[Python-Dev] defaultdict proposal round three

Guido van Rossum guido at python.org
Tue Feb 21 03:06:13 CET 2006

On 2/20/06, Guido van Rossum <guido at python.org> wrote:
> [stuff with typos]

Here's the proofread version:

I have a patch ready that implements this. I've assigned it to Raymond
for review. I'm just reusing the same SF patch as before:
http://python.org/sf/1433928 .

One subtlety: for maximal flexibility and speed, the standard dict
type now defines an on_missing(key) method; however this version
*just* raises KeyError and the implementation actually doesn't call it
unless the class is a subtype (with the possibility of overriding

collections.defaultdict overrides on_missing(key) to insert and return
self.default_factory() if it is not None; otherwise it raises
KeyError. (It should really call the base class on_missing() but I
figured I'd just in-line it which is easier to code in C than a

The defaultdict signature takes an optional positional argument which
is the default_factory, defaulting to None. The remaining positional
and all keyword arguments are passed to the dict constructor. IOW:

 d = defaultdict(list, [(1, 2)])

is equivalent to:

 d = defaultdict()
 d.default_factory = list
 d.update([(1, 2)])

At this point, repr(d) will be:

 defaultdict(<type 'list'>, {1: 2})

Once Raymond approves the patch I'll check it in.

--Guido van Rossum (home page: http://www.python.org/~guido/)

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