[Python-Dev] PEP 563: Postponed Evaluation of Annotations

[Guido van Rossum]

IMO the inability of referencing class-level definitions from annotations
on methods pretty much kills this idea.

On Thu, Nov 2, 2017 at 11:27 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 3 November 2017 at 04:39, Jukka Lehtosalo <jlehtosalo at gmail.com> wrote:
> >> > * forward references: when a type hint contains names that have not
> been
> >> >   defined yet, that definition needs to be expressed as a string
> >> >   literal;
> >>
> >> After all the discussion, I still don't see why this is an issue.
> >> Strings makes perfectly fine forward references. What is the problem
> >> that needs solving? Is this about people not wanting to type the leading
> >> and trailing ' around forward references?
> >
> >
> > Let's make a thought experiment. What if every forward reference would
> > require special quoting? Would Python programmers be happy with this?
> Say,
> > let's use ! as a suffix to mark a forward reference. They make perfectly
> > fine forward references. They are visually pretty unobtrusive (I'm not
> > suggesting $ or other ugly perlisms):
> >
> > def main():
> >     args = parse_args!()  # A forward reference
> >     do_stuff!(args)  # Explicit is better than implicit
> >
> > def parse_args():
> >     ...
> >
> > def do_stuff(args):
> >     ...
> >
> > Of course, I'm not seriously proposing this, but this highlights the fact
> > that in normal code forward references "just work" (at least usually),
> and
> > if we'd require a special quoting mechanism to use them anywhere, Python
> > would look uglier and more inconsistent. Nobody would be happy with this
> > change, even though you'd only have to type a single ! character extra --
> > that's not a lot work, right?
> >
> > I think that the analogy is reasonable.
>
> I think it also makes a pretty decent argument that pushing function
> annotations into implicit lambda expressions will be easier to explain
> to people than converting them into strings, and then having to
> explain an entirely new complex set of name resolution rules.
>
> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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-- 
--Guido van Rossum (python.org/~guido)
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