[Python-Dev] Accepting PEP 572, Assignment Expressions

[Chris Angelico]

On Fri, Jul 13, 2018 at 11:36 PM, Random832 <random832 at fastmail.com> wrote:
> On Wed, Jul 11, 2018, at 20:10, Guido van Rossum wrote:
>> As anticippated, after a final round of feedback I am hereby accepting PEP
>> 572, Assignment Expressions: https://www.python.org/dev/peps/pep-0572/
>
> I know everyone else is probably sick of discussing this (I somehow completely missed the discussion until it was almost done) but I have a question...
>
> Why does this not allow assignment to attributes and subscripts? I think this is, at least, surprising enough that there should be a rationale section explaining it in the PEP. As it is, it's barely even explicitly stated, other than the use of 'NAME' in a few places and an offhand mention "and [the assignment statement] can assign to attributes and subscripts."
>

It was discussed at some length, yes. Assignment to arbitrary targets
would also mean permitting iterable unpacking, which is not desired
("x, y := 3, 4"??), and there weren't enough use-cases for
attribute/item assignment to justify creating a rule of "you can
assign to any single target, but can't unpack". In the future, if such
use-cases are found, the grammar can be expanded.

ChrisA