[Python-Dev] assignment expressions: an alternative proposal

[Yury Selivanov]

On Tue, Apr 24, 2018 at 11:31 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 25 April 2018 at 00:54, Eric Snow <ericsnowcurrently at gmail.com> wrote:
>> Regardless, your 3 rules would benefit either syntax.  Nick may have a
>> point that the rules might be an excessive burden, but I don't think
>> it's too big a deal since the restrictions are few (and align with the
>> most likely usage) and are limited to syntax so the compiler will be
>> quick to point mistakes.
>
> I think the "single name target only" rule should be in place no
> matter the syntax for the name binding operator itself.
>
> I don't mind too much either way on the mandatory parentheses question
> (it's certainly an easy option to actively discourage use of binding
> expressions as a direct alternative to assignment statements, but as
> with the single-name-only rule, it's independent of the choice of
> syntax)

Mandatory parenthesis around `(name := expr)` would at least solve the
problem of users mixing up '=' and ':=' in statements.

>
> I *do* think the "no name rebinding except in a while loop header"
> restriction would be annoying for the if/elif use case and the while
> use case:
>
>     while (item = get_item()) is not first_delimiter:
>         # First processing loop
>     while (item = get_item()) is not second_delimiter:
>         # Second processing loop
>     # etc...
>
>     if (target = get_first_candidate()) is not None:
>         ...
>     elif (target = get_second_candidate()) is not None:
>         ...
>     elif (target = get_third_candidate()) is not None:
>         ...

Yes, it would force users to come up with better names *iff* they want
to use this new sugar:

  if (first_target = get_first_candidate()) ...
  elif (second_target = get_second_candidate()) ...

Yury