[Python-Dev] Tricky way of of creating a generator via a comprehension expression (original) (raw)
Ivan Levkivskyi levkivskyi at gmail.com
Fri Nov 24 13:20:25 EST 2017
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OK, so my 24 hours are over :-)
On 24 November 2017 at 01:50, Nick Coghlan <ncoghlan at gmail.com> wrote:
On 23 November 2017 at 23:04, Ivan Levkivskyi <levkivskyi at 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 "", line 1, in File "", 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.
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