langchain/vectorstores/faiss.py/FAISS/__add
class FAISS:    def __add(
        self,
        texts: Iterable[str],
        embeddings: Iterable[List[float]],
        metadatas: Optional[List[dict]] = None,
        ids: Optional[List[str]] = None,
        **kwargs: Any,
    ) -> List[str]:
        if not isinstance(self.docstore, AddableMixin):
            raise ValueError(
                "If trying to add texts, the underlying docstore should support "
                f"adding items, which {self.docstore} does not"
            )
        documents = []
        for i, text in enumerate(texts):
            metadata = metadatas[i] if metadatas else {}
            documents.append(Document(page_content=text, metadata=metadata))
        if ids is None:
            ids = [str(uuid.uuid4()) for _ in texts]
        # Add to the index, the index_to_id mapping, and the docstore.
        starting_len = len(self.index_to_docstore_id)
        faiss = dependable_faiss_import()
        vector = np.array(embeddings, dtype=np.float32)
        if self._normalize_L2:
            faiss.normalize_L2(vector)
        self.index.add(vector)
        # Get list of index, id, and docs.
        full_info = [(starting_len + i, ids[i], doc) for i, doc in enumerate(documents)]
        # Add information to docstore and index.
        self.docstore.add({_id: doc for _, _id, doc in full_info})
        index_to_id = {index: _id for index, _id, _ in full_info}
        self.index_to_docstore_id.update(index_to_id)
        return [_id for _, _id, _ in full_info]