RuntimeError: Never call result.get() within a task Celery

Question

I am using celery to send a task to remote server and trying to get the result back. The state of task is constantly updated using update_state method on remote server.

I am sending task using

app.send_task('task_name')

getting results of celery task is a blocking call and i don’t want my django app to wait for result and timeout.

So i tried running another celery task for getting results.

@app.task(ignore_result=True)
def catpure_res(task_id):
    task_obj = AsyncResult(task_id)
    task_obj.get(on_message=on_msg)

But it results in the error below.

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/celery/app/trace.py", line 367, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/celery/app/trace.py", line 622, in __protected_call__
    return self.run(*args, **kwargs)
  File "/home/arpit/project/appname/tasks/results.py", line 42, in catpure_res
    task_obj.get(on_message=on_msg)
  File "/usr/local/lib/python2.7/dist-packages/celery/result.py", line 168, in get
    assert_will_not_block()
  File "/usr/local/lib/python2.7/dist-packages/celery/result.py", line 44, in assert_will_not_block
    raise RuntimeError(E_WOULDBLOCK)
RuntimeError: Never call result.get() within a task!
See http://docs.celeryq.org/en/latest/userguide/tasks.html#task-synchronous-subtasks

Is there any workaround for this error. Do I have to run a daemon process for getting the results?

Asked By: Arpit Solanki

||

Source

Answer 1

As your title explain, calling get within a task is a bad practice and can lead to deadlock.
instead, you can check for the task status and get it result whenever it’s ready:

result = catpure_res.AsyncResult(task_id, app=app)
    if result.ready():
        return result.get()

    return result.state

You can wrap the above snippet within a function and request for it every x seconds.

EDIT: regard your comment:

You can get the result.state instead, and use the retry mechanism with countdown until the task result.state == SUCCESS.
You can add celery beat to run periodic task that check if the primary task ends.
Note that using such heavy task (of long duration) is also a bad practice. consider to break it apart into a small tasks and use canvas to combine them.

Answered By: ItayB

Answer 2

Use allow_join_result. See the snippet below.

@app.task(ignore_result=True)
def catpure_res(task_id):
    task_obj = AsyncResult(task_id)
    with allow_join_result():
        task_obj.get(on_message=on_msg)

Note: As mentioned in the other answers it can cause performance issue and even deadlock, but if your task is well written and doesn’t cause unexpected errors than it should work like a charm.

Answered By: Gautam Kumar

Answer 3

from celery.result import allow_join_result
task_obj = send_task("do_something", [arg1, arg2, arg3])

with allow_join_result():
    def on_msg(*args, **kwargs):
        print(f"on_msg: {args}, {kwargs}")
    try:
        result = task_obj.get(on_msg=on_msg, timeout=timeout_s)
    except TimeoutError as exc:
        print("Timeout!")

Answered By: Terry Tú Nguyễn

RuntimeError: Never call result.get() within a task Celery

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