2018-04-26 13:20 GMT+03:00 Steve Holden <steve@holdenweb.com>:
On Thu, Apr 26, 2018 at 8:56 AM, Steven D'Aprano <steve@pearwood.info> wrote:
On Thu, Apr 26, 2018 at 03:31:13AM -0400, Terry Reedy wrote:
> On 4/25/2018 8:20 PM, Chris Angelico wrote:
> >On Thu, Apr 26, 2018 at 10:11 AM, Yury Selivanov
> ><yselivanov.ml@gmail.com> wrote:
> >>Just yesterday this snippet was used on python-dev to show how great the
> >>new syntax is:
> >>
> >>           my_func(arg, buffer=(buf := [None]*get_size()), size=len(buf))
>
> What strikes me as awful about this example is that len(buf) is
> get_size(), so the wrong value is being named and saved.
> 'size=len(buf)' is, in a sense, backwards.

Terry is absolutely right, and I'm to blame for that atrocity. Mea
culpa.

​Perhaps a better spelling would be

    my_func(arg, buffer=[None]*(buflen := get_size()), size=buflen)


I know it is non productive and spamy (I promise, this is the last)  since `as` syntax is dead. In many cases, there is not much difference in perception between `:=` and `as`. But in several situations, like this one and as Ethan pointed up-thread - the expression first syntax makes obvious the intent and linearly readable:

my_func(arg, buffer=[None]*get_size() as buf, size=buf) 

In any case, it is rather an anti-pattern than a good example to follow.

p.s.: as Victor Stinner wrote on twitter that previously, there was a similar PEP in spirit - "PEP 379 -- Adding an Assignment Expression", which was withdrawn. May be it is worth to make a link to it in the current PEP.


With kind regards, 
-gdg