Why can't I create a default, ordered dict by inheriting OrderedDict and defaultdict?

Question

My first attempt to combine the features of two dictionaries in the collections module was to create a class that inherits them:

from collections im         


        

Answer 1

The reason is that the init method of defaultdict instead of calling __init__ of the next class in MRO calls init of PyDict_Type hence some of the attributes like __map that are set in OrderedDict's __init__ are never initialized, hence the error.

>>> DefaultOrderedDict.mro()
[,
 ,
 ,
 , ]

And defaultdict don't have their own __setitem__ method:

>>> defaultdict.__setitem__

>>> dict.__setitem__

>>> OrderedDict.__setitem__

So, when you called d['a'] = 1, in search of __setitem__ Python reached OrdereredDict's __setitem__ and their the access of uninitialized __map attribute raised the error:

---

A fix will be to call __init__ on both defaultdict and OrderedDict explicitly:

class DefaultOrderedDict(defaultdict, OrderedDict):
    def __init__(self, default_factory=None, *a, **kw):
        for cls in DefaultOrderedDict.mro()[1:-2]:
            cls.__init__(self, *a, **kw)