[Python-ideas] PEP 572: Assignment Expressions (post #4)

[Chris Angelico]

On Sat, Apr 14, 2018 at 12:36 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 13 April 2018 at 22:35, Chris Angelico <rosuav at gmail.com> wrote:
>> On Fri, Apr 13, 2018 at 10:22 PM, Steven D'Aprano <steve at pearwood.info> wrote:
>>> On Wed, Apr 11, 2018 at 11:50:44PM +1000, Chris Angelico wrote:
>>>
>>>> > Previously, there was an alternative _operator form_  `->`  proposed by
>>>> > Steven D'Aprano. This option is no longer considered? I see several
>>>> > advantages with this variant:
>>>> > 1. It does not use `:` symbol which is very visually overloaded in Python.
>>>> > 2. It is clearly distinguishable from the usual assignment statement and
>>>> > it's `+=` friends
>>>> > There are others but they are minor.
>>>>
>>>> I'm not sure why you posted this in response to the open question, but
>>>> whatever. The arrow operator is already a token in Python (due to its
>>>> use in 'def' statements) and should not conflict with anything;
>>>> however, apart from "it looks different", it doesn't have much to
>>>> speak for it.
>>>
>>> On the contrary, it puts the expression first, where it belongs
>>> *semi-wink*.
>>
>> The 'as' syntax already has that going for it. What's the advantage of
>> the arrow over the two front-runners, ':=' and 'as'?
>
> I stumbled across
> https://www.hillelwayne.com/post/equals-as-assignment/ earlier this
> week, and I think it provides grounds to reconsider the suitability of
> ":=", as that symbol has historically referred to *re*binding an
> already declared name. That isn't the way we're proposing to use it
> here: we're using it to mean both implicit local variable declaration
> *and* rebinding of an existing name, the same as we do for "=" and
> "as".

I'm not bothered by that. Assignment semantics vary from one language
to another; the fact that Python marks as local anything that's
assigned to is independent of the way you assign to it. ("print(x)"
followed by "for x in ..." is going to bomb with UnboundLocalError,
for instance.) If Python had any form of local variable declarations,
it wouldn't change the behaviour of the := operator.

ChrisA