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

Steve Holden steve at holdenweb.com
Mon Jun 25 17:13:52 EDT 2018

On Mon, Jun 25, 2018 at 8:37 PM, Terry Reedy <tjreedy at udel.edu> wrote:

> On 6/24/2018 7:25 PM, Guido van Rossum wrote:
>> I'd wager that the people who might be most horrified about it
> the (b) scoping rule change
> would be people who feel strongly that the change to the
>> comprehension scope rules in Python 3 is a big improvement,
> I might not be one of those 'most horrified' by (b), but I increasingly
> don't like it, and I was at best -0 on the comprehension scope change. To
> me, iteration variable assignment in the current scope is a non-problem.
> So to me the change was mostly useless churn.  Little benefit, little
> harm.  And not worth fighting when others saw a benefit.
> However, having made the change to nested scopes, I think we should stick
> with them.  Or repeal them.  (I believe there is another way to isolate
> iteration names -- see  below).  To me, (b) amounts to half repealing the
> nested scope change, making comprehensions half-fowl, half-fish chimeras.

> --
> Terry Jan Reedy
> ​I'd like to ask: how many readers of ​

​this email have ever deliberately taken advantage of the limited Python 3
scope in comprehensions and generator expressions to use what would
otherwise be a conflicting local variable name?​

I appreciate that the scope limitation can sidestep accidental naming
errors, which is a good thing.

Unfortunately, unless we anticipate Python 4 (or whatever) also making for
loops have an implicit scope, I am left wondering whether it's not too
large a price to pay. After all, special cases aren't special enough to
break the rules, and unless the language is headed towards implicit scope
for all uses of "for" one could argue that the scope limitation is a
special case too far. It certainly threatens to be yet another confusion
for learners, and while that isn't the only consideration, it should be
given due weight.
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