_task, it is first queued with status Pending. Then the task runs asynchronously and its status changes to Running. When the task finishes, the result and status (Success or Failed) are saved in the database. You can later retrieve the task object from the database to check its status and get the result. This lets your app know if the task succeeded and what output it produced.

Practice

1. In Django with Celery, which object do you use to check the status and result of a background task by its ID?

easy

A. AsyncResult

B. TaskStatus

C. TaskResult

D. ResultChecker

Solution

1. Step 1: Identify the object for task tracking

   Celery provides AsyncResult to track task status and results using the task ID.

2. Step 2: Confirm usage in Django context

   In Django projects using Celery, AsyncResult is the standard way to check if a task is pending, running, or finished.

3. Final Answer:

   AsyncResult -> Option A

4. Quick Check:

   Task status and results = AsyncResult [OK]

Hint: Remember: AsyncResult tracks task status by ID [OK]

Common Mistakes:

• Confusing AsyncResult with task function names
• Using non-existent classes like TaskStatus
• Trying to access results directly without AsyncResult

2. Which of the following is the correct way to create an AsyncResult instance for a task with ID stored in task_id?

easy

A. result = AsyncResult(task=task_id)

B. result = AsyncResult.get(task_id)

C. result = AsyncResult.fetch(task_id)

D. result = AsyncResult(task_id)

Solution

1. Step 1: Recall AsyncResult constructor usage

   The AsyncResult class is instantiated by passing the task ID as the first argument.

2. Step 2: Check each option's syntax

   Only AsyncResult(task_id) correctly creates the instance. Methods like .get() or .fetch() are not constructors.

3. Final Answer:

   result = AsyncResult(task_id) -> Option D

4. Quick Check:

   Instantiate AsyncResult with task ID directly [OK]

Hint: Use AsyncResult(task_id) to create result object [OK]

Common Mistakes:

• Calling get() or fetch() as constructor
• Passing keyword argument 'task' instead of positional
• Confusing AsyncResult with task function calls

3. Given the code:

result = AsyncResult('abc123')
status = result.status
output = result.result

What will status and output represent if the task is still running?

medium

A. status is 'PENDING', output is None

B. status is 'RUNNING', output is None

C. status is 'FAILURE', output is the error info

D. status is 'SUCCESS', output is the task result

Solution

1. Step 1: Understand AsyncResult status values

   By default, while a task is running without calling update_state inside the task, result.status remains 'PENDING'.

2. Step 2: Check result property during running

   result.result returns None until the task completes.

3. Final Answer:

   status is 'PENDING', output is None -> Option A

4. Quick Check:

   Running task (default): status='PENDING', result=None [OK]

Hint: Default running tasks show status 'PENDING' and result None [OK]

Common Mistakes:

• Mistaking for 'RUNNING' status (doesn't exist)
• Confusing with 'STARTED' which requires explicit update_state
• Thinking result is available before completion

4. You wrote:

result = AsyncResult(task_id)
if result.status == 'SUCCESS':
    print(result.result)
else:
    print('Task not done')

But it always prints 'Task not done' even after task completion. What is the likely issue?

medium

A. You must call result.get() instead of accessing result.result

B. You should check for 'COMPLETED' instead of 'SUCCESS'

C. The task ID is incorrect or expired

D. AsyncResult does not have a status attribute

Solution

1. Step 1: Understand status checking logic

   The code checks if result.status equals 'SUCCESS' to print the result.

2. Step 2: Identify why status never shows 'SUCCESS'

   If the task ID is wrong or expired, AsyncResult will not find the task and status stays 'PENDING' or similar.

3. Final Answer:

   The task ID is incorrect or expired -> Option C

4. Quick Check:

   Wrong task ID causes status never to be 'SUCCESS' [OK]

Hint: Check task ID validity if status never changes [OK]

Common Mistakes:

• Using wrong status string like 'COMPLETED'
• Assuming result.result always updates without completion
• Ignoring task ID correctness

5. You want to handle a task result in Django only if it succeeded, otherwise log the error. Which code snippet correctly checks the task status and safely accesses the result or error?

hard

A. result = AsyncResult(task_id)
if result.ready():
handle(result.result)
else:
log_error('Task not ready')

B. result = AsyncResult(task_id)
try:
handle(result.get(timeout=1))
except Exception as e:
log_error(e)

C. result = AsyncResult(task_id)
if result.status == 'PENDING':
handle(result.result)
else:
log_error('Task failed')

D. result = AsyncResult(task_id)
if result.status == 'SUCCESS':
handle(result.result)
elif result.status == 'FAILURE':
log_error(result.get())

Solution

1. Step 1: Understand safe result retrieval

   Using result.get() with a timeout waits for completion and raises exceptions on failure.

2. Step 2: Check error handling approach

   Wrapping result.get() in try-except catches task failures and allows logging errors safely.

3. Step 3: Compare other options

   Options B, C, and D do not handle exceptions properly; C incorrectly treats 'PENDING' as success.

4. Final Answer:

   Use try-except with result.get() to handle success and failure -> Option B

5. Quick Check:

   Use result.get() with try-except for safe task result handling [OK]

Hint: Use try-except with result.get() to catch errors [OK]

Common Mistakes:

• Checking only status strings without exception handling
• Assuming ready() means success
• Treating PENDING as success