[Python-Dev] Tricky way of of creating a generator via a comprehension expression (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Sat Nov 25 02:24:31 EST 2017

On Sat, 25 Nov 2017 17:10:12 +1000 Nick Coghlan <ncoghlan at gmail.com> wrote:

On 25 November 2017 at 16:18, Nathaniel Smith <njs at pobox.com> wrote: > On Fri, Nov 24, 2017 at 9:39 PM, Nick Coghlan <ncoghlan at gmail.com> wrote: >> On 25 November 2017 at 15:27, Nathaniel Smith <njs at pobox.com> wrote: >>> On Fri, Nov 24, 2017 at 9:04 PM, Nick Coghlan <ncoghlan at gmail.com> wrote: >>>> def example(): >>>> comp1 = yield from [(yield x) for x in ('1st', '2nd')] >>>> comp2 = yield from [(yield x) for x in ('3rd', '4th')] >>>> return comp1, comp2 >>> >>> Isn't this a really confusing way of writing >>> >>> def example(): >>> return [(yield '1st'), (yield '2nd')], [(yield '3rd'), (yield '4th')] >> >> A real use case > > Do you have a real use case? This seems incredibly niche...

That's not how backwards compatibility works - we were suggesting getting rid of this syntax, because there was no current way to make it do anything sensible. It turns out there is a way to make it behave reasonably - you just need to stick "yield from" in front of it, and it goes back to being equivalent to the corresponding for loop (the same as the synchronous version).

I don't know if it behaves reasonably, but it's entirely unreadable to me. I think there's a case for discouraging unreadable constructs.

Regards

Antoine.

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