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

[Ivan Levkivskyi]

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.

--
Ivan
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