CODEWITHSUNDEEP
pythonflaskcelerytypeerrorcelery-task
Martin Pichler
Martin Pichler

Reputation: 315

Celery | Flask error: expected a bytes-like object, AsyncResult found

I am currently working on a flask application (Python 3.6) and wanted to integrate celery because I have multiple long running background tasks.

Edit: Celery 4.1

The integration was no problem and the celery tasks are executed correctly, but I cannot access the current state of a running task.

Celery, Flask setup:

def make_celery(app):
    celery = Celery(app.import_name,
                    backend=app.config["result_backend"],
                    broker=app.config["broker_url"])

    celery.conf.update(app.config)

    TaskBase = celery.Task

    class ContextTask(TaskBase):
        abstract = True

        def __call__(self, *args, **kwargs):
            with app.app_context():
                return TaskBase.__call__(self, *args, **kwargs)

    celery.Task = ContextTask
    return celery


app = Flask(__name__)
app.config["broker_url"] = "redis://localhost:6379"
app.config["result_backend"] = "redis://localhost:6379"
app.config["DB"] = "../pyhodl.sqlite"

celery_app = make_celery(app)

Celery task:

@celery_app.task(bind=True, name="server.tasks.update_trading_pair")
def update_trading_pair(self, exchange, currency_a, currency_b):
    print(exchange, currency_a, currency_b)
    time.sleep(50)

Call the task and store the value in a dictionary:

task_id = update_trading_pair.delay(exchange, currency_a, currency_b)
print("NEW TASK")
print(task_id)
id = exchange_mnemonic + "_" + currency_a + "_" + currency_b
TASK_STATES[id] = task_id

Get state of a task:

result = update_trading_pair.AsyncResult(TASK_STATES[market.__id__()])
print(result.state)
print(result) # works but only prints the task_id

This is were the error is raised. When i print only the result object it simply prints the task_id. And if I try to retrieve the current state i raises following exception:

TypeError: sequence item 1: expected a bytes-like object, AsyncResult found

Upvotes: 3

Views: 3277

Answers (1)

arthur
arthur

Reputation: 2399

EXPLANATION :

When you call your task : 

task_id = update_trading_pair.delay(exchange, currency_a, currency_b)

your variable task_id is an instance of AsyncResult, it is not a string.

So, your variable TASK_STATES[market.__id__()] is also an instance of AsyncResult whereas it should be a string.

And then your are trying to instantiate an AsyncResult object with it 

result = update_trading_pair.AsyncResult(TASK_STATES[market.__id__()])

So you are instantiating an AsyncResult object with another AsyncResult object whereas it should be instantiated with a string.

Maybe your confusion comes from your print(task_id) which shows you a string, but when you do this, under the hood the __str__ method of the AsyncResult object is called and if you look at it in the source code here, 

def __str__(self):
    """`str(self) -> self.id`."""
    return str(self.id)

it justs print the id attribute of your task_id object.

SOLUTION :

You can fix it with either by doing TASK_STATES[id] = task_id.id or by doing 

result = update_trading_pair.AsyncResult(str(TASK_STATES[market.__id__()]))

Upvotes: 3

Related Questions