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

[Guido van Rossum]

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 UserDict is superseded by MutableMapping.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~
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>> >>
>> >>
>> >>
>> >> --
>> >> --Guido van Rossum (python.org/~guido)
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>>
>>
>>
>> --
>> --Guido van Rossum (python.org/~guido)



-- 
--Guido van Rossum (python.org/~guido)