On Sun, Oct 15, 2017 at 5:34 PM, Amit Green <amit.mi...@gmail.com> wrote:
> On Sun, Oct 15, 2017 at 9:44 AM, Koos Zevenhoven <k7ho...@gmail.com> > wrote: > >> So, see below for some more discussion between (it would be useful if >> some people could reply to this email and say if and why they agree or >> disagree with something below -- also non-experts that roughly understand >> what I'm talking about): >> > > Yes, I understand what you are roughly talking about. > > Also, yes, generators are co-routines [though when starting to work with > generators, people don't fully realize this]. > > But then how to address "my" original problem where the context would >> propagate through awaits, and next/send? From what others have written, it >> seems there are also other situations where that is desired. There are >> several ways to solve the problem as an extension to PEP 555, but below is >> one: >> >> >> >>> We need both versions: the one that propagates first_context() into the >>> coroutine, and the one that propagates second_context() into it. Or, using >>> my metaphor from the other thread, we need "both the forest and the trees". >>> >>> >>> A solution to this would be to have two types of context arguments: >>> >>> 1. (calling) context arguments >>> >>> and >>> >>> 2. execution context arguments >>> >>> >>> >> So yes, I'm actually serious about this possibility. Now it would be up >> to library and framework authors to pick the right variant of the two. And >> this is definitely something that could be documented very clearly. >> >> > This is an interesting idea. I would add you also need: > > 3. Shared context, the generator shares the context with it's caller > which means: > > - If the caller changes the context, the generator, see the changed > context next time it's __next__ function is called > - If the generator changes the context, the caller sees the changed > context. > - [This clearly make changing the context using 'with' totally > unusable in both the caller & the generator -- unless we add even odder > semantics, that the generator restores the original context when it > exists???] > - (As per previous email by me, I claim this is the most natural way > beginners are going to think it works; and needs to be supported; also in > real code this is not often useful] > - I'm not sure if this would even work with async or not -- *IF* not, > I would still have a syntax for the user to attempt this -- and throw a > Syntax Error when they do, with a good explanation of why this combination > doesn't work for async. I believe good explanations are a great way for > people to learn which features can't be combined together & why. > > Just as a quick note, after skimming through your bullet points: All of this is indeed covered with decorators and other explicit mechanisms in the PEP 555 approach. I don't think we need syntax errors, though. > >> If something was not clear, but seems relevant to what I'm trying to >> discuss here, please ask :) >> > > > I looked for you quote "we need both the forest & the trees", but didn't > find it here. I quite strongly agree we need both; in fact need also the > third case I highlighted above. > > The ordering of the archive was indeed thoroughly destroyed. Ordering by date might help. But the quote you ask for is here: https://mail.python.org/pipermail/python-ideas/2017-October/047285.html –-Koos > As for what Guido wrote, that we might be trying to solve too many > problems -- probably. However, these are real issues with context's, not > edge cases. > > Thus Guido writing we don't want to allow yield within a 'with' clause (as > it leaks context) .. I would argue two things: > > - There are use cases where we *DO* want this -- rare -- true -- but > they exist (i.e.: my #3 above) > > - IF, for simplicity, sake, it is decided not to handle this case now; > then make it a syntax error in the language; i.e.: > > def f(): > with context() as x: > yield 1 > > Syntax error: 'yield' may not be used inside a 'with' clause. > > This would really help new users not to make a mistake that takes hours to > debug; & help correct their [initial mistaken] thinking on how contexts & > generators interact. > > -- + Koos Zevenhoven + http://twitter.com/k7hoven +
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