id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
7496	Getting cached instance of SortedDict using db cache backend throws AttributeError	John Huddleston <huddlej@…>	John Huddleston	"Using database caching, I set a non-empty SortedDict instance in the cache.  When I try to retrieve it, I get an !AttributeError with the message ""'SortedDict' object has no attribute 'keyOrder'"".  Here is the code to recreate the problem:

{{{
# Using database cache backend
from django.core.cache import cache
from django.utils.datastructures import SortedDict
value = SortedDict()
value['1'] = 1
cache.set('my_dict', value)
cache.get('my_dict')
}}}

I looked into the database cache backend and traced the problem to the pickling of the SortedDict instance which can be recreated like this:

{{{
import base64
import pickle
from django.utils.datastructures import SortedDict
value = SortedDict()
value['1'] = 1
encoded = base64.encodestring(pickle.dumps(value, 2)).strip()
decoded = pickle.loads(base64.decodestring(encoded))
}}}

The call to pickle.loads results in the !AttributeError.  If the pickling protocol is changed in dumps to 0, the call to loads works. The problem appears to be that protocol 2 calls SortedDict.!__new!__ instead of creating an empty SortedDict.  When !__new!__ is called, !__init!__ is not being called so keyOrder is not being initialized.

Possible Solutions:

If this is really unexpected behavior, the problem could be solved by changing the pickle protocol in the database caching backend from 2 to 0.  It looks like the database cache backend is the only backend using pickle protocol 2 while the local memory backend is using protocol 0.

Another solution would be to subclass SortedDict.!__new!__ so keyOrder is always set:

{{{
def __new__(cls, *args, **kwargs):
    instance = super(SortedDict, cls).__new__(cls, *args, **kwargs)
    instance.keyOrder = []
    return instance 
}}}

I have attached patches reflecting both of these solutions (db.py.diff and datastructures.py.diff, respectively)."		closed	Core (Cache system)	dev		fixed			Accepted	1	0	0	0	0	0