OK, so my 24 hours are over :-)
On 24 November 2017 at 01:50, Nick Coghlan <ncogh...@gmail.com> wrote: > On 23 November 2017 at 23:04, Ivan Levkivskyi <levkivs...@gmail.com> > wrote: > >> 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 :-) >> > > I've posted in more detail about this to the issue tracker, but the > argument here is: because making it behave differently from the way it does > now while still hiding the loop iteration variable potentially requires > even more radical revisions to the lexical scoping rules :) > If somebody can come up with a clever trick to allow yield inside a > comprehension to jump levels in a relatively intuitive way, that would > actually be genuinely cool, but the lexical scoping rules mean it's > trickier than it sounds. > "potentially" is the key word here. The plan is to avoid "more radical revisions". > Now that I frame the question that way, though, I'm also remembering that > we didn't have "yield from" yet when I wrote the current comprehension > implementation, and given that, it may be as simple as having an explicit > yield expression in a comprehension imply delegation to a subgenerator. > My understanding is that this is exactly how async comprehensions are currently implemented and why they work as one would naively expect, i.e. `await` is "bound" to the surrounding async def, not to the implicit scope async def. So that an async comprehension is just equivalent to a for-loop. However, although "implicit yield from" solution is simpler than the one proposed by Serhiy, I still more like the latter one. The former has its strange cases, for example I mentioned before in this thread: >>> async def f(): ... for i in range(3): ... yield i ... >>> async def g(): ... return [(yield i) async for i in f()] ... >>> g().send(None) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 2, in g TypeError: object async_generator can't be used in 'await' expression My understanding is that the strange error is exactly because of the implicit `yield from`. With Serhiy's approach this would work. Another minor bonus of Serhiy's idea is a performance gain: we will not push an execution frame. -- Ivan
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