BetweenDict, a Python dict for value ranges

I was doing some programming today (no, really?) and had need of a data structure that would return a value based on the key falling within a given range. Kind of like a dict, but each key in the dict would be two values, between which the querying key would fall. Thus was born BetweenDict. It’s short and sweet, and to the point. And works for what I need.

Update: this now has a repo. https://github.com/jkugler/between_dict

class BetweenDict(dict):
    def __init__(self, d = {}):
        for k,v in d.items():
            self[k] = v

    def __getitem__(self, key):
        for k, v in self.items():
            if k[0] <= key < k[1]:
                return v
        raise KeyError("Key '%s' is not between any values in the BetweenDict" % key)

    def __setitem__(self, key, value):
        try:
            if len(key) == 2:
                if key[0] < key[1]:
                    dict.__setitem__(self, (key[0], key[1]), value)
                else:
                    raise RuntimeError('First element of a BetweenDict key '
                                       'must be strictly less than the '
                                       'second element')
            else:
                raise ValueError('Key of a BetweenDict must be an iterable '
                                 'with length two')
        except TypeError:
            raise TypeError('Key of a BetweenDict must be an iterable '
                             'with length two')

    def __contains__(self, key):
        try:
            return bool(self[key]) or True
        except KeyError:
            return False

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