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

Guido van Rossum guido at python.org
Mon Jul 2 14:19:53 EDT 2018

Thank you all. I will accept the PEP as is. I am happy to accept
*clarification* updates to the PEP if people care to submit them as PRs to
the peps repo (https://github.com/python/peps), and that could even (to
some extent) include summaries of discussion we've had, or outright
rejected ideas. But even without any of those I think the PEP is very clear
so I will not wait very long (maybe a week).

On Mon, Jul 2, 2018 at 8:38 AM Steven D'Aprano <steve at pearwood.info> wrote:

> On Wed, Jun 27, 2018 at 07:29:52PM -0500, Tim Peters wrote:
> [...]
> > For example, if the name is declared "global" in the outer scope, you'll
> > get a compile-time error if you try to declare it "nonlocal" in the
> > contained scope.  "parentlocal" adjusts its meaning accordingly,
> becoming a
> > synonym for "global" in that specific case.
> "Parentlocal" is only a thing if we buy into the paradigm that inside
> comprehensions is a separate "local". And *that* is only true under
> two circumstances:
> - if you are utterly immersed in the implementation of comprehensions
>   as invisible, implicit functions;
> - or if you start from the premise that comprehensions ought to
>   encapsulate not just the loop variable, but anything else as well.
> But experimenting with locals() inside comprehensions shows that
> comprehension-scope *isn't* a well-defined thing. It already bleeds out
> of the comprehension, and so would some (but only some!) assignment
> expressions.
> Instead, if we start from the premise that comprehensions (like any
> other expression) run in the current scope, then there is no need to
> invent a term "parentlocal". There's just the usual LEGB scopes, plus
> class (which people usually forget).
> With no sublocal scopes (a term we never even had prior to this PEP)
> assignments inside the comprehension are no more special than
> assignments inside any other expression. They bind in the current scope,
> same as always, and keep the sensible identity that these two
> expressions are exactly equivalent in their visible semantics:
>     [x:=0, x:=1, x:=2]
>     [x:=i for i in (0, 1, 2)]
> including assignments.
> What about the loop variable?
> They ARE special, which is completely justified by the Zen:
> Although practicality beats purity.
> We can take a series of ever-more-detailed explanations, starting from
> the highest "bird's eye" view and gradually dropping further into the
> murky details of the implementation when, and if, required:
> - assignment within comprehensions is no different from assignment
>   in any other expression, it occurs in the local scope;
> - loop variables? they're a special case, for good reason, and are
>   encapsulated inside the comprehension;
> - how? they're hidden in an implicit, invisible scope, same as .0
>   the implicit, invisible iterator object;
> - oh, you didn't know about the .0 variable? well forget about it,
>   it's an undocumented implementation detail, just like the invisible,
>   implicit function used by comprehensions;
> - oh, you didn't know about that either? read the source code.
> Only the first two levels of explanation are part of Python the
> language. The rest is CPython implementation.
> --
> Steve
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--Guido van Rossum (python.org/~guido)
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