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[Python-Dev] Accepting PEP 572, Assignment Expressions

Nick Coghlan ncoghlan at gmail.com
Sun Jul 15 01:32:05 EDT 2018 
On 13 July 2018 at 23:36, 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's in the same category as disallowing assignment to attributes and
subscripts in other name binding operations like "def" and "class"
statements: there isn't a technical limitation preventing it, there's
a design decision that we currently believe allowing it would reduce
code clarity rather than increase it. It's also the case that unlike
local name bindings, assigning to attributes and subscripts as an
expression can already be formulated as a helper function with side
effects.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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