On 2/17/06, Ian Bicking
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'm not going to do a point-by-point to your longer post (I don't have the time); let's (again) agree to disagree and I'll sleep on it. -- --Guido van Rossum (home page: http://www.python.org/~guido/)