langchain/vectorstores/faiss.py/FAISS/save_local
class FAISS:    def save_local(self, folder_path: str, index_name: str = "index") -> None:
        """Save FAISS index, docstore, and index_to_docstore_id to disk.

        Args:
            folder_path: folder path to save index, docstore,
                and index_to_docstore_id to.
            index_name: for saving with a specific index file name
        """
        path = Path(folder_path)
        path.mkdir(exist_ok=True, parents=True)

        # save index separately since it is not picklable
        faiss = dependable_faiss_import()
        faiss.write_index(
            self.index, str(path / "{index_name}.faiss".format(index_name=index_name))
        )

        # save docstore and index_to_docstore_id
        with open(path / "{index_name}.pkl".format(index_name=index_name), "wb") as f:
            pickle.dump((self.docstore, self.index_to_docstore_id), f)