On Sat, Nov 25, 2017 at 8:07 AM, Ivan Levkivskyi <levkivs...@gmail.com> wrote:
> On 25 November 2017 at 16:57, Guido van Rossum <gu...@python.org> wrote: > >> On Sat, Nov 25, 2017 at 6:55 AM, Ivan Levkivskyi <levkivs...@gmail.com> >> wrote: >> >>> On 25 November 2017 at 04:30, Guido van Rossum <gu...@python.org> wrote: >>> >>>> On Fri, Nov 24, 2017 at 4:22 PM, Guido van Rossum <gu...@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. >>>> >>> >>> [...] >>> If the first example will be allowed, then one will be surprised why it >>> can't be rewritten as >>> >>> def pack_two(): >>> return [(yield) for _ in range(2)] >>> >> >> And yet Nick's example shows that that is not equivalent! >> >> [...] >> >> In this example each thing that looks syntactically like a list >> comprehension becomes actually a generator expression at at runtime! And so >> does your example, so instead of a list of two items, it returns a >> generator that will produce two values when iterated over. >> >> That's not referential transparency to me, it feels more like a bug in >> the code generator. >> >> I want to ban this because apparently nobody besides Nick knows about >> this behavior (I certainly didn't, and from the above it seems you don't >> either). >> > > This whole thread started as a proposal to fix this bug and to make the > two forms equivalent, so I don't know what you are talking about. > I see. I misread your equivalence example as "this how it works" -- you meant it as "this is how I propose we fix it". The fix is not unreasonable but I still would like to retreat to the territory where `yield [from]` in comprehensions (and generator expressions) is deemed invalid. > Also as there appeared arguments of authority (thanks Antoine) its time to > stop this discussion for me. > I'd be happy to stop with the conclusion that we're going to rip out some confusing syntax rather than trying to generate code for it -- IMO we've proved to ourselves that this stuff is too complicated to be useful. -- --Guido van Rossum (python.org/~guido)
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