fix StopIteration behaviour in AsCompleted.batches

[slavoutich]

PEP 479: generators should return instead of raise StopIteration

Fixes #1069

[pitrou]

Wouldn't it better to fix next_batch to return an empty list instead of raising StopIteration?

[slavoutich]

It's rather matter of taste. Either we catch exception, or check next_batch for length. Exception is caught once, and len(self.next_batch()) > 0 will be checked every iteration.

[pitrou]

next_batch isn't an iterator, so I wouldn't expect it to raise StopIteration in any case. Either return an empty list, or raise an error such as ValueError or EOFError.

[slavoutich]

I'd prefer raising in that case, but I think that ValueError or EOFError are even worse here than StopIteration. GeneratorExit might be better.

[pitrou]

GeneratorExit, as StopIteration, is not supposed to be raised by normal functions. Actually GeneratorExit is not even an Exception subclass, so an except Exception clause won't catch it...

[slavoutich]

Correct. Anyway, I would prefer either stay with StopIteration, or create custom exception, otherwise people can accidentally enter infinite loop, when use AsCompleted.next_batch() directly.

[mrocklin]

@pitrou ok to merge? Are there any tests that would be appropriate here?

[pitrou]

I don't think it's easy to test this, except perhaps try to catch and check warnings on 3.6+. I'm going to merge.