Sorry for the double post then :-)
As for get(). Current implementation of UserDict returns the default value
for get() if __missing__() raises KeyError. Which sounds reasonable to me.
After all, you would logically expect __missing__ to be called for get() as
you tried to get a non-existing value.
If __missing__ is only a dict thing, then I'll add a note to dict's .get()
method saying it doesn't call __missing__ for missing values.
I can also modify UserDict to behave in a consistent manner.
Does that make sense?
On Wed, Jan 8, 2020 at 7:42 PM Guido van Rossum <gu...@python.org> wrote:
> (Note: We received your email twice.)
>
> I don't think __missing__ should be called by get() -- get() already has a
> way to deal with missing keys, and making it use two different mechanisms
> would be weird (e.g. if get() calls __missing__, is the default value ever
> used?).
>
> To answer your second question, __missing__ is only a dict thing, it is
> not part of Mapping.
>
> On Wed, Jan 8, 2020 at 8:07 AM Bar Harel <bzvi7...@gmail.com> wrote:
>
>> Hey guys,
>>
>> I was just about to fix dict's get() to call __missing__ if a key doesn't
>> exist (before returning the default value) but realized that although
>> small, that patch can cause future issues.
>> Right now there's an inconsistency:
>>
>> >>> from collections import UserDict
>> >>> class A(dict):
>> ... def __missing__(self, key):
>> ... print(key)
>> ...
>> >>> class B(UserDict):
>> ... def __missing__(self, key):
>> ... print("UserDict", key)
>> ...
>> >>> a = A()
>> >>> b = B()
>> >>> a.get(123)
>> >>> b.get(123)
>> UserDict 123
>> >>> a.get(123, "abc")
>> 'abc'
>> >>> b.get(123, "abc")
>> UserDict 123
>>
>> The reason for this inconsistency is because the Mapping abc and dict
>> behave differently.
>> Dict's get doesn't call __getitem__ which causes the call not to route to
>> __missing__.
>> MutableMapping's get calls __getitem__, which UserDict implements as a
>> check to __missing__ as well.
>>
>> According to the doc, the specification requires dict's __getitem__ to
>> call __missing__. It doesn't say anything about get().
>>
>> Should get() call __missing__?
>>
>> If it does, things like defaultdict.get() might break. It will however be
>> more consistent with dict's specification.
>> If it doesn't, we expect Mapping to not care about __missing__ as it's
>> only a dict thing, which will require UserDict to override get(). Dict's
>> get() will need to receive a doc update as well stating __missing__ is not
>> called.
>>
>> Second question is: Is __missing__ only a dict thing, or is it part of
>> the Mapping ABC?
>>
>> I would expect it to be a part of the Mapping ABC, with subclasses not
>> having to implement it. Right now it's not.
>>
>> Looking forward for your inputs,
>> Bar Harel
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>
>
> --
> --Guido van Rossum (python.org/~guido)
> *Pronouns: he/him **(why is my pronoun here?)*
> <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
>
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