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

[Paul Moore]

On 25 June 2018 at 13:17, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 25 June 2018 at 09:25, Guido van Rossum <guido at python.org> wrote:
>> A "neutral" argument about (b) is that despite the "horrified" reactions
>> that Nick saw, in practice it's going to confuse very few people (again, due
>> to my point about Python's scope rules). I'd wager that the people who might
>> be most horrified about it would be people who feel strongly that the change
>> to the comprehension scope rules in Python 3 is a big improvement, and who
>> are familiar with the difference in implementation of comprehensions (though
>> not generator expressions) in Python 2 vs. 3.
>
> FWIW, the most cryptic parent local scoping related exception I've
> been able to devise so far still exhibits PEP 572's desired "Omitting
> the comprehension scope entirely would give you the same name lookup
> behaviour" semantics:
>
>     >>> def outer(x=1):
>     ...     def middle():
>     ...         return [x := x + i for i in range(10)]
>     ...     return middle()
>     ...
>     >>> outer()
>     Traceback (most recent call last):
>         ...
>     NameError: free variable 'x' referenced before assignment in enclosing scope
>
> It isn't the parent local scoping, or even the assignment expression,
> that's at fault there, since you'd get exactly the same exception for:
>
>     def outer(x=1):
>         def middle():
>             x = x +1
>             return x
>         return middle()
>

Once again offering an "intuition" based response:

1. That definition of outer() is very complicated, I don't *expect* to
understand it without checking the details. So the NameError is simply
"hmm, wonder what triggered that?" not "OMG that's not what I'd
expect!" :-)
2. Given that your version with no assignment expression or
comprehension exhibits the same behaviour, I'm not sure what your
argument is here anyway...

Paul