[Python-Dev] defaultdict proposal round three

Ian Bicking ianb at colorstudy.com
Mon Feb 20 22:13:27 CET 2006

Steven Bethard wrote:
>>Alternative A: add a new method to the dict type with the semantics of
>>__getattr__ from the last proposal, using default_factory if not None
>>(except on_missing is inlined).
> I'm not certain I understood this right but (after
> s/__getattr__/__getitem__) this seems to suggest that for keeping a
> dict of counts the code wouldn't really improve much:
> dd = {}
> dd.default_factory = int
> for item in items:
>     # I want to do ``dd[item] += 1`` but with a regular method instead
>     # of __getitem__, this is not possible
>     dd[item] = dd.somenewmethod(item) + 1

This would be better done with a bag (a set that can contain multiple 
instances of the same item):

dd = collections.Bag()
for item in items:

Then to see how many there are of an item, perhaps something like:

No collections.Bag exists, but of course one should.  It has nice 
properties -- inclusion is done with __contains__ (with dicts it 
probably has to be done with get), you can't accidentally go below zero, 
the methods express intent, and presumably it will implement only a 
meaningful set of methods.

Ian Bicking  /  ianb at colorstudy.com  /  http://blog.ianbicking.org

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