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Sat Jan 10 14:37:46 EST 2004 · https://mail.python.org/pipermail/python-dev/2004-January/041905.html

Raymond H. quoted Martin L. thusly:
> > Both applications could be implemented if dictionaries had
> > the notion of a default value, which would be generated
> > by a callable (whether with or without argument is debatable)
> > 
> > # count things
> > d = {}
> > d.defaultmaker = lambda k:0
> > for t in things:
> >    d[t] += 1
> > 
> > # sort things by category
> > d = {}
> > d.defaultmaker = lambda k:[]
> > for t in things:
> >    d[category(t)].append(t)

Then answered with:
> These both read cleanly.
> 
> I really like this idea as a general purpose solution with broad
> applicability.  It fully encapsulates the ideas behind bag 
> construction and building dicts of lists.
> 
> Instead of a method, it may be better to use a keyword argument in the
> constructor:
> 
> d = dict(default = lambda k:[])
> for t in things:
>     d[category(t)].append(t)
> 
> If lambda is made parameterless, it allows type constructors 
> to be used
> for the most common cases:
> 
> dict(default=list)	# this is clear enough
> dict(default=int)       # this may be too cute

This seems a bit too easy to do "as needed" to warrant a new builtin,
but that's why we have proposals, I guess:

class DefaultingDict(dict):

    def __init__(self, default):
        self.default = default

    def __getitem__(self, key):
        return self.get(key, self.default())

>>> d = DefaultingDict(lambda: 0)
>>> d.update({'a': 1, 'b': 2, 'c': 3})
>>> d
{'a': 1, 'c': 3, 'b': 2}
>>> for x in ['a', 'b', 'c', 'd']:
... 	d[x] += 1
... 	
>>> d
{'a': 2, 'c': 4, 'b': 3, 'd': 1}

Robert Brewer
MIS
Amor Ministries
fumanchu at amor.org

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[Python-Dev] collections module