docs: def default_listener(col_attr, default):
      """Establish a default-setting listener."""
  
      @event.listens_for(col_attr, "init_scalar", retval=True, propagate=True)
      def init_scalar(target, value, dict_):
  
          if default.is_callable:
              # the callable of ColumnDefault always accepts a context argument
              value = default.arg(None)
          elif default.is_scalar:
              value = default.arg
          else:
              raise NotImplementedError(
                  "Can't invoke pre-default for a SQL-level column default")
  
          dict_[col_attr.key] = value
  
          return value