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

Guido van Rossum guido at python.org
Thu Apr 30 19:38:16 CEST 2015

On Thu, Apr 30, 2015 at 9:15 AM, Ethan Furman <ethan at stoneleaf.us> 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". This is because they have different priorities: 'not' has a very low priority so it binds less tight than comparisons (which bind less tight than arithmetic), but '-' has a high priority. (There are other quirks, e.g. -22 means -(22) and you can write 2**-2; but you can't write 2**not x.)

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