On Wed, Oct 27, 2021 at 5:05 AM 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. >
Wouldn't cases like this be most likely to use *args and/or **kwargs? Simply omitting the argument from those would mean "use the default". Or am I misunderstanding your example here? ChrisA _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/ONZR547NULZKF74H7VRVXTTWRVFZ324Y/ Code of Conduct: http://python.org/psf/codeofconduct/
