Question about defaultdict

[Frank Millman]

Hi all

I use a dictionary as a cache, and I thought that I could replace it 
with collections.defaultdict, but it does not work the way I expected 
(python 3.3.0).

my_cache = {}
def get_object(obj_id):
     if obj_id not in my_cache:
         my_object = fetch_object(obj_id)  # expensive operation
         my_cache[obj_id] = my_object
     return my_cache[obj_id]
my_obj = get_object('a')

I thought I could replace this with -

from collections import defaultdict
my_cache = defaultdict(fetch_object)
my_obj = my_cache['a']

It does not work, because fetch_object() is called without any arguments.

It is not a problem, but it would be neat if I could get it to work. Am 
I missing anything?

Frank Millman