On 2015-04-29 11:01 PM, Nick Coghlan wrote:
On 30 April 2015 at 12:31, Guido van Rossum<gu...@python.org> wrote:
>On Wed, Apr 29, 2015 at 7:07 PM, Nick Coghlan<ncogh...@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(this_is_a_function_call)
>>
>>Unless I'm misreading the proposed grammar in the PEP (which is
>>entirely possible), I believe PEP 492 would reinterpret that as:
>>
>> x = await this_is_not_a_function_call_any_more
>
>
>Ah, but here's the other clever bit: it's only interpreted this way*inside*
>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 print_function __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 be*redundant*, 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 <expr>" 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
async_functions" 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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