[Python-Dev] Tricky way of of creating a generator via a comprehension expression

[Ivan Levkivskyi]

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 _make_list(it):
> >> >          result = []
> >> >          for i in it:
> >> >              result.append(yield i)
> >> >          return result
> >> >      g = _make_list(iter(range(3)))
> >> >
> >> > Due to "yield" in the expression _make_list() 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.

--
Ivan
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