Python: finding keys with unique values in a dictionary?

Question

I receive a dictionary as input, and want to return a list of keys for which the dictionary values are unique in the scope of that dictionary.

I will clarify with an exa

Answer 1

What about subclassing?

class UniqueValuesDict(dict):

    def __init__(self, *args):
        dict.__init__(self, *args)
        self._inverse = {}

    def __setitem__(self, key, value):
        if value in self.values():
            if value in self._inverse:
                del self._inverse[value]
        else:
            self._inverse[value] = key
        dict.__setitem__(self, key, value)

    def unique_values(self):
        return self._inverse.values()

a = UniqueValuesDict()

a['cat'] =      1
a['fish'] =     1
a[None] =       1
a['duck'] =     1
a['dog'] =      2  # <-- unique
a['bat'] =      3
a['aardvark'] = 3
a['snake'] =    4  # <-- unique
a['wallaby'] =  5
a['badger'] =   5

assert a.unique_values() == ['dog', 'snake']