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Overview

LangChain is a popular framework for building LLM applications. LemonData works seamlessly with LangChain’s OpenAI integration.

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Installation

pip install langchain langchain-openai

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Basic Configuration

from langchain_openai import ChatOpenAI

llm = ChatOpenAI(
    model="gpt-4o",
    api_key="sk-your-lemondata-key",
    base_url="https://api.lemondata.cc/v1"
)

response = llm.invoke("Hello, how are you?")
print(response.content)

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Using Different Models

Access any LemonData model: 
# OpenAI GPT-4o
gpt4 = ChatOpenAI(
    model="gpt-4o",
    api_key="sk-your-key",
    base_url="https://api.lemondata.cc/v1"
)

# Anthropic Claude
claude = ChatOpenAI(
    model="claude-3-5-sonnet-20241022",
    api_key="sk-your-key",
    base_url="https://api.lemondata.cc/v1"
)

# Google Gemini
gemini = ChatOpenAI(
    model="gemini-2.0-flash",
    api_key="sk-your-key",
    base_url="https://api.lemondata.cc/v1"
)

# DeepSeek
deepseek = ChatOpenAI(
    model="deepseek-r1",
    api_key="sk-your-key",
    base_url="https://api.lemondata.cc/v1"
)

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Chat with Message History

from langchain_core.messages import HumanMessage, SystemMessage

messages = [
    SystemMessage(content="You are a helpful assistant."),
    HumanMessage(content="What is the capital of France?")
]

response = llm.invoke(messages)
print(response.content)

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Streaming

for chunk in llm.stream("Write a poem about coding"):
    print(chunk.content, end="", flush=True)

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Async Usage

import asyncio

async def main():
    response = await llm.ainvoke("Hello!")
    print(response.content)

asyncio.run(main())

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Chains

from langchain_core.prompts import ChatPromptTemplate
from langchain_core.output_parsers import StrOutputParser

prompt = ChatPromptTemplate.from_messages([
    ("system", "You are a helpful assistant that translates {input_language} to {output_language}."),
    ("human", "{text}")
])

chain = prompt | llm | StrOutputParser()

result = chain.invoke({
    "input_language": "English",
    "output_language": "French",
    "text": "Hello, how are you?"
})
print(result)

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RAG (Retrieval Augmented Generation)

from langchain_openai import OpenAIEmbeddings
from langchain_community.vectorstores import FAISS
from langchain_core.prompts import ChatPromptTemplate
from langchain_core.runnables import RunnablePassthrough

# Embeddings
embeddings = OpenAIEmbeddings(
    model="text-embedding-3-small",
    api_key="sk-your-key",
    base_url="https://api.lemondata.cc/v1"
)

# Create vector store
texts = ["LemonData supports 300+ AI models", "API is OpenAI compatible"]
vectorstore = FAISS.from_texts(texts, embeddings)
retriever = vectorstore.as_retriever()

# RAG chain
template = """Answer based on context:
{context}

Question: {question}
"""
prompt = ChatPromptTemplate.from_template(template)

rag_chain = (
    {"context": retriever, "question": RunnablePassthrough()}
    | prompt
    | llm
)

response = rag_chain.invoke("How many models does LemonData support?")
print(response.content)

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Agents

from langchain.agents import create_openai_tools_agent, AgentExecutor
from langchain_core.prompts import ChatPromptTemplate
from langchain_core.tools import tool

@tool
def search(query: str) -> str:
    """Search for information."""
    return f"Search results for: {query}"

tools = [search]

prompt = ChatPromptTemplate.from_messages([
    ("system", "You are a helpful assistant with access to tools."),
    ("human", "{input}"),
    ("placeholder", "{agent_scratchpad}")
])

agent = create_openai_tools_agent(llm, tools, prompt)
executor = AgentExecutor(agent=agent, tools=tools)

result = executor.invoke({"input": "Search for LemonData pricing"})
print(result["output"])

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Environment Variables

For cleaner code, use environment variables: 
export OPENAI_API_KEY="sk-your-lemondata-key"
export OPENAI_API_BASE="https://api.lemondata.cc/v1"

from langchain_openai import ChatOpenAI

# Will automatically use environment variables
llm = ChatOpenAI(model="gpt-4o")

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Callbacks and Tracing

from langchain_core.callbacks import StdOutCallbackHandler

llm = ChatOpenAI(
    model="gpt-4o",
    api_key="sk-your-key",
    base_url="https://api.lemondata.cc/v1",
    callbacks=[StdOutCallbackHandler()]
)

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Best Practices

Use cheaper models (GPT-4o-mini) for simple tasks in chains.
LangChain has built-in retry logic for transient errors.
Use callbacks to track token consumption.