def _lookup(self, x=None, y=None, kwargs=None):
mapping = _Mapping(x=x, y=y, kwargs=kwargs)<EOL>subkey = mapping.subkey<EOL>if x is not None:<EOL><INDENT>mapping = self._from_x[x].get(subkey, mapping).merge(x=x)<EOL><DEDENT>if y is not None:<EOL><INDENT>mapping = self._from_y[y].get(subkey, mapping).merge(y=y)<EOL><DEDENT>return mapping<EOL>
Helper which retrieves mapping info from forward/inverse dicts.