The constructor is the the right place for documenting the "output"; it should be either in the class docstring's or in a method's.

It's also unclear what you call an output (is it a return value of one of the methods?)

The constructor is the the right place for documenting the "output"; it should be either in the class docstring's or in a method's.

(I guess you meant not the right place)
I started in the class's docstring so i can put it back there.

It's also unclear what you call an output (is it a return value of one of the methods?)

Well, it's the lister's goal. It output tasks for loaders.

But it's not clearly said anywhere.

It's not a return value per say, it's written to the scheduler's db.
We can "somehow" have a glimpse when we look at the def task_dict(self, origin_type, origin_url, **kwargs): method implementations.
But it's not that apparent.