Question

Did defaultdict’s become not marshal’able as of Python 2.6? The following works under 2.5, fails under 2.6 with ‘ValueError: unmarshallable object’ on OS X 1.5.6, python-2.6.1-macosx2008-12-06.dmg from python.org:

from collections import defaultdict import marshal dd = defaultdict(list) marshal.dump(dd, file('/tmp/junk.bin','wb') ) 

Answer 1

Marshal was deliberately changed to not support subclasses of built-in types. Marshal was never supposed to handle defaultdicts, but happened to since they are a subclass of dict. Marshal is not a general ‘persistence’ module; only None, integers, long integers, floating point numbers, strings, Unicode objects, tuples, lists, sets, dictionaries, and code objects are supported.

Python 2.5:

>>> marshal.dumps(defaultdict(list)) '{0' >>> marshal.dumps(dict()) '{0' 

If for some reason you really want to marshal a defaultdict you can convert it to a dict first, but odds are you should be using a different serialization mechanism, like pickling.