in the method DictCache.getitem(key), it use self._cache_dict.get(key, NULL) to get value, but LRUDict direct subclass the builtin dict, the get()/set() does not function correctly. I reimplement the LRUDict as a subclass of collections.MutableMapping. Not sure if it is a bug, or I am wrong.

Reference:

• http://stackoverflow.com/questions/3387691/python-how-to-perfectly-override-a-dict
• http://stackoverflow.com/questions/7148419/subclass-dict-userdict-dict-or-abc
• http://docs.python.org/library/abc.html