Short History:
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(Ram Rachum)
What do you think about adding a method: `Executor.filter`?
I was using something like this: 

    my_things = [thing for thing in things if some_condition(thing)]

But the problem was that `some_condition` took a long time to run waiting on I/O, which is a great candidate for parallelizing with ThreadPoolExecutor. I made it work using `Executor.map` and some improvizing, but it would be nicer if I could do:

    with concurrent.futures.ThreadPoolExecutor(100) as executor:
        my_things = executor.filter(some_condition, things)

And have the condition run in parallel on all the threads.

(Nick Coughlan)
I think this is sufficiently tricky to get right that it's worth adding filter() as a parallel to the existing map() API.