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