Accessing key in factory of defaultdict

16
votes

I am trying to do something similar to this:

from   collections import defaultdict
import hashlib

def factory():
    key = 'aaa'
    return { 'key-md5' : hashlib.md5('%s' % (key)).hexdigest() }

a = defaultdict(factory)
print a['aaa']

(actually, the reason why I need access to the key in the factory is not to compute an md5, but for other reasons; this is just an example)

As you can see, in the factory I have no access to the key: I am just forcing it, which makes no sense whatsoever.

Is it possible to use defaultdict in a way that I can access the key in the factory?

2
pythonpython-2.7dictionarydefaultdict
Out of curiosity, why do you need defaultdict, is it only for correcting missing values? Because that's all it does aside from just returning {}? - Torxed
Yes, it is to provide values for missing keys in the dictionary. That is the whole point of a defaultdict, isn't it? The problem is that my (real) data structure, which I am storing in the defaultdict, has fields depending on the key. So, whenever I am trying to access a non-existing element, I need to create it in the factory using as a parameter the key of the defaultdict. - blueFast
Was afraid of you doing something a little more over the ordinary. See @falsetru's solution because that was what i was going to suggest to you in case of custom building usage of dictionaries :) - Torxed
Possible duplicate of Is there a clever way to pass the key to defaultdict's default_factory? - eickenberg

2 Answers

27
votes

__missing__ of defaultdict does not pass key to factory function.

If default_factory is not None, it is called without arguments to provide a default value for the given key, this value is inserted in the dictionary for the key, and returned.

Make your own dictionary class with custom __missing__ method.

>>> class MyDict(dict):
...     def __init__(self, factory):
...         self.factory = factory
...     def __missing__(self, key):
...         self[key] = self.factory(key)
...         return self[key]
... 
>>> d = MyDict(lambda x: -x)
>>> d[1]
-1
>>> d
{1: -1}
6
votes

Unfortunately not directly, as defaultdict specifies that default_factory must be called with no arguments:

http://docs.python.org/2/library/collections.html#collections.defaultdict

But it is possible to use defaultdict as a base class that has the behavior you want:

class CustomDefaultdict(defaultdict):
    def __missing__(self, key):
        if self.default_factory:
            dict.__setitem__(self, key, self.default_factory(key))
            return self[key]
        else:
            defaultdict.__missing__(self, key)

This works for me:

>>> a = CustomDefaultdict(factory)
>>> a
defaultdict(<function factory at 0x7f0a70da11b8>, {})
>>> print a['aaa']
{'key-md5': '47bce5c74f589f4867dbd57e9ca9f808'}
>>> print a['bbb']
{'key-md5': '08f8e0260c64418510cefb2b06eee5cd'}