[Python-Dev] Generator objects and list comprehensions?

[Brett Cannon]

On Sun, 29 Jan 2017 at 16:39 Craig Rodrigues <rodrigc at freebsd.org> wrote:

> On Thu, Jan 26, 2017 at 4:09 AM, Ivan Levkivskyi <levkivskyi at gmail.com>
> wrote:
>
>
>
> Concerning list/set/dict comprehensions, I am much more in favor of making
> comprehensions simply equivalent to for-loops (more or less like you
> proposed using yield from). The only reason to introduce auxiliary function
> scope was to prevent the loop variables from leaking outside comprehensions.
> Formally, this is indeed backward incompatible, but I doubt many people
> depend on the current counter-intuitive behavior.
>
> Concerning generator expressions, probably it is indeed better to simply
> prohibit yield inside them.
>
>
> Thank you to everyone who responded to my post and provided excellent
> analysis.
>
> For Python, I don't know what the best way to proceed is:
>
> OPTION 1
> ========
>
> Make a SyntaxError:  [(yield 1) for x in range(10)]
> and update the documentation to explain that this is an invalid construct.
>
> This would have certainly helped me identify the source of the problem as
> I tried porting buildbot 0.9 to Python 3.
>
> However, while not very common, there is Python 2.x code that uses that.
> I found these cases in the buildbot code which I changed so as to work on
> Python 2 and 3:
>
> https://github.com/buildbot/buildbot/pull/2661
> https://github.com/buildbot/buildbot/pull/2673
>
>
>  OPTION 2
> =========
> Make this return a list on Python 3, like in Python 2:  [
> (yield 1) for x in range(10)]
>
> As pointed out by others on the this mailing list, there are some
> problems associated with that.  I don't know if there are many Python 2
> codebases out there
> with this construct, but it would be nice to have one less Python 2 -> 3
> porting gotcha.
>
>
> I'm OK with either approach.  Leaving things the way they are in Python 3
> is no good, IMHO.
>

My vote is it be a SyntaxError since you're not getting what you expect
from the syntax.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-dev/attachments/20170130/ab1a3c2a/attachment.html>