Can I do an ordered, default dict in Python?

Question

I would like to combine OrderedDict() and defaultdict() from collections in one object, which shall be an ordered, default dict. Is this possible?

Answer 1

The following (using a modified version of this recipe) works for me:

from collections import OrderedDict, Callable

class DefaultOrderedDict(OrderedDict):
    # Source: http://stackoverflow.com/a/6190500/562769
    def __init__(self, default_factory=None, *a, **kw):
        if (default_factory is not None and
           not isinstance(default_factory, Callable)):
            raise TypeError('first argument must be callable')
        OrderedDict.__init__(self, *a, **kw)
        self.default_factory = default_factory

    def __getitem__(self, key):
        try:
            return OrderedDict.__getitem__(self, key)
        except KeyError:
            return self.__missing__(key)

    def __missing__(self, key):
        if self.default_factory is None:
            raise KeyError(key)
        self[key] = value = self.default_factory()
        return value

    def __reduce__(self):
        if self.default_factory is None:
            args = tuple()
        else:
            args = self.default_factory,
        return type(self), args, None, None, self.items()

    def copy(self):
        return self.__copy__()

    def __copy__(self):
        return type(self)(self.default_factory, self)

    def __deepcopy__(self, memo):
        import copy
        return type(self)(self.default_factory,
                          copy.deepcopy(self.items()))

    def __repr__(self):
        return 'OrderedDefaultDict(%s, %s)' % (self.default_factory,
                                               OrderedDict.__repr__(self))

Answer 2

Another simple approach would be to use dictionary get method

>>> from collections import OrderedDict
>>> d = OrderedDict()
>>> d['key'] = d.get('key', 0) + 1
>>> d['key'] = d.get('key', 0) + 1
>>> d
OrderedDict([('key', 2)])
>>> 

Answer 3

A simpler version of @zeekay 's answer is:

from collections import OrderedDict

class OrderedDefaultListDict(OrderedDict): #name according to default
    def __missing__(self, key):
        self[key] = value = [] #change to whatever default you want
        return value