[Python-Dev] (name := expression) doesn't fit the narrative of PEP 20

[Kirill Balunov]

2018-04-26 13:20 GMT+03:00 Steve Holden <steve at holdenweb.com>:

> On Thu, Apr 26, 2018 at 8:56 AM, Steven D'Aprano <steve at 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 at 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was withdrawn.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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-dev/attachments/20180426/83ebb98a/attachment-0001.html>