code: def label_field(self, f):
          
          if f is None:
              raise ValueError("Label field name cannot be None.")
          self._assert_ml_fields_valid(f)
          return _change_singleton_roles(self, {_get_field_name(f): FieldRole.LABEL}, clear_feature=True)
docs: Select one field as the label field.
  
          Note that this field will be exclude from feature fields.
  
          :param f: Selected label field
          :type f: str
          :rtype: DataFrame