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

[Ethan Furman]

On 04/30, Guido van Rossum wrote:
> On Thu, Apr 30, 2015 at 9:15 AM, Ethan Furman wrote:
> 
>>  [...]
>>  Both you and Paul are correct on this, thank you.  The proper resolution
>>  of
>> 
>>    await -coro()
>> 
>>  is indeed to get the result of coro(), call it's __neg__ method, and then
>>  await on that.
>> 
>>  And that is perfectly reasonable, and should not be a SyntaxError; what it
>>  might be is an AttributeError (no __neg__ method) or an AsyncError (__neg__
>>  returned non-awaitable object), or might even just work [1]... but it
>>  definitely should /not/ be a SyntaxError.
>> 
> 
> Why not? Unlike some other languages, Python does not have uniform
> priorities for unary operators, so it's reasonable for some unary
> operations to have a different priority than others, and certain things
> will be SyntaxErrors because of that. E.g. you can write "not -x" but you
> can't write "- not x".

For one, Yury's answer is "- await x" which looks just as nonsensical as
"- not x".

For another, an error of some type will be raised if either __neg__ doesn't
exist or it doesn't return an awaitable, so a SyntaxError is unnecessary.

For a third, by making it a SyntaxError you are forcing the use of parens to
get what should be the behavior anyway.

In other words, a SyntaxError is nat any clearer than "AttributeError: obj
has no __neg__ method" and it's not any clearer than "AwaitError: __neg__
returned not-awaitable".  Those last two errors tell you exactly what you
did wrong.

--
~Ethan~