code: def _construct_key(previous_key, separator, new_key):
      
      if previous_key:
          return u"{}{}{}".format(previous_key, separator, new_key)
      else:
          return new_key
docs: Returns the new_key if no previous key exists, otherwise concatenates
      previous key, separator, and new_key
      :param previous_key:
      :param separator:
      :param new_key:
      :return: a string if previous_key exists and simply passes through the
      new_key otherwise