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use cases for a defaultdict

P: n/a
Steven Bethard wrote:
Agreed. I really hope that Python 3.0 applies Raymond Hettinger's
suggestion "Improved default value logic for Dictionaries" from
http://wiki.python.org/moin/Python3%2e0Suggestions

This would allow you to make the setdefault() call only once, instead
of on every lookup:

class meh(dict):
def __init__(self, *args, **kwargs):
super(meh, self).__init__(*args, **kwargs)
self.setdefault(function=meh)
Steve Holden wrote: In fact, why not go one better and also add a "default" keyword
parameter to dict()?
Steven Bethard wrote: It's not backwards compatible:
>>> dict(default=4) {'default': 4}
Steve Holden wrote: Nyargle. Thanks, you're quite right, of course: I was focussing on the
list-of-pairs argument style when I wrote that. So the best we could
do is provide a subtype, defaultdict(default, *args, *kw).
Steven D'Aprano wrote: I vote to leave dict just as it is, and add a subclass, either in a
module or as a built in (I'm not fussed either way) for
dicts-with-defaults.


Yeah, I like that idea too. That's a lot of Steven's agreeing on this
-- do we realy need agreement from people with other names too? ;)

I do think defaultdict() is probably the right way to go, though I'm not
certain about the signature -- it needs to support value-based defaults
(e.g. 0) and function based defaults (e.g. a new list each time).
Perhaps we need to introduce two new collections objects,
defaultvaluedict and defaultfunctiondict:
class defaultdict(dict): .... def __init__(self, default, *args, **kwargs):
.... super(defaultdict, self).__init__(*args, **kwargs)
.... self._default = default
.... def __repr__(self):
.... type_name = type(self).__name__
.... super_str = super(defaultdict, self).__repr__()
.... return '%s(%r, %s)' % (type_name, self._default, super_str)
.... class defaultvaluedict(defaultdict): .... def __getitem__(self, key):
.... if key not in self:
.... self[key] = self._default
.... return super(defaultvaluedict, self).__getitem__(key)
.... class defaultfunctiondict(defaultdict): .... def __getitem__(self, key):
.... if key not in self:
.... self[key] = self._default()
.... return super(defaultfunctiondict, self).__getitem__(key)
.... counts = defaultvaluedict(0)
counts['Steve'] += 1
counts['Steve'] += 1
counts['Steven'] += 1
counts defaultvaluedict(0, {'Steve': 2, 'Steven': 1}) groups = defaultfunctiondict(list)
groups['Steve'].append('Holden')
groups['Steve'].append('Bethard')
groups['Steven'].append("D'Aprano")
groups

defaultfunctiondict(<type 'list'>, {'Steve': ['Holden', 'Bethard'],
'Steven': ["D'Aprano"]})

I didn't override of get() or setdefault(), which means they don't use
the default associated with the dict. I think this is right because the
only time you'd use get() or setdefault() with a defaultdict is if you
wanted to override the default normally associated with it.

The question, I guess, is whether or not there are enough use cases to
merit introducing these types into the collections module. Some of my
use cases:

* Counting numbers of items, using defaultvaluedict(0). Currently, I
support this by having a counts() function in my dicttools module:

def counts(iterable, key=None):
result = {}
for item in iterable:
# apply key function if necessary
if key is None:
k = item
else:
k = key(item)
# increment key's count
try:
result[k] += 1
except KeyError:
result[k] = 1
return result

Raymond Hettinger has also proposed a bag_ recipe for similar purposes.

... _bag: http://aspn.activestate.com/ASPN/Coo.../Recipe/259174

* Grouping items into lists, using defaultfunctiondict(list).
Currently, I support this by having a groupby() function in my dicttools
module:

def groupby(iterable, key=None, value=None):
result = {}
for item in iterable:
# apply key function if necessary
if key is None:
k = item
else:
k = key(item)
# apply value function if necessary
if value is None:
v = item
else:
v = value(item)
# append value to key's list
try:
result[k].append(v)
except KeyError:
result[k] = [v]
return result

Note that for both of my use cases, the appropriate defaultdict() could
take the try/except (equivalent to a setdefault() call) out of my code.
It's nice, but not a huge gain -- I definitely can't drop my code
completely...

Do others have compelling use-cases for a defaultdict-like class?

STeVe
Jan 18 '06 #1
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