Convert dict fields into objects, where appropriate

def _update_fields_with_objects(self):
        """ Convert dict fields into objects, where appropriate """
        # Update the data with photo objects
        if self.type is not None:
            if self.type.startswith("photo"):
                self.data = Photo(self._client, self.data)
            else:
                raise NotImplementedError("Unrecognised activity type: %s"
                                          % self.type)