LangChain · Lesson 2 of 5

Prompts, Parsers and Chains

Compose prompts, models and parsers with the pipe operator.

• Beginner
• 16 min read
• 3 objectives

Before this lessonLesson 1: LLM Apps and LangChain Basics

Hard-coding prompts as strings becomes messy fast. LangChain gives you reusable prompt templates, output parsers and a way to connect them into chains with the pipe operator, called LCEL (LangChain Expression Language).

Prompt templates

from langchain_core.prompts import ChatPromptTemplate

prompt = ChatPromptTemplate.from_messages([
    ("system", "You are a helpful {role}. Answer in at most {words} words."),
    ("human", "{question}"),
])

messages = prompt.invoke({"role": "teacher", "words": 30, "question": "What is recursion?"})

A first chain

The pipe | feeds the output of one step into the next: prompt, then model, then parser.

from langchain_core.output_parsers import StrOutputParser

chain = prompt | llm | StrOutputParser()

print(chain.invoke({"role": "teacher", "words": 30, "question": "What is recursion?"}))

StrOutputParser turns the model's message object into a plain string. Every chain built this way automatically supports invoke, stream, batch and async versions.

Structured output

Free text is hard for programs to use. Ask the model for data that matches a schema, and LangChain returns a validated object.

from pydantic import BaseModel, Field

class Review(BaseModel):
    sentiment: str = Field(description="positive, negative or neutral")
    score: int = Field(ge=1, le=5)
    summary: str

structured = llm.with_structured_output(Review)
result = structured.invoke("The battery is great but the screen scratches easily.")
print(result.sentiment, result.score)
print(result.model_dump())

neutral 3
{'sentiment': 'neutral', 'score': 3, 'summary': 'Great battery but a screen that scratches easily.'}

Composing bigger chains

from langchain_core.runnables import RunnablePassthrough, RunnableParallel

summarize = ChatPromptTemplate.from_template("Summarize in one line: {text}") | llm | StrOutputParser()
translate = ChatPromptTemplate.from_template("Translate to French: {text}") | llm | StrOutputParser()

both = RunnableParallel(summary=summarize, french=translate)
print(both.invoke({"text": "LangChain helps build LLM applications."}))

RunnableParallel runs branches at the same time. Other useful runnables: RunnableLambda to wrap your own function, and RunnablePassthrough to carry inputs alongside results.

Prompt-writing tips

• Be specific about role, task, format and length.
• Give one or two examples (few-shot) when the format matters.
• Put untrusted user text in its own clearly labeled section to reduce prompt-injection risk.
• Version and test your prompts like code.

# Write your solution here