How to store result of long running Celery job in a JSON file without blocking my REST endpoint on Flask?

I created a long running Celery worker that handles web scraping and populates a dictionary object. I created multiple jobs that run asynchronously and all of which populate the same dict object. I need a way to dump the result in my worker to a JSON file. It can be done periodically or once the jobs are done, but it should not block my API response.

@celery.task(name="tasks.workertask")
def worker(d, x):
    """
    Do some long task on x and populate the dictionary d
    """

def collect(x_list)
    d = dict()
    for x in x_list:
        worker.delay(d,x)
    return d.copy()

@application.route('/api')
def api():
    #Store x_list
    x_list = [...]
    d = collect(x_list)
    with open('result.json','w') as fp:
        json.dump(d,fp)
    return "Dumping into JSON"

The dump into result.json is empty because the task is running in the background. I don't want this task to be blocking my API response either as the task is very very long (more than 12 hours)