:param attributes_a:    A dict of functions to their attributes
        :param attributes_b:    A dict of functions to their attributes

        The following parameters are optional.

        :param filter_set_a:    A set to limit attributes_a to the functions in this set.
        :param filter_set_b:    A set to limit attributes_b to the functions in this set.
        :returns:               A list of tuples of matching objects.

def _get_function_matches(attributes_a, attributes_b, filter_set_a=None, filter_set_b=None):
        """
        :param attributes_a:    A dict of functions to their attributes
        :param attributes_b:    A dict of functions to their attributes

        The following parameters are optional.

        :param filter_set_a:    A set to limit attributes_a to the functions in this set.
        :param filter_set_b:    A set to limit attributes_b to the functions in this set.
        :returns:               A list of tuples of matching objects.
        """
        # get the attributes that are in the sets
        if filter_set_a is None:
            filtered_attributes_a = {k: v for k, v in attributes_a.items()}
        else:
            filtered_attributes_a = {k: v for k, v in attributes_a.items() if k in filter_set_a}

        if filter_set_b is None:
            filtered_attributes_b = {k: v for k, v in attributes_b.items()}
        else:
            filtered_attributes_b = {k: v for k, v in attributes_b.items() if k in filter_set_b}

        # get closest
        closest_a = _get_closest_matches(filtered_attributes_a, filtered_attributes_b)
        closest_b = _get_closest_matches(filtered_attributes_b, filtered_attributes_a)

        # a match (x,y) is good if x is the closest to y and y is the closest to x
        matches = []
        for a in closest_a:
            if len(closest_a[a]) == 1:
                match = closest_a[a][0]
                if len(closest_b[match]) == 1 and closest_b[match][0] == a:
                    matches.append((a, match))

        return matches