config.py

import bisect
import collections

class Configuration(collections.Mapping):
    '''dict-like object that allows multiple values per key. each key is
    associated with a 'priority' __getitem__ always return the entry with the
    highest priority
    '''
    def __init__(self):
        self._items = collections.defaultdict(lambda : [])
        self._priorities = collections.defaultdict(lambda : [])

    def __getitem__(self, key):
        return self._items[key][-1]

    def __contains__(self, key):
        return key in self._items

    def set(self, key, value, priority=0):
        i = bisect.bisect_right(self._priorities[key], priority)
        self._priorities[key].insert(i, priority)
        self._items[key].insert(i, value)

    def __len__(self):
        return len(self._items)

    def update(self, other):
        if not isinstance(other, Configuration):
            raise TypeError()
        for k, v, p in other:
            self.set(k, v, p)
    
    def __iter__(self):
        for k in self._items.keys():
            for v, p in zip(self._items[k], self._priorities[k]): 
                yield k, v, p


if __name__ == '__main__':
    c = Configuration()
    c.set('key', 'b', 1)
    c.set('key', 'a', 0)
    c.set('otherkey', 'o', 1)
    assert c['key'] == 'b'
    assert c['otherkey'] == 'o'
    
    c2 = Configuration()
    c2.set('key', 'c', 2)
    
    c2.update(c)
    expected = iter([('key', 'a', 0), ('key', 'b', 1), ('key', 'c', 2)])
    for i, (k, v, p) in enumerate(c2):
        if k == 'key':
            assert next(expected) == (k, v, p) 
        

Some code adapted from http://code.activestate.com/recipes/577197-sortedcollection/