[Python-Dev] Tricky way of of creating a generator via a comprehension expression (original) (raw)

Ivan Levkivskyi levkivskyi at gmail.com
Wed Nov 22 09:15:49 EST 2017

On 22 November 2017 at 15:09, Paul Moore <p.f.moore at gmail.com> wrote:

On 22 November 2017 at 13:53, Ivan Levkivskyi <levkivskyi at gmail.com> wrote: > On 22 November 2017 at 14:38, Antoine Pitrou <solipsis at pitrou.net> wrote: >> >> On Wed, 22 Nov 2017 15:03:09 +0200 >> Serhiy Storchaka <storchaka at gmail.com> wrote: >> > From >> > >> > https://stackoverflow.com/questions/45190729/ differences-between-generator-comprehension-expressions. >> > >> > g = [(yield i) for i in range(3)] >> > >> > Syntactically this looks like a list comprehension, and g should be a >> > list, right? But actually it is a generator. This code is equivalent to >> > the following code: >> > >> > def makelist(it): >> > result = [] >> > for i in it: >> > result.append(yield i) >> > return result >> > g = makelist(iter(range(3))) >> > >> > Due to "yield" in the expression makelist() is not a function >> > returning a list, but a generator function returning a generator. >> > >> > This change in semantic looks unintentional to me. It looks like leaking >> > an implementation detail. >> >> Perhaps we can deprecate the use of "yield" in comprehensions and make >> it a syntax error in a couple versions? >> >> I don't see a reason for writing such code rather than the more >> explicit variants. It looks really obscure, regardless of the actual >> semantics. > > > People actually try this (probably simply because they like comprehensions) > see two mentioned Stackoverflow questions, plus there are two b.p.o. issues. > So this will be a breaking change. Second, recent PEP 530 allowed writing a > similar comprehensions with await: > > async def process(funcs): > result = [await fun() for fun in funcs] # OK > ... > > Moreover, it has the semantics very similar to the proposed by Serhiy for > yield (i.e. equivalent to for-loop without name leaking into outer scope). > Taking into account that the actual fix is not so hard, I don't think it > makes sense to have all the hassles of deprecation period.

I agree with Antoine. This (yield in a comprehension) seems far too tricky, and I'd rather it just be rejected. If I saw it in a code review, I'd certainly insist it be rewritten as an explicit loop. There are many things that I would reject in code review, but they are still allowed in Python, this is one of the reasons why code reviews exist. Also I am not sure how yield in a comprehension is more tricky than await in a comprehension. Anyway, this looks more like a taste question.

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