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

[Nathaniel Smith]

On Wed, Apr 29, 2015 at 3:46 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> Yury Selivanov wrote:
>>
>> I'm not sure
>> why Greg is pushing his Grammar idea so aggressively.
>
>
> Because I believe that any extra complexity in the grammar
> needs a very strong justification. It's complexity in the
> core language, like a new keyword, so it puts a burden on
> everyone's brain.
>
> Saying "I don't think anyone would ever need to write this,
> therefore we should disallow it" is not enough, given that
> there is a substantial cost to disallowing it.
>
> If you don't think there's a cost, consider that we *both*
> seem to be having trouble predicting the consequences of
> your proposed syntax, and you're the one who invented it.
> That's not a good sign!

FWIW, now that I've seen the precedence table in the updated PEP, it
seems really natural to me:
   https://www.python.org/dev/peps/pep-0492/#updated-operator-precedence-table
According to that, "await" is just a prefix operator that binds more
tightly than any arithmetic operation, but less tightly than
indexing/funcall/attribute lookup, which seems about right.

However, if what I just wrote were true, then that would mean that
"await -foo" and "await await foo" would be syntactically legal
(though useless). The fact that they apparently are *not* legal means
that in fact there is still some weird thing going on in the syntax
that I don't understand. And the PEP gives no further details, it just
suggests I go read the parser generator source.

My preference would be that the PEP be updated so that my one-sentence
summary above became correct. But like Guido, I don't necessarily care
about the exact details all that much. What I do feel strongly about
is that whatever syntax we end up with, there should be *some*
accurate human-readable description of *what it is*. AFAICT the PEP
currently doesn't have that.

-n

-- 
Nathaniel J. Smith -- http://vorpus.org