[Python-Dev] Informal educator feedback on PEP 572 (was Re: 2018 Python Language Summit coverage, last part)

[Chris Angelico]

On Sun, Jun 24, 2018 at 4:33 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 24 June 2018 at 15:56, Steven D'Aprano <steve at pearwood.info> wrote:
>> On Sun, Jun 24, 2018 at 02:33:59PM +1000, Nick Coghlan wrote:
>>
>>> Given that PEP 572 *is* proposing implicit comprehension state export,
>>
>> "Implicit" and "explicit" are two terms which often get misused to mean
>> "I don't like it" and "I do like it".
>>
>> Making the intentional choice to use an assignment expression is not
>> really "implicit" in any meaningful sense.
>
> No, it's actually implicit: there's an extra "global NAME" or
> "nonlocal NAME" in the equivalent code for a comprehension that isn't
> there in the as-written source code, and doesn't get emitted for a
> regular assignment expression or for the iteration variable in a
> comprehension - it only shows up due to the defined interaction
> between comprehensions and assignment expressions.

The implicit "nonlocal NAME" is only because there is an equally
implicit function boundary. Why is there a function boundary marked by
square brackets? It's not saying "def" or "lambda", which obviously
create functions. It's a 'for' loop wrapped inside a list display.
What part of that says "hey, I'm a nested function"?

So if there's an implicit function, with implicit declaration of a
magical parameter called ".0", why can't it have an equally implicit
declaration that "spam" is a nonlocal name?

ChrisA