code: def handle_missing(self, instance, ctx):
          
          if self.missing is not None:
              value = self.missing(instance, ctx)
              if value is not None:
                  self._set_from_code(instance, value)
                  return
  
          if self.default is _PropBase.NO_DEFAULT:
              raise ValueError("missing attribute {} on {}".format(
                  tag_to_str(self.tag),
                  tag_to_str(instance.TAG),
              ))
docs: Handle a missing attribute on `instance`. This is called whenever no
          value for the attribute is found during parsing. The call to
          :meth:`missing` is independent of the value of `required`.
  
          If the `missing` callback is not :data:`None`, it is called with the
          `instance` and the `ctx` as arguments. If the returned value is not
          :data:`None`, it is used as the value of the attribute (validation
          takes place as if the value had been set from the code, not as if the
          value had been received from XML) and the handler returns.
  
          If the `missing` callback is :data:`None` or returns :data:`None`, the
          handling continues as normal: if `required` is true, a
          :class:`ValueError` is raised.