code: def _check_stop_list(stop):
      
      if stop == "thai":
          return THAI_STOP_WORDS
      elif isinstance(stop, six.string_types):
          raise ValueError("not a built-in stop list: %s" % stop)
      elif stop is None:
          return None
      # assume it's a collection
      return frozenset(stop)
docs: Check stop words list
      ref: https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/feature_extraction/text.py#L87-L95