[Python-ideas] PEP 572: Assignment Expressions (post #4)

[Jacco van Dorp]

2018-04-12 15:02 GMT+02:00 Nick Coghlan <ncoghlan at gmail.com>:
> On 12 April 2018 at 22:22, Jacco van Dorp <j.van.dorp at deonet.nl> wrote:
>> I've looked through PEP 343, contextlib docs (
>> https://docs.python.org/3/library/contextlib.html ), and I couldn't
>> find a single case where "with (y := f(x))" would be invalid.
>
> Consider this custom context manager:
>
>     @contextmanager
>     def simple_cm():
>         yield 42
>
> Given that example, the following code:
>
>     with cm := simple_cm() as value:
>         print(cm.func.__name__, value)
>
> would print "'simple_cm 42", since the assignment expression would
> reference the context manager itself, while the with statement binds
> the yielded value.
>
> Another relevant example would be `contextlib.closing`: that returns
> the passed in argument from __enter__, *not* self.
>
> And that's why earlier versions of PEP 572 (which used the "EXPR as
> NAME" spelling) just flat out prohibited top level name binding
> expressions in with statements: "with (expr as name):" and "with expr
> as name:" were far too different semantically for the only syntactic
> difference to be a surrounding set of parentheses.
>
> Cheers,
> Nick.

Makes sense. However, couldn't you prevent that by giving with
priority over the binding ? As in "(with simple_cm) as value", where
we consider the "as" as binding operator instead of part of the with
statement ? Sure, you could commit suicide by parenthesis, but by
default it'd do exactly what the "with simple_cm as value" currently
does. This does require use of as instead of :=, though. (which was
the point I was trying to make, apologies for the confusion)