How to subclass an OrderedDict?

Question

Subclassing a Python dict works as expected:

>>> class DictSub(dict): ...     def __init__(self): ...         self[1] = 10 ...          >>> DictSub() {1: 10} 

However, doing the same thing with a collections.OrderedDict does not work:

>>> import collections >>> class OrdDictSub(collections.OrderedDict): ...     def __init__(self): ...         self[1] = 10 ...          >>> OrdDictSub() (…) AttributeError: 'OrdDictSub' object has no attribute '_OrderedDict__root' 

Thus, the OrderedDict implementation uses a private __root atribute, which prevents the subclass OrdDictSub from behaving like the DictSub subclass. Why? How can one inherit from an OrderedDict?

Answer 1

You need to invoke OrderedDict.__init__ from your __init__:

class OrdDictSub(collections.OrderedDict):     def __init__(self):         super(OrdDictSub, self).__init__() 

You haven't given OrderedDict a chance to initialize itself. Technically, you want to do this for your dict subclass as well, since you want a fully initialized dict. The fact that dict works without it is just luck.

Answer 2

Try initializing the superclass in the __init__ method:

def __init__(self):     collections.OrderedDict.__init__(self)     self[1] = 10 

This is the normal way to initialize a subclass. You don't have to call the superclass's __init__ method in general, but if you have no knowledge of the superclass's implementation you really should call __init__.