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

[Ivan Levkivskyi]

On 22 November 2017 at 17:16, Paul Moore <p.f.moore at gmail.com> wrote:

> On 22 November 2017 at 16:08, Ivan Levkivskyi <levkivskyi at gmail.com>
> wrote:
> > On 22 November 2017 at 16:56, Yury Selivanov <yselivanov.ml at gmail.com>
> > wrote:
> >>
> >> On Wed, Nov 22, 2017 at 10:10 AM, Ivan Levkivskyi <levkivskyi at gmail.com
> >
> >> wrote:
> >> > On 22 November 2017 at 15:47, Paul Moore <p.f.moore at gmail.com> wrote:
> >> [...]
> >> I'm all for prohibiting using 'yield' expression in generator
> >> expressions or comprehensions.  The semantics is way to hard to
> >> understand and hence be of any value.
> >>
> >> Making 'await' a SyntaxError is absolutely not an option.  Async
> >> generator expressions are a shorthand syntax for defining asynchronous
> >> generators (PEP 525), and it's already being used in the wild.
> >
> >
> > OK, makes sense, so it looks like we may have the following plan:
> >
> > - fix `yield` in comprehensions
>
> I'm still not clear what "fix" would actually mean, but you propose
> clarifying the docs below, so I assume it means "according to whatever
> the updated docs say"...
>
>
I mean the initial proposal: make comprehensions equivalent to a for-loop


> > - update PEP 530 and docs re generator expressions vs comprehensions
>
> Docs more importantly than PEP IMO. And are you implying that there's
> a difference between generator expressions and comprehensions? I
> thought both were intended to behave as if expanded to a function
> containing nested for loops? Nothing said in this thread so far (about
> semantics, as opposed to about current behaviour) implies there's a
> deliberate difference.
>

I think there may be a difference:

comprehension `g = [(yield i) for i in range(3)]` is defined as this code:

    __result = []
    __i = None
    try:
        for __i in range(3):
            __result.append(yield __i)
        g = __result
    finally:
        del __result, __i

while `g = list((yield i) for i in range(3))` is defined as this code:

    def __gen():
       for i in range(3):
         yield (yield i)
    g = list(__gen())

 Although these two definitions are equivalent in simple cases (like having
`f(i)` instead of `yield i`)

But this is debatable, I think before we move to other points we need to
agree on the clear definitions of semantics of generator expressions and
comprehensions.

--
Ivan
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