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

[Guido van Rossum]

+1 for option 3.

On Sat, Apr 25, 2015 at 1:18 PM, Yury Selivanov <yselivanov.ml at gmail.com>
wrote:

> Hi Guido,
>
> On 2015-04-24 1:03 PM, Guido van Rossum wrote:
>
>> *3. syntactic priority of `await`*
>>
>> Yury, could you tweak the syntax for `await` so that we can write the most
>> common usages without parentheses? In particular I'd like to be able to
>> write
>> ```
>> return await foo()
>> with await foo() as bar: ...
>> foo(await bar(), await bletch())
>> ```
>> (I don't care about `await foo() + await bar()` but it would be okay.)
>> ```
>> I think this is reasonable with some tweaks of the grammar (similar to
>> what
>> Greg did for cocall, but without requiring call syntax at the end).
>>
>
> I've done some experiments with grammar, and it looks like
> we indeed can parse await quite differently from yield. Three
> different options:
>
>
> Option #1. Parse 'await' exactly like we parse 'yield'.
> This how we parse 'await' in the latest version of reference
> implementation.
>
> Required grammar changes:
> https://gist.github.com/1st1/cb0bd257b04adb87e167#file-option-1-patch
>
> Repo to play with:
> https://github.com/1st1/cpython/tree/await
>
> Syntax:
>
>   await a()
>   res = (await a()) + (await b())
>   res = (await (await a()))
>   if (await a()): pass
>   return (await a())
>   print((await a()))
>   func(arg=(await a()))
>   await a() * b()
>
>
> Option #2. Keep 'await_expr' in 'atom' terminal, but don't
> require parentheses.
>
> Required grammar changes:
> https://gist.github.com/1st1/cb0bd257b04adb87e167#file-option-2-patch
>
> Repo to play with (parser module is broken atm):
> https://github.com/1st1/cpython/tree/await_noparens
>
> Syntax:
>
>   await a()
>   res = (await a()) + await b() # w/o parens (a() + await b())
>   res = await await a()
>   if await a(): pass
>   return await a()
>   print(await a())
>   func(arg=await a())
>   await a() * b()
>
>
> Option #3. Create a new terminal for await expression between
> 'atom' and 'power'.
>
> Required grammar changes:
> https://gist.github.com/1st1/cb0bd257b04adb87e167#file-option-3-patch
>
> Repo to play with (parser module is broken atm):
> https://github.com/1st1/cpython/tree/await_noparens2
>
> Syntax:
>
>   await a()
>   res = await a() + await b()
>   res = await (await a()) # invalid syntax w/o parens
>   if await a(): pass
>   return await a()
>   print(await a())
>   func(arg=await a())
>   await (a() * b()) # w/o parens '(await a() * b())
>
>
> I think that Option #3 is a clear winner.
>
> Thanks,
> Yury
>
>


-- 
--Guido van Rossum (python.org/~guido)
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-dev/attachments/20150425/b7dc7c50/attachment.html>