code: def _check_input(self, input):
          
          if isinstance(input, str):
              return 'st'
          elif isinstance(input, list):
              if all(isinstance(item, str) for item in input):
                  return 'gst'
  
          raise ValueError("String argument should be of type String or"
                                       " a list of strings")
docs: Checks the validity of the input.
  
          In case of an invalid input throws ValueError.