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. In the
current implementation, I've kept inspect.signature() consistent with
the dunders, but if there's good reason to change, I wouldn't be
averse to it. But I would need feedback from people who make heavy use
of inspect.signature, as I don't do much with it other than the basics
of help().
Everything in the reference implementation that isn't part of the PEP
should be considered provisional at best :)
ChrisA
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