def _findUniqueMappingKeys(mapping):
    """Find mapping keys that only have one entry (value length of 1.

    :param mapping: dict, for each key contains a set of entries
    :returns: a set of unique mapping keys
    """

    uniqueMappingKeys = set()
    for key, entries in viewitems(mapping):
        if len(entries) == 1:
            uniqueMappingKeys.add(key)
    return uniqueMappingKeys