Preface: I'm not sure if this is a bug or not, but I could not find
any mention of it in the tickets or the mailing lists.
Problem:
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 caching
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 caching 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.
Solution:
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
Does this seem ticket-worthy?
Thanks!
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