(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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--
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*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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