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

Nick Coghlan ncoghlan at gmail.com
Mon Jun 25 08:17:09 EDT 2018

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()


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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