Types define how instances of it get pickled by defining one or more of a (fairly large) set of methods. Each has its own subtle behaviour. See the docs on the pickle protocol. In the case of collections.defaultdict
, it uses the __reduce__
method:
>>> l = collections.defaultdict(list)
>>> l.__reduce__()
(<type 'collections.defaultdict'>, (<type 'list'>,), None, None, <dictionary-itemiterator object at 0x7f031fb3c470>)
The first item in the tuple there is the type, and the second item is the tuple of arguments to pass to the type when instantiating it. If you don't override __reduce__
, the first item will correctly change to your type, but the second item will not. This causes the error you see. A crude example of how you could fix it:
>>> import collections
>>> import pickle
>>> class C(collections.defaultdict):
... def __init__(self):
... collections.defaultdict.__init__(self, list)
... def __reduce__(self):
... t = collections.defaultdict.__reduce__(self)
... return (t[0], ()) + t[2:]
...
>>> c = C()
>>> c[1].append(2)
>>> c[2].append(3)
>>> c2 = pickle.loads(pickle.dumps(c))
>>> c2 == c
True
It's only a crude example because there's more to pickling (like __reduce_ex__
) and it's all fairly intricate. In this case, using __getinitargs__
may be more convenient.
Alternatively, you could make your class's __init__
method take an optional callable, defaulting to list
, or you could just use a function instead of a class:
def listdict():
return collections.defaultdict(list)
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