On Sat, Nov 25, 2017 at 12:17 PM Brett Cannon <brett@python.org> wrote:

On Fri, Nov 24, 2017, 19:32 Guido van Rossum, <guido@python.org> wrote:
On Fri, Nov 24, 2017 at 4:22 PM, Guido van Rossum <guido@python.org> wrote:
The more I hear about this topic, the more I think that `await`, `yield` and `yield from` should all be banned from occurring in all comprehensions and generator expressions. That's not much different from disallowing `return` or `break`.

From the responses it seems that I tried to simplify things too far. Let's say that `await` in comprehensions is fine, as long as that comprehension is contained in an `async def`. While we *could* save `yield [from]` in comprehensions, I still see it as mostly a source of confusion, and the fact that the presence of `yield [from]` *implicitly* makes the surrounding `def` a generator makes things worse. It just requires too many mental contortions to figure out what it does.

I still propose to rule out all of the above from generator expressions, because those can escape from the surrounding scope.

+1 from me

On Sat, Nov 25, 2017 at 9:21 AM, Yury Selivanov <yselivanov.ml@gmail.com> wrote:

So we are keeping asynchronous generator expressions as long as they are defined in an 'async def' coroutine?

I would be happy to declare that `await` is out of scope for this thread. It seems that it is always well-defined and sensible what it does in comprehensions and in genexprs. (Although I can't help noticing that PEP 530 does not appear to propose `await` in generator expressions -- it proposes `async for` in comprehensions and in genexprs, and `await` in comprehensions only -- but they appear to be accepted nevertheless.)

So we're back to the original issue, which is that `yield` inside a comprehension accidentally makes it become a generator rather than a list, set or dict. I believe that this can be fixed. But I don't believe we should fix it. I believe we should ban `yield` from comprehensions and from genexprs. We don't need it, and it's confused most everyone. And the ban should extend to `yield from` in those same contexts. I think we have a hope for consensus on this.

(I also think that if we had invented `await` earlier we wouldn't have gone down the path of `yield` expressions -- but historically it appears we wouldn't have invented `await` at all if we hadn't first tried `yield` and then `yield from` to build coroutines, so I don't think this so bad after all. :-)

--Guido van Rossum (python.org/~guido)