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

Eric V. Smith eric at trueblade.com
Wed Jun 27 15:01:03 EDT 2018

> On Jun 27, 2018, at 9:49 AM, Steven D'Aprano <steve at pearwood.info> wrote:
>> On Wed, Jun 27, 2018 at 08:00:20AM -0400, Eric V. Smith wrote:
>>> On 6/27/2018 7:08 AM, Chris Angelico wrote:
>>> It gets funnier with nested loops. Or scarier. I've lost the ability
>>> to distinguish those two.
>>> def test():
>>>    spam = 1
>>>    ham = 2
>>>    vars = [key1+key2 for key1 in locals() for key2 in locals()]
>>>    return vars
>>> Wanna guess what that's gonna return?
>> I'm not singling out Chris here, but these discussions would be easier 
>> to follow and more illuminating if the answers to such puzzles were 
>> presented when they're posed.
> You can just copy and paste the function into the interactive 
> interpreter and run it :-)

Not on my phone when I’m riding a bus, I can’t. I’m trying to more or less follow the discussion, but the “guess what this will do” aspect of the discussion makes it hard.


> But where's the fun in that? The point of the exercise is to learn first 
> hand just how complicated it is to try to predict the *current* scope 
> behaviour of comprehensions. Without the ability to perform assignment 
> inside them, aside from the loop variable, we've managed to avoid 
> thinking too much about this until now.
> It also demonstrates the unrealisticness of treating comprehensions as a 
> separate scope -- they're hybrid scope, with parts of the comprehension 
> running in the surrounding local scope, and parts running in an sublocal 
> scope.
> Earlier in this thread, Nick tried to justify the idea that 
> comprehensions run in their own scope, no matter how people think of 
> them -- but that's an over-simplification, as Chris' example above 
> shows. Parts of the comprehension do in fact behave exactly as the naive 
> model would suggest (even if Nick is right that other parts don't).
> As complicated and hairy as the above example is, (1) it is a pretty 
> weird thing to do, so most of us will almost never need to consider it; 
> and (2) backwards compatibility requires that we live with it now (at 
> least unless we introduce a __future__ import).
> If we can't simplify the scope of comprehensions, we can at least 
> simplify the parts that actually matters. What matters are the loop 
> variables (already guaranteed to be sublocal and not "leak" out of the 
> comprehension) and the behaviour of assignment expressions (open to 
> discussion).
> Broadly speaking, there are two positions we can take:
> 1. Let the current implementation of comprehensions as an implicit 
> hidden function drive the functionality; that means we duplicate the 
> hairiness of the locals() behaviour seen above, although it won't be 
> obvious at first glance.
> What this means in practice is that assignments will go to different 
> scopes depending on *where* they are in the comprehension:
>    [ expr   for x in iter1  for y in iter2  if cond   ...]
>    [ BBBBBB for x in AAAAAA for y in BBBBBB if BBBBBB ...]
> Assignments in the section marked "AAAAAA" will be in the local scope; 
> assignments in the BBBBBB sections will be in the sublocal scope. That's 
> not too bad, up to the point you try to assign to the same name in 
> AAAAAA and BBBBBB. And then you are likely to get confusing hard to 
> debug UnboundLocalErrors.
> 2. Or we can keep the current behaviour for locals and the loop 
> variables, but we can keep assignment expressions simple by ensuring 
> they always bind to the enclosing scope. Compared to the complexity of 
> the above, we have the relatively straight forward:
>    [ AAAAAA for x in AAAAAA for y in AAAAAA if AAAAAA ...]
> The loop variables continue to be hidden away in the invisible, implicit 
> comprehension function, where they can't leak out, while explicit 
> assignments to variables (using := or given or however it is spelled) 
> will always go into the surrounding local scope, like they do in every 
> other expression.
> Does it matter that the implementation of this requires an implicit 
> nonlocal declaration for each assignment? No more than it matters that 
> comprehensions themselves require an implicit function.
> And what we get out of this is simpler semantics at the Python level:
> - Unless previous declared global, assignment expressions always bind to 
> the current scope, even if they're inside a comprehension;
> - and we don't have to deal with the oddity that different bits of a 
> comprehension run in different scopes (unless we go out of our way to 
> use locals()); merely using assignment expressions will just work 
> consistently and simply, and loop variables will still be confined to 
> the comprehension as they are now.
> -- 
> Steve
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