[Python-Dev] Tricky way of of creating a generator via a comprehension expression (original) (raw)
Paul Moore p.f.moore at gmail.com
Wed Nov 22 09:09:33 EST 2017
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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 foryield(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.
Paul
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