code: def _deleter(self, obj):
          
          if self._fdel_ is not None:
              self._fdel_(obj)
  
          else:
              delattr(obj, self._attrname())
  
          # notify parent schema about value deletion.
          if isinstance(obj, Schema):
              obj._delvalue(self)
docs: Called when the parent element tries to delete this property value.
  
          :param obj: parent object.