On Wed, Oct 27, 2021 at 12:50 PM Rob Cliffe <[email protected]> wrote:
>
>
>
> On 27/10/2021 00:50, Chris Angelico wrote:
> > On Wed, Oct 27, 2021 at 10:38 AM Rob Cliffe <[email protected]>
> > wrote:
> >
> >> There has been support for evaluating all early-bound defaults before
> >> all late-bound defaults. I have been persuaded that this is a
> >> reasonable option. AFAICS there would be little practical difference
> >> from straight left-to-right evaluation of defaults, since assigning an
> >> early-bound default should not have a side effect. So it could even be
> >> an implementation choice.
> > If it's an implementation choice, then it'll mean that code is legal
> > on some interpreters and not others. Whether that's a major problem or
> > not I don't know, but generally, when you can't depend on your code
> > working on all interpreters, it's not best practice (eg
> > "open(fn).read()" isn't recommended, even if it happens to close the
> > file promptly in CPython).
> >
> > ChrisA
> I guess I wasn't clear. I'm not sure if you understood what I meant to
> say, or not.
> First I should have said "binding" rather than "evaluating".
> What I meant was that in cases like
> def f(a=earlydefault1, b:= latedefault2, c=earlydefault3,
> d:=latedefault4):
> it makes no real difference if the bindings are done in the order
> a, c, b, d (early ones before late ones)
> or
> a, b, c, d (strict left-to-right)
> since binding an early default should have no side effects, so (I
> thought, wrongly) it could be an implementation detail.
> Of course there IS a difference: it allows late default values to refer
> to subsequent early default values, e.g. in the example above
> `latedefault2' could refer to `c`. So yes, then that code would be
> legal on some interpreters and not others, as you said. If you
> understood exactly what I meant, I apologise.
Yep, that's precisely the distinction that matters: whether it's legal
to refer to parameters further to the right. If we consider tuple
unpacking as an approximate parallel:
def f():
a = [10,20,30]
i, a[i] = 1, "Hi"
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