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Wed Apr 29 21:27:24 CEST 2015 · https://mail.python.org/pipermail/python-dev/2015-April/139646.html

Hi Nathaniel,

BTW, I'm going to reply to you in the other thread about
context-local objects soon.  I have some thoughts on the
topic.

On 2015-04-29 3:14 PM, Nathaniel Smith wrote:
> On Apr 29, 2015 11:49 AM, "Yury Selivanov" <yselivanov.ml at gmail.com> wrote:
>> Hi Ethan,
>>
>>
>> On 2015-04-29 2:32 PM, Ethan Furman wrote:
>>> On 04/29, Yury Selivanov wrote:
>>>> On 2015-04-29 1:25 PM, Ethan Furman wrote:
>>>>> cannot also just work and be the same as the parenthesized
>>>>> version.
>>>> Because it does not make any sense.
>>> I obviously don't understand your position that "it does not make
>>> any sense" -- perhaps you could explain a bit?
>>>
>>> What I see is a suspension point that is waiting for the results of
>>> coro(), which will be negated (and returned/assigned/whatever).
>>> What part of that doesn't make sense?
>>>
>> Because you want operators to be resolved in the
>> order you see them, generally.
>>
>> You want '(await -fut)' to:
>>
>> 1. Suspend on fut;
>> 2. Get the result;
>> 3. Negate it.
>>
>> This is a non-obvious thing. I would myself interpret it
>> as:
>>
>> 1. Get fut.__neg__();
>> 2. await on it.
>>
>> So I want to make this syntactically incorrect:
> As a bystander, I don't really care either way about whether await -fut is
> syntactically valid (since like you say it's semantically nonsense
> regardless and no one will ever write it). But I would rather like to
> actually know what the syntax actually is, not just have a list of examples
> (which kinda gives me perl flashbacks). Is there any simple way to state
> what the rules for parsing await are? Or do I just have to read the parser
> code if I want to know that?

There is a summary of grammar changes in the PEP:
https://www.python.org/dev/peps/pep-0492/#grammar-updates

You can also see the actual grammar file from the reference
implementation:
https://github.com/1st1/cpython/blob/await/Grammar/Grammar

Thanks,
Yury

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[Python-Dev] PEP 492 vs. PEP 3152, new round