On 25 November 2017 at 04:30, Guido van Rossum <[email protected]> wrote:
> On Fri, Nov 24, 2017 at 4:22 PM, Guido van Rossum <[email protected]> > 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. > There were some arguments that `await` is like a function call, while `yield` is like `return`. TBH, I don't really like these arguments since to me they are to vague. Continuing this logic one can say that `return` is just a fancy function call (calling continuation with the result). To me there is one clear distinction: `return` and `break` are statements, while `yield`, `yield from`, and `await` are expressions. Continuing the topic of the ban, what exactly should be banned? For example will this still be valid? def pack_two(): return [(yield), (yield)] # Just a list display I don't see how this is controversial. It is clear that `pack_two` is a generator. If this is going to be prohibited, then one may be surprised by lack of referential transparency, since this will be valid: def pack_two(): first = (yield) second = (yield) return [first, second] 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)] I have found several other examples where it is not clear whether they should be prohibited with `yield` or not. I still propose to rule out all of the above from generator expressions, > because those can escape from the surrounding scope. > Here I agree. Also note that the above problem does not apply to generator expressions since (x, x) and (x for _ in range(2)) are two very different expressions. -- Ivan
_______________________________________________ Python-Dev mailing list [email protected] https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com