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

[Yury Selivanov]

Greg,

On 2015-04-29 1:40 AM, Greg Ewing wrote:
> Yury Selivanov wrote:
>
>> I don't want this: "await a() * b()" to be parsed, it's not meaningful.
>
> Why not? If a() is a coroutine that returns a number,
> why shouldn't I be able to multiply it by something?

Sorry, I thought you meant parsing "await a()*b()" as
"await (a() * b())".
>
> I don't think your currently proposed grammar prevents
> that anyway. We can have
>
> <expr> --> <factor> '*' <factor>
>    --> <power> '*' <power>
>    --> <atom_expr> '*' <atom_expr>
>    --> 'await' <atom> <trailer>* '*' <atom> <trailer>*
>    --> 'await' 'a' '(' ')' '*' 'b' '(' ')'
>
> It does, on the other hand, seem to prevent
>
>   x = - await a()

This works just fine:
https://github.com/1st1/cpython/commit/33b3cd052243cd71c064eb385c7a557eec3ced4b

Current grammar prevents this: "await -fut", and this:
"await fut ** 2" being parsed as "await (fut ** 2)


>
> which looks perfectly sensible to me.
>
> I don't like the idea of introducing another level
> of precedence. Python already has too many of those
> to keep in my brain. Being able to tell people "it's
> just like unary minus" makes it easy to explain (and
> therefore possibly a good idea!).
>

It's just like unary minus ;)

Yury