fix map reduce chain (#550)

langchain/chains/combine_documents/map_reduce.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import Any, Callable, Dict, List, Optional, Tuple
+from typing import Any, Callable, Dict, List, Optional, Protocol, Tuple
 
 from pydantic import BaseModel, Extra, root_validator
 
@@ -11,6 +11,13 @@ from langchain.chains.llm import LLMChain
 from langchain.docstore.document import Document
 
 
+class CombineDocsProtocol(Protocol):
+    """Interface for the combine_docs method."""
+
+    def __call__(self, docs: List[Document], **kwargs: Any) -> Tuple[str, dict]:
+        """Interface for the combine_docs method."""
+
+
 def _split_list_of_docs(
     docs: List[Document], length_func: Callable, token_max: int, **kwargs: Any
 ) -> List[List[Document]]:
@@ -38,10 +45,10 @@ def _split_list_of_docs(
 
 def _collapse_docs(
     docs: List[Document],
-    combine_document_func: Callable,
+    combine_document_func: CombineDocsProtocol,
     **kwargs: Any,
 ) -> Document:
-    result = combine_document_func(docs, **kwargs)
+    result, _ = combine_document_func(docs, **kwargs)
     combined_metadata = {k: str(v) for k, v in docs[0].metadata.items()}
     for doc in docs[1:]:
         for k, v in doc.metadata.items():

tests/unit_tests/chains/test_combine_documents.py

@@ -1,6 +1,6 @@
 """Test functionality related to combining documents."""
 
-from typing import List
+from typing import Any, List, Tuple
 
 import pytest
 
@@ -12,11 +12,11 @@ from langchain.docstore.document import Document
 
 
 def _fake_docs_len_func(docs: List[Document]) -> int:
-    return len(_fake_combine_docs_func(docs))
+    return len(_fake_combine_docs_func(docs)[0])
 
 
-def _fake_combine_docs_func(docs: List[Document]) -> str:
-    return "".join([d.page_content for d in docs])
+def _fake_combine_docs_func(docs: List[Document], **kwargs: Any) -> Tuple[str, dict]:
+    return "".join([d.page_content for d in docs]), {}
 
 
 def test__split_list_long_single_doc() -> None: