micropython-lib/collections.defaultdict/collections/defaultdict.py

class defaultdict:

    @staticmethod
    def __new__(cls, default_factory=None, **kwargs):
        # Some code (e.g. urllib.urlparse) expects that basic defaultdict
        # functionality will be available to subclasses without them
        # calling __init__().
        self = super(defaultdict, cls).__new__(cls)
        self.d = {}
        return self

    def __init__(self, default_factory=None, **kwargs):
        self.d = kwargs
        self.default_factory = default_factory

    def __getitem__(self, key):
        try:
            return self.d[key]
        except KeyError:
            v = self.__missing__(key)
            self.d[key] = v
            return v

    def __setitem__(self, key, v):
        self.d[key] = v

    def __delitem__(self, key):
        del self.d[key]

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