update keys for chain (#5164)

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/chains/combine_documents/stuff.py

@@ -100,6 +100,13 @@ class StuffDocumentsChain(BaseCombineDocumentsChain):
                 )
         return values
 
+    @property
+    def input_keys(self) -> List[str]:
+        extra_keys = [
+            k for k in self.llm_chain.input_keys if k != self.document_variable_name
+        ]
+        return super().input_keys + extra_keys
+
     def _get_inputs(self, docs: List[Document], **kwargs: Any) -> dict:
         """Construct inputs from kwargs and docs.
 

libs/langchain/tests/unit_tests/chains/test_combine_documents.py

@@ -9,8 +9,10 @@ from langchain.chains.combine_documents.reduce import (
     _collapse_docs,
     _split_list_of_docs,
 )
+from langchain.chains.qa_with_sources import load_qa_with_sources_chain
 from langchain.docstore.document import Document
 from langchain.schema import format_document
+from tests.unit_tests.llms.fake_llm import FakeLLM
 
 
 def _fake_docs_len_func(docs: List[Document]) -> int:
@@ -21,6 +23,11 @@ def _fake_combine_docs_func(docs: List[Document], **kwargs: Any) -> str:
     return "".join([d.page_content for d in docs])
 
 
+def test_multiple_input_keys() -> None:
+    chain = load_qa_with_sources_chain(FakeLLM(), chain_type="stuff")
+    assert chain.input_keys == ["input_documents", "question"]
+
+
 def test__split_list_long_single_doc() -> None:
     """Test splitting of a long single doc."""
     docs = [Document(page_content="foo" * 100)]