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
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