[Python-Dev] PEP 492 quibble and request (original) (raw)

Yury Selivanov yselivanov.ml at gmail.com
Thu Apr 30 05:19:56 CEST 2015

On 2015-04-29 11:01 PM, Nick Coghlan wrote:

On 30 April 2015 at 12:31, Guido van Rossum<guido at python.org> wrote:

>On Wed, Apr 29, 2015 at 7:07 PM, Nick Coghlan<ncoghlan at gmail.com> wrote:

>> >>[...] >>Yeah, I'm coming around to the idea. For the async pseudo-keyword, I >>can see that the proposal only allows its use in cases that were >>previously entirely illegal, but I'm not yet clear on how the PEP >>proposes to avoid changing the meaning of the following code: >> >> x = await(thisisafunctioncall) >> >>Unless I'm misreading the proposed grammar in the PEP (which is >>entirely possible), I believe PEP 492 would reinterpret that as: >> >> x = await thisisnotafunctioncallanymore > > >Ah, but here's the other clever bit: it's only interpreted this wayinside >a function declared with 'async def'. Outside such functions, 'await' is not >a keyword, so that grammar rule doesn't trigger. (Kind of similar to the way >that the printfunction future disables the keyword-ness of 'print', >except here it's toggled on or off depending on whether the nearest >surrounding scope is 'async def' or not. The PEP could probably be clearer >about this; it's all hidden in the Transition Plan section.) Ah, nice, although even reading the Transition Plan section didn't clue me in to that particular aspect of the idea :) Given that clarification, I think the rationale for "no future statement needed" can be strengthened by focusing on the fact that such a statement would largely beredundant, given that: * "async def", "async with", and "async for" are all currently syntax errors, and hence adding them is backwards compatible if "async" is otherwise treated as a normal variable name * "await " only gains its new interpretation when used inside an "async def" statement, so "async def" fills the role that a module level compiler declaration like "from future import asyncfunctions" would otherwise fill

Thanks, Nick.

I've fixed the Transition Plan section, and rewrote the "why not future" one too.

https://hg.python.org/peps/rev/552773d7e085 https://hg.python.org/peps/rev/5db3ad3d540b

Yury

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