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From: Peter Otten <__peter__@web.de>
Subject: Re: Question about defaultdict
Date: Sat, 23 Feb 2013 11:34:21 +0100
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Xref: csiph.com comp.lang.python:39656

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]