code: def _is_utf_8(txt):
      
      assert isinstance(txt, six.binary_type)
  
      try:
          _ = six.text_type(txt, 'utf-8')
      except (TypeError, UnicodeEncodeError):
          return False
      else:
          return True
docs: Check a string is utf-8 encoded
  
      :param bytes txt: utf-8 string
      :return: Whether the string\
      is utf-8 encoded or not
      :rtype: bool