sequential chain from prompts

tests/unit_tests/chains/test_sequential.py

@@ -5,7 +5,14 @@ import pytest
 from pydantic import BaseModel
 
 from langchain.chains.base import Chain
-from langchain.chains.sequential import SequentialChain, SimpleSequentialChain
+from langchain.chains.llm import LLMChain
+from langchain.chains.sequential import (
+    SequentialChain,
+    SimpleSequentialChain,
+    construct_sequential_llm_chain,
+)
+from langchain.prompts.prompt import PromptTemplate
+from tests.unit_tests.llms.fake_llm import FakeLLM
 
 
 class FakeChain(Chain, BaseModel):
@@ -138,3 +145,21 @@ def test_multi_output_errors() -> None:
     chain_2 = FakeChain(input_variables=["bar"], output_variables=["baz"])
     with pytest.raises(ValueError):
         SimpleSequentialChain(chains=[chain_1, chain_2])
+
+
+def test_construct_sequential_llm_chain() -> None:
+    """Test constructing simple sequential chain."""
+    prompt = PromptTemplate(template="what is {foo}?", input_variables=["foo"])
+    llm_chain = LLMChain(llm=FakeLLM(), prompt=prompt, output_key="bar")
+    add_ons = [("{bar} and what does it do?", ["bar"], "baz")]
+    chain = construct_sequential_llm_chain(llm_chain, add_ons)
+
+    expected_new_prompt = PromptTemplate(
+        template="what is {foo}?{bar} and what does it do?",
+        input_variables=["foo", "bar"],
+    )
+    expected_new_chain = LLMChain(
+        llm=FakeLLM(), prompt=expected_new_prompt, output_key="baz"
+    )
+    expected_chains = [llm_chain, expected_new_chain]
+    assert chain.chains == expected_chains