PydanticOutputParser for typed objects in LangChain

Introduction

PydanticOutputParser helps turn text into typed Python objects easily. It makes sure the output matches the expected data structure.

When you want to convert text responses from language models into Python objects.

When you need to validate and parse data automatically into typed classes.

When working with APIs or tools that return JSON or structured text you want as Python objects.

When you want to avoid manual parsing and error checking of output data.

When you want clear, typed data for easier coding and fewer bugs.

Syntax

LangChain

from langchain.output_parsers import PydanticOutputParser
from pydantic import BaseModel

class MyData(BaseModel):
    name: str
    age: int

parser = PydanticOutputParser(pydantic_object=MyData)

You define a Pydantic model class to describe the data structure.

Pass the model class to PydanticOutputParser to create a parser for that type.

Examples

This creates a parser for a User object with username and active fields.

LangChain

from pydantic import BaseModel

class User(BaseModel):
    username: str
    active: bool

parser = PydanticOutputParser(pydantic_object=User)

Parser for Product with id and price fields, ensuring correct types.

LangChain

from pydantic import BaseModel

class Product(BaseModel):
    id: int
    price: float

parser = PydanticOutputParser(pydantic_object=Product)

Sample Program

This example shows how to parse a JSON string into a typed Person object using PydanticOutputParser. It prints the whole object and individual fields.

LangChain

from langchain.output_parsers import PydanticOutputParser
from pydantic import BaseModel

class Person(BaseModel):
    name: str
    age: int

# Create parser for Person
parser = PydanticOutputParser(pydantic_object=Person)

# Example text output from a language model
text_output = '{"name": "Alice", "age": 30}'

# Parse text into Person object
person_obj = parser.parse(text_output)

print(person_obj)
print(person_obj.name)
print(person_obj.age)

Important Notes

Make sure the text input is valid JSON matching the Pydantic model structure.

PydanticOutputParser raises errors if the data does not match the expected types.

This parser helps keep your code clean by automating data validation and conversion.

Summary

PydanticOutputParser converts text into typed Python objects using Pydantic models.

It validates data automatically, reducing bugs and manual parsing.

Use it when you want structured, typed output from language model responses or APIs.