Question about collections.defaultdict

Robert Kern robert.kern at
Mon Mar 26 17:52:47 CEST 2012

On 3/26/12 4:33 PM, Steven W. Orr wrote:
> On 3/26/2012 9:44 AM, Robert Kern wrote:
>> On 3/26/12 2:33 PM, Steven W. Orr wrote:
>>> I created a new class called CaseInsensitiveDict (by stealing from code I found
>>> on the web, thank you very much). The new class inherits from dict. It makes it
>>> so that if the key has a 'lower' method, it will always access the key using
>>> lower
>>> I'd like to change the place where I previously declared a dict
>>> self.lookup = defaultdict(list)
>>> so that the new code will allow this new dict to be used instead. But then I
>>> realized I may have painted myself into a small corner:
>>> Is there a way to use defaultdict so that I can override what *kind* of dict it
>>> will use?
>> No.
>>> I would like the value to still be a list be default, but it seems like I can't
>>> tell defaultdict to use *my* new dict.
>>> Do I give up on defaultdict?
>> Assuming that your CaseInsensitiveDict subclasses from dict or UserDict, it's
>> relatively easy to make a subclass of your CaseInsensitiveDict act like a
>> defaultdict. Just implement the __missing__(key) method appropriately (and
>> modify the constructor to take the callable, of course).
> I'm not quite getting what you're telling me, but I'm sure you have the right
> idea. Here's the beginning of my class:
> class CaseInsensitiveDict(dict):
> def __init__(self, init=None):
> if isinstance(init, (dict, list, tuple)):
> for kk, vv in init.items():
> self[self.key_has_lower(kk)] = vv
> It sounds like you want me to subclass defaultdict to create something like this?
> class CaseInsensitiveDictDef(defaultdict):
> def __init__(self, init=None):
> super(CaseInsensitiveDictDef, self).__init__(list)
> self.__missing__ = list
> I think I'm way off base. I'm not clear on what the calling sequence is for
> defaultdict or how to get it to use my CaseInsensitiveDict instead of regular dict.
> Can you help?

You need to make a subclass of CaseInsensitiveDict, implement the 
__missing__(key) method, and override the __init__() method to take the factory 
function as an argument instead of data. defaultdict is just a subclass of dict 
that does this.

class CaseInsensitiveDictDef(CaseInsensitiveDict):
     def __init__(self, default_factory):
         super(CaseInsensitiveDictDef, self).__init__()
         self.default_factory = default_factory

     def __missing__(self, key):
         return self.default_factory()

Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco

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