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

[Ivan Levkivskyi]

On 23 November 2017 at 15:30, Paul Moore <p.f.moore at gmail.com> wrote:

> On 23 November 2017 at 14:24, Ivan Levkivskyi <levkivskyi at gmail.com>
> wrote:
> >> My main concern is that comprehension is not equivalent to a for loop
> >> for a specific reason - the scope issue. Has anyone looked back at the
> >> original discussions to confirm *why* a function was used?
> >>
> >> My recollection:
> >>
> >> >>> i = 1
> >> >>> a = [i for i in (1,2,3)]
> >> >>> print(i)
> >> 1
> >>
> >> Serihy's approach (and your described expansion) would have print(i)
> >> return NameError.
> >
> >
> > Absolutely no, it will still print 1. The internal implementation will
> use
> > unique ids internally (see https://bugs.python.org/issue10544 for
> details).
> >
>
> Ok, cool. My main point still applies though - has anyone confirmed
> why a function scope was considered necessary at the time of the
> original implementation, but it's apparently not now? I'm pretty sure
> it was a deliberate choice, not an accident.


>From what Nick explained on b.p.o. I understand that this is closer to the
"accident" definition.
Also the original issue https://bugs.python.org/issue1660500 doesn't have
any discussion of the implementation _strategy_.
So I tried to dig the mailing list, in the latest Guido's message I have
found
https://mail.python.org/pipermail/python-3000/2006-December/005218.html
he still likes the idea of unique hidden ids (like Serhiy proposes now) and
no function scopes. After that there is Nick's message
https://mail.python.org/pipermail/python-3000/2006-December/005229.html
where he says that he still likes pseudo-scopes more.
Then I lost the track of discussion.

It may well be Nick's intentional decision (and it has its merits) but I am
not sure it was a conscious consensus.
Nick could probably add more. Also I propose to wait and see when Serhiy
will show us his complete implementation.

--
Ivan
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