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

[Guido van Rossum]

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 UserDict is superseded by MutableMapping.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)