Discussion on: defaultdicts: Never Check If a Key is Present Again!

[László Károlyi]

I know dicts are ordered by default from 3.7 onwards (so you could just use defaultdict), but I needed to mix the defaultdict and OrderedDict at one point. Introducing the DefaultOrderedDict:

from collections import OrderedDict
from copy import deepcopy


class DefaultOrderedDict(OrderedDict):
    """Extending getter functions to provide defaults."""
    _default = dict()

    def __init__(self, *args, default, **kwargs):
        self._default = default
        super().__init__(*args, **kwargs)

    def __getitem__(self, key):
        if key in self:
            return self.get(key)
        value = self[key] = deepcopy(self._default)
        return value

    def __deepcopy__(self, memo):
        """Return a deep copy of self."""
        obj = DefaultOrderedDict(default=self._default)
        for key, value in self.items():
            obj[key] = deepcopy(value)
        return obj

one example of how it can be used:

terms_by_func = DefaultOrderedDict(default=[])

it will basically deepcopy() the contents of the default value.

[Ryan Palo]

Neat! My first thought was to wonder if it could be done with multiple inheritance between defaultdict and OrderedDict, but that seems like that could be a big nightmare, so your method is probably cleaner. :)