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

Barry Warsaw barry at python.org
Tue Feb 21 05:11:26 CET 2006

On Sat, 2006-02-18 at 12:53 +0100, Pierre Barbier de Reuille wrote:

> Guido> Over lunch with Alex Martelli, he proposed that a subclass of > Guido> dict with this behavior (but implemented in C) would be a good > Guido> addition to the language.

I agree that .setdefault() is a well-intentioned failure, although I'm much less concerned about any potential performance impact than the fact that it's completely unreadable. And while I like the basic idea, I also agree that deriving from dict is problematic, both because of the constructor signature is tough to forward, but also because dict is such a fundamental type that APIs that return dicts may have to be changed to allow passing in a factory type.

I'd rather like to see what Pierre proposes, with a few minor differences.

Well, first not ot break the current interface, and second because I think it reads better I would prefer :

d = {'a': 1}' d['b'] # raises KeyError d.get('c') # evaluates to None d.default = 42 d['b'] # evaluates to 42 d.get('c') # evaluates to 42

So far so good.

And to undo the default, you can simply do :

del d.default

Although this I'm not crazy about. If you let .default be a callable, you could also write this as

def keyerror(): raise KeyError d.default = keyerror

or possibly just this as a shortcut:

d.default = KeyError

> The only question in my mind is whether or not getting a non-existent value > under the influence of a given default value should stick that value in the > dictionary or not.

Agreed. I'm not sure whether .get(onearg) should return None or .default. I /think/ I want the latter, but I'd have to play with some real code to know for sure.

-Barry

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