[Python-Dev] defaultdict proposal round three (original) (raw)

Guido van Rossum guido at python.org
Tue Feb 21 03:03:34 CET 2006

On 2/20/06, Alex Martelli <aleaxit at gmail.com> wrote:

> [Alex] >>> I see d[k]+=1 as a substantial improvement -- conceptually more >>> direct, "I've now seen one more k than I had seen before". > > [Guido] >> Yes, I now agree. This means that I'm withdrawing proposal A (new >> method) and championing only B (a subclass that implements >> getitem() calling onmissing() and onmissing() defined in that >> subclass as before, calling defaultfactory unless it's None). I don't >> think this crisis is big enough to need two solutions, and this >> example shows B's superiority over A.

[Raymond]

> FWIW, I'm happy with the proposal and think it is a nice addition > to Py2.5.

[Alex]

OK, sounds great to me. collections.defaultdict, then?

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: python.org/sf/1433928.

One subtlety: for maximul 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 on_missing()).

collections.defaultdict overrides on_missing(key) to insert and return self.fefault_factory() if it is not empty; 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 super-call.)

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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