On Tue, Oct 26, 2021 at 2:05 PM Paul Moore <p.f.mo...@gmail.com> wrote:
> On Tue, 26 Oct 2021 at 16:48, Eric V. Smith <e...@trueblade.com> wrote:
> >
> > And also the "No Loss of Abilities If Removed" section sort of applies
> > to late-bound function arguments: there's nothing proposed that can't
> > currently be done in existing Python. I'll grant you that they might
> > (might!) be more newbie-friendly, but I think the bar is high for
> > proposals that make existing things doable in a different way, as
> > opposed to proposals that add new expressiveness to the language.
>
> One issue with not having an introspection capability, which has been
> bothering me but I've not yet had the time to come up with a complete
> example, is the fact that with this new feature, you have functions
> where there's no way to express "just use the default" without knowing
> what the default actually *is*.
>
> Take for example
>
> def f(a, b=None):
> if b is None:
> b = len(a)
> ...
>
> def g(a, b=>len(a)):
> ...
>
> Suppose you want to call f as follows:
>
> args = [
> ([1,2,3], 2),
> ([4,5,6], None),
> ([7,8,9], 4),
> ]
>
> for a, b in args:
> f(a, b)
>
> That works fine. But you cannot replace f by g, because None doesn't
> mean "use the default", and in fact by design there's *nothing* that
> means "use the default" other than "know what the default is and
> supply it explicitly". So if you want to do something similar with g
> (allowing the use of None in the list of tuples to mean "use the
> default"), you need to be able to introspect g to know what the
> default is. You may also need to manipulate first-class "deferred
> expression" objects as well, just to have something you can return as
> the default value (you could return a string and require the user to
> eval it, I guess, but that doesn't seem particularly user-friendly...)
>
> I don't have a good solution for this, unfortunately. And maybe it's
> something where a "good enough" solution would be sufficient. But
> definitely, it should be discussed in the PEP so what's being proposed
> is clear.
>
> Paul
>
I had drafted an entire reply last night trying to explain this same
concern; Paul Moore, you did a better job. Just want to say I agree: being
able to say "I want the defaults" seems like a useful ability.
But on the other hand, we also must recognize that right now there isn't
really a great, UNIVERSAL way to say "I want the defaults". It varies from
API to API. Many times getting the default means passing None, but many it
is True, or False, or a MISSING sentinel. You have to read the docs to find
out.
Taking PM's examples of f and g, the way you'd have to dynamically call g
this way be:
for arg_group in args:
g(*arg_group)
But this is going to be limiting because maybe you could also have a
function j, like this:
def j(a, b=None, c=None):
if b is None:
b = len(a)
...
args = [
([1,2,3], 2, "spam"),
([4,5,6], None, "eggs"),
([7,8,9], 4, "bacon"),
]
for a, b, c in args:
j(a, b, c)
But with function k below, where the b parameter is deferred, you can't get
the default b parameter by dynamically unpacking some values; you would
have to pass c as a kwd arg:
def k(a, b=>len(a), c=None):
...
Seems like it would be- needed? convenient?- to be able to "ask" for the
default in a dynamic way... Am I making more of this than is justified?
---
Ricky.
"I've never met a Kentucky man who wasn't either thinking about going home
or actually going home." - Happy Chandler
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