On Sat, Dec 4, 2021 at 2:52 PM Rob Cliffe via Python-ideas
<python-ideas@python.org> wrote:
>
>
>
> On 04/12/2021 01:06, Chris Angelico wrote:
> > On Sat, Dec 4, 2021 at 11:59 AM Rob Cliffe via Python-ideas
> > <python-ideas@python.org> wrote:
> >>
> >>
> >> On 03/12/2021 22:38, Chris Angelico wrote:
> >>> On Sat, Dec 4, 2021 at 8:18 AM Rob Cliffe via Python-ideas
> >>> <python-ideas@python.org> wrote:
> >>>>
> >>>> On 03/12/2021 19:32, Adam Johnson wrote:
> >>>>> The first unwelcome surprise was:
> >>>>>
> >>>>> >>> def func(a=>[]):
> >>>>> ... return a
> >>>>> ...
> >>>>>
> >>>>> >>> import inspect
> >>>>> >>> inspect.signature(func).parameters['a'].default
> >>>>> Ellipsis
> >>>>>
> >>>>> Here the current behaviour of returning `Ellipsis` is very unfortunate,
> >>>>> and I think could lead to a lot of head scratching — people wondering
> >>>>> why they are getting ellipses in their code, seemingly from nowhere.
> >>>>> Sure, it can be noted in the official documentation that `Ellipsis` is
> >>>>> used as the indicator of late bound defaults, but third-party resources
> >>>>> which aim to explain the uses of `Ellipsis` would (with their current
> >>>>> content) leave someone clueless.
> >>>>>
> >>>> +1.
> >>>> This may be a very naive question, apologies if it's nonsense.
> >>>> Instead of Ellipsis, would it be possible to have a built-in LateBound
> >>>> class and use instances of that class instead of Ellipsis?
> >>>> The __str__ method of the inspect.Parameter class could be modified to
> >>>> return something like
> >>>> "a=>[]"
> >>>> (or whatever syntax is adopted for specifying late-bound defaults) in
> >>>> such cases.
> >>>> The __repr__ and __str__ methods of a LateBound object could return
> >>>> something like, respectively,
> >>>> "LateBound('[]')"
> >>>> "[]"
> >>>> I am sure there is code that uses inspect.signature that would be
> >>>> broken, but isn't that inevitable anyway?
> >>>>
> >>> That's a possibility for the inspect module. For the core language -
> >>> and therefore for anything that directly inspects the function's
> >>> dunders - it's much more efficient to use a well-known object.
> >> OK.
> >> I'm guessing that by "well-known" you mean pre-existing. Python has a
> >> number of built-in singleton objects (None, True, False, Ellipsis).
> > Mainly by "well-known" I mean "not private to any particular module",
> > so those four you mention are all well-known, but a singleton as part
> > of the inspect module would be a pain, since the core language would
> > have to import that (or it would have to be magically created).
> >
> >> What about adding a new one called LateBound (or other bikeshed colour)?
> > What would be gained? You would still be able to use LateBound as an
> > early-bound default, so you would still need the same dual check.
> I'm struggling here. Yes you could use LateBound as an early-bound
> default (or as a parameter value to explicitly pass to a function) but
> ISTM that such usages would be perverse.
> I've a gut feeling that a solution can be found (to avoid the "Ellipsis
> from nowhere" problem) but I can't put my finger on it. Maybe explicitly
> specifying LateBound could be an error, perhaps even a SyntaxError?
> Help, please!
It would be extremely odd if something could fail due to the precise
object chosen.
x = LateBound
def f(x=x): pass # SyntaxError? Runtime error?
Awkward. Problematic. And it wouldn't buy you anything anyway -
consumers of inspect.Signature would still have to be aware of this
special object, so you still have the same problems.
ChrisA
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