dict subclass and pickle bug (?)

Christian Heimes lists at cheimes.de
Sun Dec 7 20:18:21 CET 2008

James Stroud wrote:
> Hello All,
> I subclassed dict and overrode __setitem__. When instances are 
> unpickled, the __setstate__ is not called before the keys are assigned 
> via __setitem__ in the unpickling protocol.
> I googled a bit and found that this a bug filed in 2003:
> http://bugs.python.org/issue826897
> It is still "open" with "normal" priority.
> Am I missing something? Is there a workaround for this bug that makes 
> fixing it pointless or has it just fallen through the cracks for the 
> last 5 years?

Try this:

def __newobj__(cls, *args):
     return cls.__new__(cls, *args)

class DictPlus(dict):
     def __init__(self, *args, **kwargs):
         dict.__init__(self, *args, **kwargs)
         self.extra_thing = ExtraThingClass()

     def __setitem__(self, k, v):
         do_something_with(self.extra_thing, k, v)
         dict.__setitem__(self, k, v)

     def __setstate__(self, state):
         dict.update(self, state[0])

     def __reduce__(self):
         state = (dict(self), self.__dict__)
         return (__newobj__, (self.__class__,), state)


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