concurrency - Python: Wait on all of `concurrent.futures.ThreadPoolExecutor`'s futures

Question

I've given concurrent.futures.ThreadPoolExecutor a bunch of tasks, and I want to wait until they're all completed before proceeding with the flow. How can I do that, without having to save all the futures and call wait on them? (I want an action on the executor.)

Answer 1

Just call Executor.shutdown:

shutdown(wait=True)

Signal the executor that it should free any resources that it is using when the currently pending futures are done executing. Calls to Executor.submit() and Executor.map() made after shutdown will raise RuntimeError.

If wait is True then this method will not return until all the pending futures are done executing and the resources associated with the executor have been freed.

However if you keep track of your futures in a list then you can avoid shutting the executor down for future use using the futures.wait() function:

concurrent.futures.wait(fs, timeout=None, return_when=ALL_COMPLETED)

Wait for the Future instances (possibly created by different Executor instances) given by fs to complete. Returns a named 2-tuple of sets. The first set, named done, contains the futures that completed (finished or were cancelled) before the wait completed. The second set, named not_done, contains uncompleted futures.

note that if you don't provide a timeout it waits until all futures have completed.

You can also use futures.as_completed() instead, however you'd have to iterate over it.