[Python-ideas] PEP 572: Statement-Local Name Bindings, take three!

[Nick Coghlan]

On 9 April 2018 at 01:01, Steven D'Aprano <steve at pearwood.info> wrote:
> On Sun, Apr 08, 2018 at 09:25:33PM +1000, Nick Coghlan wrote:
>
>> I was writing a new stdlib test case today, and thinking about how I
>> might structure it differently in a PEP 572 world, and realised that a
>> situation the next version of the PEP should discuss is this one:
>>
>>     # Dict display
>>     data = {
>>         key_a: 1,
>>         key_b: 2,
>>         key_c: 3,
>>     }
>>
>>     # Set display with local name bindings
>>     data = {
>>         local_a := 1,
>>         local_b := 2,
>>         local_c := 3,
>>    }
>
> I don't understand the point of these examples. Sure, I guess they would
> be legal, but unless you're actually going to use the name bindings,
> what's the point in defining them?

That *would* be the point.

In the case where it occurred to me, the actual code I'd written
looked like this:

   curdir_import = ""
   curdir_relative = os.curdir
   curdir_absolute = os.getcwd()
   all_spellings = [curdir_import, curdir_relative, curdir_absolute]

(Since I was testing the pydoc CLI's sys.path manipulation, and wanted
to cover all the cases).


>> I don't think this is bad (although the interaction with dicts is a
>> bit odd), and I don't think it counts as a rationale either, but I do
>> think the fact that it becomes possible should be noted as an outcome
>> arising from the "No sublocal scoping" semantics.
>
> If we really wanted to keep the sublocal scoping, we could make
> list/set/dict displays their own scope too.
>
> Personally, that's the only argument for sublocal scoping that I like
> yet: what happens inside a display should remain inside the display, and
> not leak out into the function.
>
> So that has taken me from -1 on sublocal scoping to -0.5 if it applies
> to displays.

Inflicting the challenges that comprehensions have at class scope on
all container displays wouldn't strike me as a desirable outcome (plus
there's also the problem that full nested scopes are relatively
expensive at runtime).

Cheers,
Nick.

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