How to use the output-fixing parser

This output parser wraps another output parser, and in the event that the first one fails it calls out to another LLM to fix any errors.

But we can do other things besides throw errors. Specifically, we can pass the misformatted output, along with the formatted instructions, to the model and ask it to fix it.

For this example, we'll use the above Pydantic output parser. Here's what happens if we pass it a result that does not comply with the schema:

from typing import List

from langchain_core.output_parsers import PydanticOutputParser
from langchain_openai import ChatOpenAI
from pydantic import BaseModel, Field

API Reference:PydanticOutputParser | ChatOpenAI

class Actor(BaseModel):
    name: str = Field(description="name of an actor")
    film_names: List[str] = Field(description="list of names of films they starred in")


actor_query = "Generate the filmography for a random actor."

parser = PydanticOutputParser(pydantic_object=Actor)

misformatted = "{'name': 'Tom Hanks', 'film_names': ['Forrest Gump']}"

parser.parse(misformatted)

---------------------------------------------------------------------------
``````output
JSONDecodeError                           Traceback (most recent call last)
``````output
File ~/workplace/langchain/libs/langchain/langchain/output_parsers/pydantic.py:29, in PydanticOutputParser.parse(self, text)
     28     json_str = match.group()
---> 29 json_object = json.loads(json_str, strict=False)
     30 return self.pydantic_object.parse_obj(json_object)
``````output
File ~/.pyenv/versions/3.10.1/lib/python3.10/json/__init__.py:359, in loads(s, cls, object_hook, parse_float, parse_int, parse_constant, object_pairs_hook, **kw)
    358     kw['parse_constant'] = parse_constant
--> 359 return cls(**kw).decode(s)
``````output
File ~/.pyenv/versions/3.10.1/lib/python3.10/json/decoder.py:337, in JSONDecoder.decode(self, s, _w)
    333 """Return the Python representation of ``s`` (a ``str`` instance
    334 containing a JSON document).
    335 
    336 """
--> 337 obj, end = self.raw_decode(s, idx=_w(s, 0).end())
    338 end = _w(s, end).end()
``````output
File ~/.pyenv/versions/3.10.1/lib/python3.10/json/decoder.py:353, in JSONDecoder.raw_decode(self, s, idx)
    352 try:
--> 353     obj, end = self.scan_once(s, idx)
    354 except StopIteration as err:
``````output
JSONDecodeError: Expecting property name enclosed in double quotes: line 1 column 2 (char 1)
``````output

During handling of the above exception, another exception occurred:
``````output
OutputParserException                     Traceback (most recent call last)
``````output
Cell In[4], line 1
----> 1 parser.parse(misformatted)
``````output
File ~/workplace/langchain/libs/langchain/langchain/output_parsers/pydantic.py:35, in PydanticOutputParser.parse(self, text)
     33 name = self.pydantic_object.__name__
     34 msg = f"Failed to parse {name} from completion {text}. Got: {e}"
---> 35 raise OutputParserException(msg, llm_output=text)
``````output
OutputParserException: Failed to parse Actor from completion {'name': 'Tom Hanks', 'film_names': ['Forrest Gump']}. Got: Expecting property name enclosed in double quotes: line 1 column 2 (char 1)

Now we can construct and use a OutputFixingParser. This output parser takes as an argument another output parser but also an LLM with which to try to correct any formatting mistakes.

from langchain.output_parsers import OutputFixingParser

new_parser = OutputFixingParser.from_llm(parser=parser, llm=ChatOpenAI())

API Reference:OutputFixingParser

new_parser.parse(misformatted)

Actor(name='Tom Hanks', film_names=['Forrest Gump'])

Find out api documentation for OutputFixingParser.