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

Paul Moore p.f.moore at gmail.com
Mon Jun 25 08:28:16 EDT 2018

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

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