fix(memory): allow internal chains to use memory (#6769)

Fixed #6768.

This is a workaround only. I think a better longer-term solution is for
chains to declare how many input variables they *actually* need (as
opposed to ones that are in the prompt, where some may be satisfied by
the memory). Then, a wrapping chain can check the input match against
the actual input variables.

@hwchase17

langchain/chains/sequential.py

@@ -62,6 +62,9 @@ class SequentialChain(Chain):
 
         for chain in chains:
             missing_vars = set(chain.input_keys).difference(known_variables)
+            if chain.memory:
+                missing_vars = missing_vars.difference(chain.memory.memory_variables)
+
             if missing_vars:
                 raise ValueError(
                     f"Missing required input keys: {missing_vars}, "

tests/unit_tests/chains/test_sequential.py

@@ -6,6 +6,7 @@ import pytest
 from langchain.callbacks.manager import CallbackManagerForChainRun
 from langchain.chains.base import Chain
 from langchain.chains.sequential import SequentialChain, SimpleSequentialChain
+from langchain.memory import ConversationBufferMemory
 from langchain.memory.simple import SimpleMemory
 
 
@@ -81,6 +82,21 @@ def test_sequential_usage_memory() -> None:
         )
 
 
+def test_sequential_internal_chain_use_memory() -> None:
+    """Test sequential usage with memory for one of the internal chains."""
+    memory = ConversationBufferMemory(memory_key="bla")
+    memory.save_context({"input": "yo"}, {"output": "ya"})
+    chain_1 = FakeChain(
+        input_variables=["foo", "bla"], output_variables=["bar"], memory=memory
+    )
+    chain_2 = FakeChain(input_variables=["bar"], output_variables=["baz"])
+    chain = SequentialChain(chains=[chain_1, chain_2], input_variables=["foo"])
+    output = chain({"foo": "123"})
+    print("HEYYY OUTPUT", output)
+    expected_output = {"foo": "123", "baz": "123 Human: yo\nAI: yafoofoo"}
+    assert output == expected_output
+
+
 def test_sequential_usage_multiple_outputs() -> None:
     """Test sequential usage on multiple output chains."""
     chain_1 = FakeChain(input_variables=["foo"], output_variables=["bar", "test"])