For the newer versions of celery(4.0 or above), we can get registered tasks as follows.
from celery import current_app
tasks = current_app.tasks.keys()
For older versions of celery, celery < 4, we can get registered tasks as follows.
from celery.task.control import inspect
i = inspect()
i.registered_tasks()
This will give a dictionary of all workers & related registered tasks.
from itertools import chain
set(chain.from_iterable( i.registered_tasks().values() ))
In case if you have multiple workers running same tasks or if you just need a set of all registered tasks, it does the job.
Alternate Way:
From terminal you can get a dump of registered tasks by using this command
celery inspect registered
To inspect tasks related to a specific app, you can pass app name
celery -A app_name inspect registered