Hum, it looks like the UserDict.get() behavior is an accident due to the way Mapping.get() is implemented. The comment there says 'D.get(k[,d]) -> D[k] if k in D, else d. d defaults to None.' but the implementation just tries D[k] and catches KeyError. The UserDict.__contains__() implementation is explicitly code to avoid __missing__, since the default Mapping.__contains__() implementation *also* just calls D[k] and catches KeyError. More indication that the UserDict.get() behavior is an accident: in test_userdict.py there's a test for __missing__, test_missing() but it doesn't test get() at all, only __getitem__(). How were you proposing to fix the situation? I could imagine changing Mapping.get() to call __contains__() first. That would slow down Mapping.get() slightly (the default implementation would end up calling __getitem__() twice if the key is present), but I don't think that's a grave concern (if people want it faster there are a zillion ways to do that). On Wed, Jan 8, 2020 at 9:49 AM Bar Harel <bzvi7919@gmail.com> wrote:
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 <guido@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 <bzvi7919@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 _______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-leave@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/SDXOEMAE... Code of Conduct: http://python.org/psf/codeofconduct/
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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/>