langchain: Make OutputFixingParser.from_llm() create a useable retry chain (#24687)

Description: OutputFixingParser.from_llm() creates a retry chain that
returns a Generation instance, when it should actually just return a
string.
Issue: https://github.com/langchain-ai/langchain/issues/24600
Twitter handle: scribu

---------

Co-authored-by: isaac hershenson <ihershenson@hmc.edu>

libs/langchain/langchain/output_parsers/fix.py

@@ -3,10 +3,9 @@ from __future__ import annotations
 from typing import Any, TypeVar, Union
 
 from langchain_core.exceptions import OutputParserException
-from langchain_core.language_models import BaseLanguageModel
+from langchain_core.output_parsers import BaseOutputParser, StrOutputParser
-from langchain_core.output_parsers import BaseOutputParser
 from langchain_core.prompts import BasePromptTemplate
-from langchain_core.runnables import RunnableSerializable
+from langchain_core.runnables import Runnable, RunnableSerializable
 from typing_extensions import TypedDict
 
 from langchain.output_parsers.prompts import NAIVE_FIX_PROMPT
@@ -42,7 +41,7 @@ class OutputFixingParser(BaseOutputParser[T]):
     @classmethod
     def from_llm(
         cls,
-        llm: BaseLanguageModel,
+        llm: Runnable,
         parser: BaseOutputParser[T],
         prompt: BasePromptTemplate = NAIVE_FIX_PROMPT,
         max_retries: int = 1,
@@ -58,7 +57,7 @@ class OutputFixingParser(BaseOutputParser[T]):
         Returns:
             OutputFixingParser
         """
-        chain = prompt | llm
+        chain = prompt | llm | StrOutputParser()
         return cls(parser=parser, retry_chain=chain, max_retries=max_retries)
 
     def parse(self, completion: str) -> T:

libs/langchain/langchain/output_parsers/retry.py

@@ -4,7 +4,7 @@ from typing import Any, TypeVar, Union
 
 from langchain_core.exceptions import OutputParserException
 from langchain_core.language_models import BaseLanguageModel
-from langchain_core.output_parsers import BaseOutputParser
+from langchain_core.output_parsers import BaseOutputParser, StrOutputParser
 from langchain_core.prompt_values import PromptValue
 from langchain_core.prompts import BasePromptTemplate, PromptTemplate
 from langchain_core.runnables import RunnableSerializable
@@ -82,7 +82,7 @@ class RetryOutputParser(BaseOutputParser[T]):
         Returns:
             RetryOutputParser
         """
-        chain = prompt | llm
+        chain = prompt | llm | StrOutputParser()
         return cls(parser=parser, retry_chain=chain, max_retries=max_retries)
 
     def parse_with_prompt(self, completion: str, prompt_value: PromptValue) -> T:

libs/langchain/tests/unit_tests/output_parsers/test_fix.py

@@ -3,6 +3,7 @@ from typing import Any, Callable, Dict, Optional, TypeVar
 
 import pytest
 from langchain_core.exceptions import OutputParserException
+from langchain_core.messages import AIMessage
 from langchain_core.prompts.prompt import PromptTemplate
 from langchain_core.runnables import Runnable, RunnableLambda, RunnablePassthrough
 from pytest_mock import MockerFixture
@@ -63,6 +64,22 @@ def test_output_fixing_parser_parse(
     # TODO: test whether "instructions" is passed to the retry_chain
 
 
+def test_output_fixing_parser_from_llm() -> None:
+    def fake_llm(prompt: str) -> AIMessage:
+        return AIMessage("2024-07-08T00:00:00.000000Z")
+
+    llm = RunnableLambda(fake_llm)
+
+    n = 1
+    parser = OutputFixingParser.from_llm(
+        llm=llm,
+        parser=DatetimeOutputParser(),
+        max_retries=n,
+    )
+
+    assert parser.parse("not a date")
+
+
 @pytest.mark.parametrize(
     "base_parser",
     [