[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 08:24:20 EDT 2018

On 25 June 2018 at 22:17, Nick Coghlan <ncoghlan at gmail.com> wrote:
> 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()

Oops, I didn't mean to say "exactly the same exception" here, as the
whole reason I'd settled on this example as the most cryptic one I'd
found so far was the fact that the doubly nested version *doesn't*
give you the same exception as the singly nested version: the version
without the comprehension throws UnboundLocalError instead.

However, the resolution is the same either way: either 'x' has to be
declared as 'nonlocal x' in 'middle', or else it has to be passed in
to 'middle' as a parameter.


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

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