def _cache_update(self, mapping):
if mapping.x is not None and mapping.subkey in self._from_x[mapping.x]:<EOL><INDENT>self._cache_by_x(mapping)<EOL><DEDENT>if mapping.y is not None and mapping.subkey in self._from_y[mapping.y]:<EOL><INDENT>self._cache_by_y(mapping)<EOL><DEDENT>
Helper which updates only those cached entries that already exist.