[Python-ideas] A comprehension scope issue in PEP 572

[Steven D'Aprano]

On Mon, May 07, 2018 at 10:38:09AM -0700, Guido van Rossum wrote:

> The only solution that makes sense to me is Steven's (2). (1) is what the
> PEP currently says and what Tim doesn't want; (3) has no precedent
> (function defaults don't really work like this) and just gets my hackles
> all up. (I can't even tell if Steven meant it as a serious proposal.)

It doesn't get my hackles up as much as you, but its not really what I 
want. It's just a compromise between what I *don't* want (1), which 
fails to solve the original motivating example that started this 
discussion, and what Chris was pushing back against (2).


> There's one corner case (again) -- class scopes. If the containing scope is
> a function, everything's fine, we can use the existing closure machinery.
> If the containing scope is global, everything's fine too, we can treat it
> as a global. But if the containing scope is a class, we can't easily extend
> the current machinery. But this breakage is similar to the existing
> breakage with comprehensions in class scope that reference class variables:
[...]
> I propose to punt on this case. If we want to fix it we can fix it in a
> number of ways and the fix can easily apply to both getting and setting --
> but this is a separate fix (and we should take it out of PEP 572).

+1

Whether the current class behaviour is "broken" or desirable or 
somewhere in between, it is what we have now and its okay if binding 
expressions have the same behaviour.


-- 
Steve