[Python-Dev] PEP 492 quibble and request

[Nick Coghlan]

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(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

That said, it may be worth having the future statement *anyway* as:

1. It gives us the runtime accessible record of the feature transition
in the __future__ module
2. It lets folks opt-in to the full keyword implementation from day 1
if they prefer, addressing the tokenisation limitation noted in the
PEP: https://www.python.org/dev/peps/pep-0492/#transition-period-shortcomings

Regards,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia