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

Ivan Pozdeev vano at mail.mipt.ru
Sun Jun 24 10:24:12 EDT 2018

On 24.06.2018 9:53, Chris Angelico wrote:
> 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.

My 2c.
An expression is intuitively thought to be self-contained i.e. without 
side effects.
if I write `a=b+1`, I'm not expecting it to do anything except assigning 

Expressions with side effects has long since proven to be problematic 
because of the implicit (thus hard to see and track) links they create
(and because the result depends on the order of evaluation).
Moreover, Python's other design elements have been consistently 
discouraging expressions with side effects, too (e.g. mutator methods 
intentionally return None instead of the new value, making them useless 
in expressions), so the proposition is in direct conflict with the 
language's design.

Assignment expressions are a grey area: they carry the full implications 
of expressions with side effects described above, but their side effect 
is their only effect, i.e. they are explicit and prominent about the 
"evil" they do.

>> 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
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