code: def _none_value(self):
          
          if self.out_type == int:
              return 0
          elif self.out_type == float:
              return 0.0
          elif self.out_type == bool:
              return False
          elif self.out_type == six.text_type:
              return u''
docs: Get an appropriate "null" value for this field's type. This
          is used internally when setting the field to None.