"""runnable_parallel_demo.py — run two chains at the same time Run with venv active: python runnable_parallel_demo.py """ from dotenv import load_dotenv from langchain_openai import ChatOpenAI from langchain_core.prompts import ChatPromptTemplate from langchain_core.output_parsers import StrOutputParser from langchain_core.runnables import RunnableParallel load_dotenv() model = ChatOpenAI(model="gpt-4o-mini", temperature=0) parser = StrOutputParser() purpose_prompt = ChatPromptTemplate.from_template( "What does the HTML '{tag}' element do? One short sentence." ) example_prompt = ChatPromptTemplate.from_messages([ ( "system", "Reply with exactly one line of HTML. " "No markdown fences. No DOCTYPE. No , , or tags. " "Only the single tag snippet.", ), ("human", "Minimal one-line example for the HTML '{tag}' tag:"), ]) purpose_chain = purpose_prompt | model | parser example_chain = example_prompt | model | parser # Run both branches in parallel — result is a dict chain = RunnableParallel( purpose=purpose_chain, example=example_chain, ) tags = ["a", "img"] for tag in tags: result = chain.invoke({"tag": tag}) print(f"Tag: <{tag}>") print(f" purpose: {result['purpose']}") print(f" example: {result['example']}\n")