Frank Millman wrote:
> 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?
You can subclass the ordinary dict:
class Cache(dict):
def __missing__(self, key):
result = self[key] = fetch_object(key)
return result
_cache = Cache()
def get_object(object_id):
return _cache[object_id]
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