docs: def ensure_utf8(str_or_unicode):
      """
      tests, if the input is ``str`` or ``unicode``. if it is ``unicode``,
      it will be encoded from ``unicode`` to ``utf-8``. otherwise, the
      input string is returned.
      """
      if isinstance(str_or_unicode, str):
          return str_or_unicode
      elif isinstance(str_or_unicode, unicode):
          return str_or_unicode.encode('utf-8')
      else:
          raise ValueError(
              "Input '{0}' should be a string or unicode, but it is of "
              "type {1}".format(str_or_unicode, type(str_or_unicode)))