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

Victor Stinner vstinner at redhat.com
Thu Jun 28 06:56:08 EDT 2018

✨ Congrats Nick on your 100 emails thread 😍! ✨ You won a virtual
piece of cake: 🍰

2018-06-22 16:22 GMT+02:00 Nick Coghlan <ncoghlan at gmail.com>:
> On 22 June 2018 at 02:26, Antoine Pitrou <solipsis at pitrou.net> wrote:
>> Indeed.  But, for a syntax addition such as PEP 572, I think it would be
>> a good idea to ask their opinion to teaching/education specialists.
>> As far as I'm concerned, if teachers and/or education specialists were
>> to say PEP 572 is not a problem, my position would shift from negative
>> towards neutral.
> I asked a handful of folks at the Education Summit the next day about it:
> * for the basic notion of allowing expression level name binding using
> the "NAME := EXPR" notation, the reactions ranged from mildly negative
> (I read it as only a "-0" rather than a "-1") to outright positive.
> * for the reactions to my description of the currently proposed parent
> local scoping behaviour in comprehensions, I'd use the word
> "horrified", and feel I wasn't overstating the response :)
> While I try to account for the fact that I implemented the current
> comprehension semantics for the 3.x series, and am hence biased
> towards considering them the now obvious interpretation, it's also the
> case that generator expressions have worked like nested functions
> since they were introduced in Python 2.4 (more than 13 years ago now),
> and comprehensions have worked the same way as generator expressions
> since Python 3.0 (which has its 10th birthday coming up in December
> this year).
> This means that I take any claims that the legacy Python 2.x
> interpretation of comprehension behaviour is intuitively obvious with
> an enormous grain of salt - for the better part of a decade now, every
> tool at a Python 3 user's disposal (the fact that the iteration
> variable is hidden from the current scope, reading the language
> reference [1], printing out locals(), using the dis module, stepping
> through code in a debugger, writing their own tracing function, and
> even observing the quirky interaction with class scopes) will have
> nudged them towards the "it's a hidden nested function" interpretation
> of expected comprehension behaviour.
> Acquiring the old mental model for the way comprehensions work pretty
> much requires a developer to have started with Python 2.x themselves
> (perhaps even before comprehensions and lexical closures were part of
> the language), or else have been taught the Python 2 comprehension
> model by someone else - there's nothing in Python 3's behaviour to
> encourage that point of view, and plenty of
> functional-language-inspired documentation to instead encourage folks
> to view comprehensions as tightly encapsulated declarative container
> construction syntax.
> I'm currently working on a concept proposal at
> https://github.com/ncoghlan/peps/pull/2 that's much closer to PEP 572
> than any of my previous `given` based suggestions: for already
> declared locals, it devolves to being the same as PEP 572 (except that
> expressions are allowed as top level statements), but for any names
> that haven't been previously introduced, it prohibits assigning to a
> name that doesn't already have a defined scope, and instead relies on
> a new `given` clause on various constructs that allows new target
> declarations to be introduced into the current scope (such that "if
> x:= f():" implies "x" is already defined as a target somewhere else in
> the current scope, while "if x := f() given x:" potentially introduces
> "x" as a new local target the same way a regular assignment statement
> does).
> One of the nicer features of the draft proposal is that if all you
> want to do is export the iteration variable from a comprehension, you
> don't need to use an assignment expression at all: you can just append
> "... given global x" or "... given nonlocal x" and export the
> iteration variable directly to the desired outer scope, the same way
> you can in the fully spelled out nested function equivalent.
> Cheers,
> Nick.
> [1] From https://docs.python.org/3.0/reference/expressions.html#displays-for-lists-sets-and-dictionaries:
> 'Note that the comprehension is executed in a separate scope, so names
> assigned to in the target list don’t “leak” in the enclosing scope.'
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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