On 2022-06-17 07:53, Chris Angelico wrote:
On Sat, 18 Jun 2022 at 00:21, Paul Moore <p.f.mo...@gmail.com> wrote:
On Fri, 17 Jun 2022 at 14:15, Chris Angelico <ros...@gmail.com> wrote:
>
> There are several ways to make this clearly sane.
>
> # Clearly UnboundLocalError
> def frob(n=>len(items), items=>[]):
Um, I didn't see that as any more obvious than the original example. I
guess I can see it's UnboundLocalError, but honestly that's not
obvious to me.
Question: Is this obvious?
def f():
x, x[0] = [2], 3
print(x)
def boom():
x[0], x = 3, [2]
# raises UnboundLocalError
Personally, I don't care so much whether the behavior is "obvious" or
even "confusing" as about whether it's defined. Both of those examples
have defined behavior in Python. That's better than having undefined
behavior where either of them might do different things depending on the
implementation.
It's unfair to say that I "refuse to assign semantics" as if I'm
permitting literally any behaviour. All I'm doing is saying that the
UnboundLocalError is optional, *at this stage*. There have been far
less-defined semantics that have remained in the language for a long
time, or cases where something has changed in behaviour over time
despite not being explicitly stated as implementation-defined. Is this
legal?
def f():
x = 1
global x
Does Python mandate whether this is legal or not? If so, how far back
in Python's history has it been defined?
If it doesn't mandate it, then it would be better if it did. In my
view, none of your arguments about "some things are undefined" are
actually providing any positive support for the idea that the behavior
in question should remained undefined in the PEP.
I think the PEP would benefit from a fully explicit definition of
exactly when and how the late-bound defaults would be evaluated. For
instance, by demonstrating an "unrolling" into something paralleling
existing Python code. Like:
def f(a=>items[0], items=[]):
# body
is equivalent to:
def f(a=FakeDefault, items=[]):
a = items[0]
# body
. . . or whatever. I mean, it's up to you as the PEP author to decide
what semantics you want. But I think the PEP should be fully explicit
about the order of evaluation of everything. I don't see any benefit to
leaving it ambiguous.
This is all academic to me, however, since even if you did that I still
wouldn't support the PEP for various more basic reasons that I've
mentioned in the earlier iteration of this thread.
--
Brendan Barnwell
"Do not follow where the path may lead. Go, instead, where there is no
path, and leave a trail."
--author unknown
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