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

[Neil Girdhar]

Is it deprecated?

I've seen this question a lot on stackoverflow:
http://stackoverflow.com/questions/7148419/subclass-dict-userdict-dict-or-abc

http://stackoverflow.com/questions/2390827/how-to-properly-subclass-dict-and-override-getitem-setitem
http://stackoverflow.com/questions/10901048/i-want-to-subclass-dict-and-set-default-values

I still have no idea what the right answer is.

On Tue, Jul 5, 2016 at 6:11 PM Guido van Rossum <guido at python.org> wrote:

> Because you shouldn't be using UserDict.
>
> On Tue, Jul 5, 2016 at 3:07 PM, Neil Girdhar <mistersheik at gmail.com>
> wrote:
> > 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 UserDict is superseded by MutableMapping.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)
>
>
>
> --
> --Guido van Rossum (python.org/~guido)
>
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