ordereddefaultdict.py

import collections


class OrderedDefaultDict(collections.OrderedDict, collections.defaultdict):
    pass


odd = OrderedDefaultDict()
odd.default_factory = list

odd['a'].append(True)
odd['b'].append(False)
odd['c'].append(False)

print(odd.items())
print(odd['d'])

If I make defaultdict the first base, then the setitems will fail because the dict has no __root (Python 2) or __map (3) attribute. This is certainly due to defaultdict not calling super(...).__init__. This can be worked around by defining OrderedDefaultDict.__init__(default_factory, items) and calling the superclasses’ __init__ methods explicitly.

If OrderedDict is the first base, then the signature of __init__ has no argument for default_factory, so I had to set it manually after creating my instance.

You could also only subclass OrderedDict and define a __missing__ method to to the same thing as default_factory in defaultdict.

FTR collections.Counter can also help depending on your need, and it combines well with collections.OrderedDict: http://rhettinger.wordpress.com/2011/05/26/super-considered-super/