Accessing key in factory of defaultdict

Question

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?

Answer 1

__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}