On Sun, Oct 31, 2021 at 11:47 PM Eric V. Smith <e...@trueblade.com> wrote:
>
> On 10/30/2021 10:08 PM, Christopher Barker wrote:
> > I'm a bit confused as to why folks are making pronouncements about
> > their support for this PEP before it's even finished, but, oh well.
> I think it's safe to say people are opposed to the PEP as it current
> stands, not in it's final, as yet unseen, shape. But I'm willing to use
> other words that "I'm -1 on PEP 671". You can read my opposition as "as
> it currently stands, I'm -1 on PEP 671".
> > As for what seems like one major issue:
> >
> > Yes, this is a kind of "deferred" evaluation, but it is not a general
> > purpose one, and that, I think, is the strength of the proposal, it's
> > small and specific, and, most importantly, the scope in which the
> > expression will be evaluated is clear and simple.
>
> And to me and others, what you see as a strength, and seem opposed to
> changing, we see as a fatal flaw.
>
> What if the walrus operator could only be used in "for" loops? What if
> f-strings were only available in function parameters? What if decorators
> could only be used on free-standing functions, but not on object methods?
>
> In all of these cases, what could be a general-purpose tool would have
> been restricted to one specific context. That would make the language
> more confusing to learn. I feel you're proposing the same sort of thing
> with late-bound function argument defaults. And I think it's a mistake.
Deferred expressions are not the same as late-bound argument defaults.
What is the correct behaviour here?
def foo(a=>[1,2,3], b=>len(a)):
a.append(4)
print(b)
And what is the correct behaviour here?
def foo(a=defer [1,2,3], b=defer len(a)):
a.append(4)
print(b)
When is 'a' evaluated and the list constructed? When is the length
calculated and stored in 'b'? With argument defaults, it's clear: this
happens as the function is called. (See other thread for a subtlety
about whether this happens during frame construction or as the
function-proper begins execution, but that is a minor distinction that
doesn't affect non-generators very much.) With deferreds, the usual
expectation is that they are evaluated on usage, which could be a very
different point.
Late-bound defaults are NOT "deferreds but limited to function
headers". They are quite different. You can think of them as a sort of
deferred expression if that helps, but they're not a specialization of
a more general feature.
ChrisA
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