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

Nathaniel Smith njs at pobox.com
Fri May 1 05:57:12 CEST 2015

On Apr 30, 2015 8:40 PM, "Guido van Rossum" <guido at python.org> wrote:
> On Thu, Apr 30, 2015 at 8:30 PM, Nathaniel Smith <njs at pobox.com> wrote:
>> The actual effect of making "await" a different precedence is to resolve
the ambiguity in
>>   await x ** 2
>> If await acted like -, then this would be
>>   await (x ** 2)
>> But with the proposed grammar, it's instead
>>   (await x) ** 2
>> Which is probably correct, and produces the IMHO rather nice invariant
that "await" binds more tightly than arithmetic in general (instead of
having to say that it binds more tightly than arithmetic *except* in this
one corner case...)
> Correct.
>> AFAICT these and the ** case are the only expressions where there's any
difference between Yury's proposed grammar and your proposal of treating
await like unary minus. But then given the limitations of Python's parser
plus the desire to disambiguate the expression above in the given way, it
becomes an arguably regrettable, yet inevitable, consequence that
>>   await -fut
>>   await +fut
>>   await ~fut
>> become parse errors.
>  Why is that regrettable? Do you have a plan for overloading one of those
on Futures? I personally consider it a feature that you can't do that. :-)

I didn't say it was regrettable, I said it was arguably regrettable. For
proof, see the last week of python-dev ;-).

(I guess all else being equal it would be nice if unary operators could
stack arbitrarily, since that really is the more natural parse rule IMO and
also if things had worked that way then I would have spent this thread less
confused. But this is a pure argument from elegance. In practice there's
obviously no good reason to write "await -fut" or "-not x", so meh,

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