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

[Yury Selivanov]

On 2015-04-30 1:56 PM, Ethan Furman wrote:
>> >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".


"- await x" is a perfectly valid code:

     result = - await compute_in_db()

(same as "result = - (yield from do_something())")

>
> 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.

I still want to see where my current grammar forces to use
parens.  See [1], there are no useless parens anywhere.

FWIW, I'll fix the 'await (await x)' expression to be parsed
without parens.

>
> 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.

This is debatable. "obj has no __neg__ method" isn't obvious
to everyone (especially to those people who aren't using
operator overloading).


[1] https://www.python.org/dev/peps/pep-0492/#examples-of-await-expressions


Yury