Open In Colab

LLM Pydantic Program#

This guide shows you how to generate structured data with our LLMTextCompletionProgram. Given an LLM as well as an output Pydantic class, generate a structured Pydantic object.

In terms of the target object, you can choose to directly specify output_cls, or specify a PydanticOutputParser or any other BaseOutputParser that generates a Pydantic object.

in the examples below, we show you different ways of extracting into the Album object (which can contain a list of Song objects)

Extract into Album class#

This is a simple example of parsing an output into an Album schema, which can contain multiple songs.

Just pass Album into the output_cls property on initialization of the LLMTextCompletionProgram.

If you’re opening this Notebook on colab, you will probably need to install LlamaIndex 🦙.

!pip install llama-index
from pydantic import BaseModel
from typing import List

from llama_index.core.program import LLMTextCompletionProgram

Define output schema

class Song(BaseModel):
    """Data model for a song."""

    title: str
    length_seconds: int

class Album(BaseModel):
    """Data model for an album."""

    name: str
    artist: str
    songs: List[Song]

Define LLM pydantic program

from llama_index.core.program import LLMTextCompletionProgram
prompt_template_str = """\
Generate an example album, with an artist and a list of songs. \
Using the movie {movie_name} as inspiration.\
program = LLMTextCompletionProgram.from_defaults(

Run program to get structured output.

output = program(movie_name="The Shining")

The output is a valid Pydantic object that we can then use to call functions/APIs.

Album(name='The Overlook', artist='Jack Torrance', songs=[Song(title='Redrum', length_seconds=240), Song(title="Here's Johnny", length_seconds=180), Song(title='Room 237', length_seconds=300), Song(title='All Work and No Play', length_seconds=210), Song(title='The Maze', length_seconds=270)])

Initialize with Pydantic Output Parser#

The above is equivalent to defining a Pydantic output parser and passing that in instead of the output_cls directly.

from llama_index.core.output_parsers import PydanticOutputParser

program = LLMTextCompletionProgram.from_defaults(
output = program(movie_name="Lord of the Rings")
Album(name='The Fellowship of the Ring', artist='Middle-earth Ensemble', songs=[Song(title='The Shire', length_seconds=240), Song(title='Concerning Hobbits', length_seconds=180), Song(title='The Ring Goes South', length_seconds=300), Song(title='A Knife in the Dark', length_seconds=270), Song(title='Flight to the Ford', length_seconds=210), Song(title='Many Meetings', length_seconds=240), Song(title='The Council of Elrond', length_seconds=330), Song(title='The Great Eye', length_seconds=180), Song(title='The Breaking of the Fellowship', length_seconds=360)])

Define a Custom Output Parser#

Sometimes you may want to parse an output your own way into a JSON object.

from llama_index.core.output_parsers import ChainableOutputParser

class CustomAlbumOutputParser(ChainableOutputParser):
    """Custom Album output parser.

    Assume first line is name and artist.

    Assume each subsequent line is the song.


    def __init__(self, verbose: bool = False):
        self.verbose = verbose

    def parse(self, output: str) -> Album:
        """Parse output."""
        if self.verbose:
            print(f"> Raw output: {output}")
        lines = output.split("\n")
        name, artist = lines[0].split(",")
        songs = []
        for i in range(1, len(lines)):
            title, length_seconds = lines[i].split(",")
            songs.append(Song(title=title, length_seconds=length_seconds))

        return Album(name=name, artist=artist, songs=songs)
prompt_template_str = """\
Generate an example album, with an artist and a list of songs. \
Using the movie {movie_name} as inspiration.\

Return answer in following format.
The first line is:
<album_name>, <album_artist>
Every subsequent line is a song with format:
<song_title>, <song_length_seconds>

program = LLMTextCompletionProgram.from_defaults(
output = program(movie_name="The Dark Knight")
> Raw output: Gotham's Reckoning, The Dark Knight
A Dark Knight Rises, 240
The Joker's Symphony, 180
Harvey Dent's Lament, 210
Gotham's Guardian, 195
The Batmobile Chase, 225
The Dark Knight's Theme, 150
The Joker's Mind Games, 180
Rachel's Tragedy, 210
Gotham's Last Stand, 240
The Dark Knight's Triumph, 180
Album(name="Gotham's Reckoning", artist=' The Dark Knight', songs=[Song(title='A Dark Knight Rises', length_seconds=240), Song(title="The Joker's Symphony", length_seconds=180), Song(title="Harvey Dent's Lament", length_seconds=210), Song(title="Gotham's Guardian", length_seconds=195), Song(title='The Batmobile Chase', length_seconds=225), Song(title="The Dark Knight's Theme", length_seconds=150), Song(title="The Joker's Mind Games", length_seconds=180), Song(title="Rachel's Tragedy", length_seconds=210), Song(title="Gotham's Last Stand", length_seconds=240), Song(title="The Dark Knight's Triumph", length_seconds=180)])