add stealable keyword

Inspired by this stackoverflow question, I want to suggest adding a idempotent keyword somewhere in the workflow, which in the case idempotent = False switches off work stealing. I would suggest to make it a keyword of a delayed object, in this case a workflow that expects tasks to be run only once could look like this:

c = distributed.Client()
delayed(mytask, idempotent = False).compute(get = c.get)

[mrocklin]

Okay, I understand that. Generally it would be nice to have a good way to react if a task runs twice. Are there alternatives that are both convenient for the user and easy to implement?

I can’t think of anything at the moment. My inclination would be to try to resolve this by making tasks idempotent (can be run multiple times without bad effect) and not depending on side effects like local files. Or, if you require local files, tie their cleanup to cleanup within the python process using __del__ or the atexit module.

Generally speaking Dask doesn’t play nicely with code that depends on side effects. My recommendations are usually going to be about avoiding or mitigating the use of side effects.

[abast]

This is awesome, thanks!