Fix combining runnable sequences (#8557)

Combining runnable sequences was dropping a step in the middle.

@nfcampos @baskaryan

libs/langchain/langchain/schema/runnable.py

@@ -214,7 +214,7 @@ class RunnableSequence(Serializable, Runnable[Input, Output]):
         if isinstance(other, RunnableSequence):
             return RunnableSequence(
                 first=self.first,
-                middle=self.middle + [self.last] + other.middle,
+                middle=self.middle + [self.last] + [other.first] + other.middle,
                 last=other.last,
             )
         else:
@@ -235,7 +235,7 @@ class RunnableSequence(Serializable, Runnable[Input, Output]):
         if isinstance(other, RunnableSequence):
             return RunnableSequence(
                 first=other.first,
-                middle=other.middle + [other.last] + self.middle,
+                middle=other.middle + [other.last] + [self.first] + self.middle,
                 last=self.last,
             )
         else:

libs/langchain/tests/unit_tests/schema/test_runnable.py

@@ -440,6 +440,64 @@ def test_prompt_with_chat_model_and_parser(
     assert tracer.runs == snapshot
 
 
+@freeze_time("2023-01-01")
+def test_combining_sequences(
+    mocker: MockerFixture, snapshot: SnapshotAssertion
+) -> None:
+    prompt = (
+        SystemMessagePromptTemplate.from_template("You are a nice assistant.")
+        + "{question}"
+    )
+    chat = FakeListChatModel(responses=["foo, bar"])
+    parser = CommaSeparatedListOutputParser()
+
+    chain = prompt | chat | parser
+
+    assert isinstance(chain, RunnableSequence)
+    assert chain.first == prompt
+    assert chain.middle == [chat]
+    assert chain.last == parser
+    assert dumps(chain, pretty=True) == snapshot
+
+    prompt2 = (
+        SystemMessagePromptTemplate.from_template("You are a nicer assistant.")
+        + "{question}"
+    )
+    chat2 = FakeListChatModel(responses=["baz, qux"])
+    parser2 = CommaSeparatedListOutputParser()
+    input_formatter: RunnableLambda[List[str], Dict[str, Any]] = RunnableLambda(
+        lambda x: {"question": x[0] + x[1]}
+    )
+
+    chain2 = input_formatter | prompt2 | chat2 | parser2
+
+    assert isinstance(chain, RunnableSequence)
+    assert chain2.first == input_formatter
+    assert chain2.middle == [prompt2, chat2]
+    assert chain2.last == parser2
+    assert dumps(chain2, pretty=True) == snapshot
+
+    combined_chain = chain | chain2
+
+    assert combined_chain.first == prompt
+    assert combined_chain.middle == [
+        chat,
+        parser,
+        input_formatter,
+        prompt2,
+        chat2,
+    ]
+    assert combined_chain.last == parser2
+    assert dumps(combined_chain, pretty=True) == snapshot
+
+    # Test invoke
+    tracer = FakeTracer()
+    assert combined_chain.invoke(
+        {"question": "What is your name?"}, dict(callbacks=[tracer])
+    ) == ["baz", "qux"]
+    assert tracer.runs == snapshot
+
+
 @freeze_time("2023-01-01")
 def test_seq_dict_prompt_llm(
     mocker: MockerFixture, snapshot: SnapshotAssertion