Langchain

LangChain Framework

Overview

LangChain is a framework for developing applications powered by language models. It provides interfaces for LLMs, prompt management, memory, chains, agents, and tools.

Installation

pip install langchain langchain-openai langchain-community
pip install chromadb  # For vector store

Core Components

Language Models

from langchain_openai import ChatOpenAI
from langchain_community.llms import HuggingFacePipeline

# OpenAI
llm = ChatOpenAI(
    model="gpt-4",
    temperature=0.7,
    api_key="your-api-key"
)

# Hugging Face
from transformers import AutoModelForCausalLM, AutoTokenizer, pipeline
model = AutoModelForCausalLM.from_pretrained("gpt2")
tokenizer = AutoTokenizer.from_pretrained("gpt2")
pipe = pipeline("text-generation", model=model, tokenizer=tokenizer)
llm = HuggingFacePipeline(pipeline=pipe)

Prompts

from langchain.prompts import PromptTemplate, ChatPromptTemplate

# Simple prompt
prompt = PromptTemplate(
    input_variables=["product"],
    template="What is a good name for a company that makes {product}?"
)

# Chat prompt
prompt = ChatPromptTemplate.from_messages([
    ("system", "You are a helpful assistant."),
    ("human", "{input}")
])

Chains

from langchain.chains import LLMChain

# Basic chain
chain = LLMChain(llm=llm, prompt=prompt)
result = chain.run(product="colorful socks")

# Sequential chain
from langchain.chains import SimpleSequentialChain

chain1 = LLMChain(llm=llm, prompt=prompt1)
chain2 = LLMChain(llm=llm, prompt=prompt2)
combined_chain = SimpleSequentialChain(chains=[chain1, chain2])

RAG (Retrieval Augmented Generation)

Basic RAG Pipeline

from langchain_community.document_loaders import TextLoader
from langchain.text_splitter import RecursiveCharacterTextSplitter
from langchain_community.vectorstores import Chroma
from langchain_openai import OpenAIEmbeddings
from langchain.chains import RetrievalQA

# Load documents
loader = TextLoader("document.txt")
documents = loader.load()

# Split documents
text_splitter = RecursiveCharacterTextSplitter(
    chunk_size=1000,
    chunk_overlap=200
)
splits = text_splitter.split_documents(documents)

# Create vector store
vectorstore = Chroma.from_documents(
    documents=splits,
    embedding=OpenAIEmbeddings()
)

# Create RAG chain
retriever = vectorstore.as_retriever(search_kwargs={"k": 3})
qa_chain = RetrievalQA.from_chain_type(
    llm=llm,
    chain_type="stuff",
    retriever=retriever
)

# Query
result = qa_chain.run("What is the document about?")

Custom RAG with Prompts

from langchain.prompts import PromptTemplate

prompt_template = """
Use the following pieces of context to answer the question at the end.
If you don't know the answer, just say that you don't know, don't try to make up an answer.

Context: {context}

Question: {question}

Answer:
"""

PROMPT = PromptTemplate(
    template=prompt_template,
    input_variables=["context", "question"]
)

qa_chain = RetrievalQA.from_chain_type(
    llm=llm,
    chain_type="stuff",
    retriever=retriever,
    chain_type_kwargs={"prompt": PROMPT}
)

Memory

Conversation Memory

from langchain.memory import ConversationBufferMemory

memory = ConversationBufferMemory()

memory.save_context({"input": "Hi"}, {"output": "Hello"})
memory.load_memory_variables({})

Memory in Chains

from langchain.chains import ConversationChain

conversation = ConversationChain(
    llm=llm,
    memory=ConversationBufferMemory()
)

response = conversation.run("Hi, I'm John")
response = conversation.run("What's my name?")

Memory Types

• ConversationBufferMemory: Stores all conversation history
• ConversationBufferWindowMemory: Keeps last k exchanges
• ConversationSummaryMemory: Summarizes conversation
• ConversationKGMemory: Uses knowledge graph

Agents

ReAct Agent

from langchain.agents import AgentExecutor, create_openai_functions_agent
from langchain.tools import Tool
from langchain_openai import ChatOpenAI

# Define tools
def calculator(input_str):
    return eval(input_str)

tools = [
    Tool(
        name="Calculator",
        func=calculator,
        description="Useful for math calculations"
    )
]

# Create agent
llm = ChatOpenAI(model="gpt-4")
agent = create_openai_functions_agent(llm, tools, prompt)

# Create executor
agent_executor = AgentExecutor(
    agent=agent,
    tools=tools,
    verbose=True
)

# Run
result = agent_executor.invoke({"input": "What is 2 + 2?"})

Custom Tools

from langchain.tools import StructuredTool
from pydantic import BaseModel, Field

class SearchInput(BaseModel):
    query: str = Field(description="Search query")

def search_func(query: str) -> str:
    # Implement search
    return f"Results for {query}"

search_tool = StructuredTool.from_function(
    func=search_func,
    name="Search",
    description="Search the web",
    args_schema=SearchInput
)

Document Loaders

Common Loaders

from langchain_community.document_loaders import (
    TextLoader,
    PyPDFLoader,
    WebBaseLoader,
    GitHubLoader,
    YoutubeLoader
)

# Text
loader = TextLoader("file.txt")

# PDF
loader = PyPDFLoader("document.pdf")

# Web
loader = WebBaseLoader("https://example.com")

# GitHub
loader = GitHubLoader(
    repo="username/repo",
    branch="main",
    folder_path="src"
)

# YouTube
loader = YoutubeLoader.from_youtube_url(
    "https://youtube.com/watch?v=xxx",
    add_video_info=True
)

Custom Loader

from langchain.schema import Document

class CustomLoader:
    def load(self) -> list[Document]:
        # Load data
        data = self._fetch_data()

        # Convert to documents
        documents = [
            Document(page_content=item["text"], metadata=item["meta"])
            for item in data
        ]
        return documents

Text Splitters

Character Text Splitter

from langchain.text_splitter import CharacterTextSplitter

splitter = CharacterTextSplitter(
    separator="\n\n",
    chunk_size=1000,
    chunk_overlap=200,
    length_function=len
)

Recursive Character Splitter

from langchain.text_splitter import RecursiveCharacterTextSplitter

splitter = RecursiveCharacterTextSplitter(
    chunk_size=1000,
    chunk_overlap=200,
    separators=["\n\n", "\n", " ", ""]
)

Code Splitter

from langchain.text_splitter import RecursiveCharacterTextSplitter, Language

splitter = RecursiveCharacterTextSplitter.from_language(
    language=Language.PYTHON,
    chunk_size=1000,
    chunk_overlap=200
)

Vector Stores

Pinecone

from langchain_community.vectorstores import Pinecone
from langchain_openai import OpenAIEmbeddings

vectorstore = Pinecone.from_documents(
    documents=documents,
    embedding=OpenAIEmbeddings(),
    index_name="my-index"
)

ChromaDB

from langchain_community.vectorstores import Chroma

vectorstore = Chroma.from_documents(
    documents=documents,
    embedding=OpenAIEmbeddings(),
    persist_directory="./chroma_db"
)

Weaviate

from langchain_community.vectorstores import Weaviate

vectorstore = Weaviate.from_documents(
    documents=documents,
    embedding=OpenAIEmbeddings(),
    index_name="MyIndex"
)

Output Parsers

Structured Output

from langchain.output_parsers import StructuredOutputParser, ResponseSchema
from langchain.prompts import PromptTemplate

# Define schema
schemas = [
    ResponseSchema(name="name", description="The person's name"),
    ResponseSchema(name="age", description="The person's age")
]

output_parser = StructuredOutputParser.from_response_schemas(schemas)

# Use in prompt
format_instructions = output_parser.get_format_instructions()
prompt = PromptTemplate(
    template="Answer the user question.\n{format_instructions}\n{question}",
    input_variables=["question"],
    partial_variables={"format_instructions": format_instructions}
)

Best Practices

1. Prompt Engineering

• Be specific and clear
• Provide examples (few-shot)
• Use appropriate delimiters
• Specify output format

2. Chain Design

• Keep chains simple and focused
• Use appropriate chain type
• Handle errors gracefully
• Add validation

3. RAG Optimization

• Chunk size: 500-1500 tokens
• Overlap: 10-20% of chunk size
• Use appropriate embeddings
• Tune retrieval (k value, search type)

4. Agent Design

• Provide clear tool descriptions
• Limit tool set to essentials
• Add error handling
• Monitor and log agent actions

5. Performance

• Cache embeddings
• Use batch processing
• Stream responses for long text
• Optimize vector store queries

Integration

• Vector Databases: Pinecone, Weaviate, Qdrant, Chroma
• LLMs: OpenAI, Anthropic, Hugging Face, Cohere
• Tools: Web search, databases, APIs, file systems
• Deployment: FastAPI, Streamlit, AWS Lambda