3. Parsers Basics

In this section, you will learn how to extract plain text and convert comma-separated string outputs from LLMs into Python list objects.

Objectives

Understand why output parsers are required.
Initialize and configure StrOutputParser to clean LLM response objects.
Configure CommaSeparatedListOutputParser and feed format instructions to the LLM.

Invoking Parsers: `.invoke()` vs `.parse()`

When using LangChain output parsers step-by-step, you have two primary methods to run them:

.invoke(response): Passes the entire model response object (like AIMessage). LangChain automatically extracts the raw string under the hood and parses it.
.parse(raw_text): Passes a raw Python str (e.g., response.content). Use this when you have manually extracted the text or are parsing raw strings from external sources.

[!NOTE] In production LCEL chains, the pipe operator | automatically runs .invoke() behind the scenes, making .invoke() the standard approach for unified LangChain components.

Code Implementation

Each step of the implementation is preceded by extensive comments explaining the code logic.

Setup and Initialization

First, we load environment variables and initialize our chat model using core abstractions (Groq provider).

from dotenv import load_dotenv
from langchain.chat_models import init_chat_model

# Load environment variables
load_dotenv()

# Initialize the Groq model using core abstractions
model = init_chat_model("llama-3.3-70b-versatile", model_provider="groq")

StrOutputParser

The simplest output parser. It extracts the raw string message content from the model’s chat response object.

from langchain.prompts import ChatPromptTemplate
from langchain.schema.output_parser import StrOutputParser

# Define prompt template
str_prompt = ChatPromptTemplate.from_messages([
    ("system", "You are a helpful assistant."),
    ("human", "Tell me a one-sentence fun fact about the ocean.")
])

# 1. Format the prompt
str_messages = str_prompt.format_messages()

# 2. Invoke the model to get the response object (AIMessage)
str_response = model.invoke(str_messages)

# 3. Use the parser to extract the string content
parser = StrOutputParser()
str_result = parser.invoke(str_response)

print("Parsed String Output:")
print(str_result)

CommaSeparatedListOutputParser

Instructs the model to return a list of items separated by commas, then parses that raw string into a standard Python list.

from langchain.prompts import ChatPromptTemplate
from langchain.output_parsers import CommaSeparatedListOutputParser

# Instantiate the list parser
list_parser = CommaSeparatedListOutputParser()

# Get format instructions generated by the parser
format_instructions = list_parser.get_format_instructions()

# Define prompt incorporating format instructions
list_prompt = ChatPromptTemplate.from_messages([
    ("system", "List the requested items. {format_instructions}"),
    ("human", "List 5 top programming languages in 2026.")
])

# 1. Format the prompt with inputs and instructions
list_messages = list_prompt.format_messages(format_instructions=format_instructions)

# 2. Invoke the model
list_response = model.invoke(list_messages)

# 3. Parse the raw string response into a Python list
list_result = list_parser.parse(list_response.content)

print("Parsed List Output:")
print(list_result)

Practice & Exercises

To reinforce what you’ve learned in this section, practice with the interactive notebook:

Practice & Exercises

Practice extracting strings and comma-separated lists from LLM outputs.💻 VS Code | 🚀 Colab | 📥 Download

Introduction

Chat Models

Templates & Parsers

Chains

RAG

Tools and Agents

Objectives

Invoking Parsers: `.invoke()` vs `.parse()`

Code Implementation

Setup and Initialization

StrOutputParser

CommaSeparatedListOutputParser

Practice & Exercises

Practice & Exercises

​Objectives

​Invoking Parsers: .invoke() vs .parse()

​Code Implementation

​Setup and Initialization

​StrOutputParser

​CommaSeparatedListOutputParser

​Practice & Exercises

Practice & Exercises

Objectives

Invoking Parsers: `.invoke()` vs `.parse()`

Code Implementation

Setup and Initialization

StrOutputParser

CommaSeparatedListOutputParser

Practice & Exercises