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

[Ivan Levkivskyi]

On 23 November 2017 at 15:21, Paul Moore <p.f.moore at gmail.com> wrote:

> On 23 November 2017 at 13:04, Ivan Levkivskyi <levkivskyi at gmail.com>
> wrote:
> > Let us forget for a moment about other problems and focus on this one:
> list
> > comprehension is currently not equivalent to a for-loop.
> > There are two options:
> > - Fix this, i.e. make comprehension equivalent to a for-loop even in edge
> > cases (Serhiy seems ready to do this)
> > - Prohibit all cases when they are not equivalent
> >
> > I still prefer option one. But I see your point, option two is also an
> > acceptable fix.
> > Note that there were not so many situations when some code became
> > SyntaxError later.
> > I don't see why this particular case qualifies for such a radical
> measure as
> > an exception to syntactic rules,
> > instead of just fixing it (sorry Nick :-)
>
> 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).

--
Ivan
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-dev/attachments/20171123/c3dcd503/attachment.html>