On Sat, 18 Feb 2006 10:44:15 +0100 (CET), "=?iso-8859-1?Q?Walter_D=F6rwald?=" <walter@livinglogic.de> wrote:
Guido van Rossum wrote:
On 2/17/06, Ian Bicking <ianb@colorstudy.com> wrote:
Guido van Rossum wrote:
d =3D {} d.default_factory =3D set ... d[key].add(value)
Another option would be:
d =3D {} d.default_factory =3D set d.get_default(key).add(value)
Unlike .setdefault, this would use a factory associated with the diction= ary, 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 functio= nality 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 P= yDict_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_m= issing() 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 v= arious 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 beha= viour 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 =3D=3D type(None))
OTOH, I forgot why it was desirable in the first place to overload d[k] with defaulting logic. E.g., why wouldn't d.defaulting[k] be ok to write when you want the d.default_factory action? on_missing feels more like a tracing hook though, so maybe it could always act either way if defined. Also, for those wanting to avoid lambda:42 as factory, would a callable test cost a lot? Of course then the default_factory name might require revision. Regards, Bengt Richter