Python nested defaultdict with mix data types

Question

So, how can I create a defaultdict for this:

{
    'branch': {
        'count': 23,
        'leaf': {
            'tag1': 30,
            'tag2': 10
        }
    },
}

so that, I'll get zeros for count, tag1 and tag2 as default? I wanna populate the dict dynamically while I'm reading the inputs. When I see a new branch I want to create a dict with count as zero and an empty dict as leaf. When I get a leaf, I want to create a key with it's name and set the value to zero.

Update: Accepted Martijn's answer as it has more upvotes but other answers are equally good.

Answer 1

You can't do this with defaultdict, because the factory doesn't have access to the key.

However, you can just subclass dict to create your own 'smart' defaultdict-like class. Provide your own __missing__ method that adds values based on the key:

class KeyBasedDefaultDict(dict):
    def __init__(self, default_factories, *args, **kw):
        self._default_factories = default_factories
        super(KeyBasedDefaultDict, self).__init__(*args, **kw)

    def __missing__(self, key):
        factory = self._default_factories.get(key)
        if factory is None:
            raise KeyError(key)
        new_value = factory()
        self[key] = new_value
        return new_value

Now you can supply your own mapping:

mapping = {'count': int, 'leaf': dict}
mapping['branch'] = lambda: KeyBasedDefaultDict(mapping)

tree = KeyBasedDefaultDict(mapping)

Demo:

>>> mapping = {'count': int, 'leaf': dict}
>>> mapping['branch'] = lambda: KeyBasedDefaultDict(mapping)
>>> tree = KeyBasedDefaultDict(mapping)
>>> tree['branch']['count'] += 23
>>> tree['branch']['leaf']['tag1'] = 30
>>> tree['branch']['leaf']['tag2'] = 10
>>> tree
{'branch': {'count': 23, 'leaf': {'tag1': 30, 'tag2': 10}}}