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