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.
With inspect.Parameter, you already have a repr and str that show that
it's late-bound, exactly like you suggest. What advantage is there
from using a dedicated sentinel instead of Ellipsis?
It wouldn't be too hard to change the inspect module, but I'd need to know why.
ChrisA
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