Guido van Rossum wrote:
> On 2/17/06, Ian Bicking <[EMAIL PROTECTED]> wrote:
>> Guido van Rossum wrote:
>> > d = {}
>> > d.default_factory = set
>> > ...
>> > d[key].add(value)
>>
>> Another option would be:
>>
>> d = {}
>> d.default_factory = set
>> d.get_default(key).add(value)
>>
>> Unlike .setdefault, this would use a factory associated with the dictionary,
>> and no default value would get passed in.
>> Unlike the proposal, this would not override __getitem__ (not overriding
>> __getitem__ is really the only difference with the proposal). It would be
>> clear reading the code that you were not
>> implicitly asserting they "key in d" was true.
>>
>> "get_default" isn't the best name, but another name isn't jumping out at me
>> at the moment. Of course, it is not a Pythonic
>> argument to say that an existing method should be overridden, or
>> functionality made nameless simply because we can't think
>> of a name (looking to anonymous functions of course ;)
>
> I'm torn. While trying to implement this I came across some ugliness in
> PyDict_GetItem() -- it would make sense if this also
> called
> on_missing(), but it must return a value without incrementing its
> refcount, and isn't supposed to raise exceptions -- so what to do if
> on_missing() returns a value that's not inserted in the
> dict?
>
> If the __getattr__()-like operation that supplies and inserts a
> dynamic default was a separate method, we wouldn't have this problem.
>
> OTOH most reviewers here seem to appreciate on_missing() as a way to do
> various other ways of alterning a dict's
> __getitem__() behavior behind a caller's back -- perhaps it could even be
> (ab)used to
> implement case-insensitive lookup.
I don't like the fact that on_missing()/default_factory can change the
behaviour of __getitem__, which upto now has been
something simple and understandable.
Why don't we put the on_missing()/default_factory functionality into get()
instead?
d.get(key, default) does what it did before. d.get(key) invokes on_missing()
(and dict would have default_factory == type(None))
Bye,
Walter Dörwald
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