[Python-Dev] PEP 492 vs. PEP 3152, new round (original) (raw)

Ethan Furman ethan at stoneleaf.us
Thu Apr 30 19:56:34 CEST 2015

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

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