[Python-Dev] Proposal: defaultdict (original) (raw)

Alex Martelli aleaxit at gmail.com
Fri Feb 17 20:34:46 CET 2006

On 2/16/06, Guido van Rossum <guido at python.org> wrote:

A bunch of Googlers were discussing the best way of doing the ... Wow, what a great discussion! As you'll recall, I had also mentioned the "callable factory" as a live possibility, and there seems to be a strong sentiment in favor of that; not really a "weakness case" for HOFs, as you feared it might be during the lunchtime discussion.

Out of all I've read here, I like the idea of having a collections.autodict (a much nicer name than defaultdict, a better collocation for 2.5 than the builtins). One point I think nobody has made is that whenever reasonably possible the setting of a callback (the callable factory here) should include *a and **k to use when calling back. So, for example:

ad = collections.autodict(copy.copy, whatever)

would easily cover the use case of Google's DefaultDict (yes, partial would also cover this use case, but having *a and **k is usefully more general). If you're convinced copy.copy is an overwhelmingly popular use case (I'm not, personally), then this specific idiom might also be summarized in a classmethod, a la

ad = collections.autodict.by_copy(whatever)

This way, all autodicts would start out empty (and be filled by update if needed). An alternative would be to have autodict's ctor have the same signature as dict's, with a separate .set_initial method to pass the factory (and *a, **k) -- this way an autodict might start out populated, but would always start with some default factory, such as lambda:None I guess. I think the first alternative (autodict always starts empty, but with a specifically chosen factory [including *a, **k]) is more useful.

Alex

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