腌制OrderedDict派生的对象

Question

I built a subclass of collections.OrderedDict standard class. When I try to unpickle an object of this class I get the following error:

Traceback (most recent call last):
  File "pickle.py", line 29, in <module>
    print cPickle.load(f)
TypeError: ('__init__() takes exactly 1 argument (2 given)', <class '__main__.ConfiguratorsDict'>, ([['toto', 20]],))

Trying to understand the reason for such behaviour I narrowed the body of collections.OrderedDict to get the following minimal code that triggers the aformentionned error. Here it is:

import cPickle

class OrderedDict(dict):
    def __reduce__(self):
        items = [[k, self[k]] for k in self]
        inst_dict = vars(self).copy()
        for k in vars(OrderedDict()):
            inst_dict.pop(k, None)
        if inst_dict:
            return (self.__class__, (items,), inst_dict)

        return self.__class__, (items,)

class ConfiguratorsDict(OrderedDict):

    def __init__(self):
        OrderedDict.__init__(self)

        self._myspec = "blabla"

if __name__ == "__main__":

    f = open("test.pickle","wb")
    c = ConfiguratorsDict()
    c["toto"] = 20
    cPickle.dump(c,f)
    f.close()    
    f = open("test.pickle","rb")
    print cPickle.load(f)
    f.close()

At this point, I really do not understand what is going wrong in there. Is there something I misunderstood with pickle mechanism or is there some trouble related to OrderedDict ?

thanks a lot for your help

Answer 1

You didn't read the documentation for __reduce__ carefully enough:

When a tuple is returned, it must be between two and five elements long. Optional elements can either be omitted, or None can be provided as their value. The contents of this tuple are pickled as normal and used to reconstruct the object at unpickling time. The semantics of each element are:

• A callable object that will be called to create the initial version of the object. The next element of the tuple will provide arguments for this callable , and later elements provide additional state information that will subsequently be used to fully reconstruct the pickled data.

You are returning the class as callable and as second element the items hence the unpickle is trying to pass the items to the class thus calling __init__ , but your __init__ doesn't take any argument and hence you get an error.

You have to either change the __init__ to accept the argument or avoid putting that as second element and you an empty tuple instead.