[Python-Dev] [PEP 3148] futures - execute computations asynchronously

Brian Quinlan brian at sweetapp.com
Wed Mar 10 09:19:38 CET 2010

On 9 Mar 2010, at 08:39, Greg Ewing wrote:

> Terry Reedy wrote:
>> Looking more close, I gather that the prime results will be printed  
>> 'in order' (waiting on each even if others are done) while the url  
>> results will be printed 'as available'.
> Seems to me that if you care about the order of the results,
> you should be able to just wait for each result separately
> in the order you want them. Something like
>  task1 = start_task(proc1)
>  task2 = start_task(proc2)
>  task3 = start_task(proc3)
>  result1 = task1.wait_for_result()
>  result2 = task2.wait_for_result()
>  result3 = task3.wait_for_result()

You can write this as:

executor = ...
future1 = executor.submit(proc1)
future2 = executor.submit(proc2)
future3 = executor.submit(proc3)
result1 = task1.result()
result2 = task2.result()
result3 = task3.result()

> This would also be a natural way to write things even if
> you don't care about the order, but you need all the results
> before proceeding. You're going to be held up until the
> longest-running task completes anyway, so it doesn't matter
> if some of them finish earlier and have to sit around
> waiting for you to collect the result.

Often you don't want to continue if there is a failure.

In the example that you gave, if "proc3" raises an exception  
immediately, you still wait for "proc1" and "proc2" to complete even  
though you will end up discarding their results.


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