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. Also as there appeared arguments of authority (thanks Antoine) its time to stop this discussion for me. -- Ivan
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