How do you implement an ordered defaultdict in Python to maintain key order and assign default values?

Implementing an Ordered Default Dict

One might want to merge the functionalities of OrderedDict() and defaultdict() from the Python Collections module to create an ordered default dict. This combination would allow for maintaining a dictionary where keys are ordered and default values are assigned when accessing non-existent keys.

To achieve this, we can utilize a modified version of a recipe provided in the Stack Overflow community. Here's the code:

<code class="python">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

    # Override necessary methods for pickling
    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))</code>

This code snippet incorporates an __init__ method, allowing you to specify a default factory function. The __missing__ method handles key access and assigns default values using the factory function. Additionally, various methods such as __reduce__, copy(), __copy__(), __deepcopy__(), and __repr__() are defined for proper pickling and representation of this custom dictionary.

By utilizing this DefaultOrderedDict class, you can construct an ordered dictionary that provides default values for missing keys. This approach combines the benefits of ordered key management and dynamic value assignment.

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