[Python-ideas] should `dict` supply a default `__missing__` implementation?

[Neil Girdhar]

Okay, that makes sense, but why isn't __missing__ in UserDict?

On Tue, Jul 5, 2016 at 6:04 PM Guido van Rossum <guido at python.org> wrote:

> What kind of question is that? If you subclass MutableMapping the
> whole feature doesn't exist (since you're not subclassing dict). It
> *only* exists for subclasses of dict.
>
> On Tue, Jul 5, 2016 at 12:18 PM, Neil Girdhar <mistersheik at gmail.com>
> wrote:
> > But neither UserDict nor MutableMapping defines __missing__ ?  What is a
> > subclasser supposed to do?
> >
> > On Wednesday, June 29, 2016 at 4:59:03 PM UTC-4, Guido van Rossum wrote:
> >>
> >> UserDict is superseded by MutableMapping.
> >>
> >> I don't think we should make dict a kitchen sink class. I also don't
> >> think we should particularly encourage subclassing it. So -1 on adding
> >> dict.__missing__.
> >>
> >> On Wed, Jun 29, 2016 at 12:30 PM, Ethan Furman <et... at stoneleaf.us>
> wrote:
> >> > On 06/29/2016 12:09 PM, Guido van Rossum wrote:
> >> >
> >> >> So providing the comprehensive base class is up to the user, not up
> to
> >> >> the stdlib. Is that such a big deal?
> >> >
> >> >
> >> > No, it's not.  But it makes for a better user experience if the base
> >> > class
> >> > has the __missing__ method that raises a KeyError already.
> >> >
> >> > Didn't we add a UserDict that could be subclassed primarily because
> >> > subclassing dict directly was such a poor user experience?
> >> >
> >> > If adding __missing__ to dict is huge (performance hit?), we don't do
> >> > it.
> >> > If it's not, I think we should.  Maybe add it to UserDict if
> performance
> >> > is
> >> > a concern?
> >> >
> >> >
> >> > --
> >> > ~Ethan~
> >> > _______________________________________________
> >> > Python-ideas mailing list
> >> > Python... at python.org
> >> > https://mail.python.org/mailman/listinfo/python-ideas
> >> > Code of Conduct: http://python.org/psf/codeofconduct/
> >>
> >>
> >>
> >> --
> >> --Guido van Rossum (python.org/~guido)
> >> _______________________________________________
> >> Python-ideas mailing list
> >> Python... at python.org
> >> https://mail.python.org/mailman/listinfo/python-ideas
> >> Code of Conduct: http://python.org/psf/codeofconduct/
>
>
>
> --
> --Guido van Rossum (python.org/~guido)
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20160705/fe441291/attachment.html>