Add improved sources splitting in BaseQAWithSourcesChain (#8716)

## Type:
Improvement

---

## Description:
Running QAWithSourcesChain sometimes raises ValueError as mentioned in
issue #7184:
```
ValueError: too many values to unpack (expected 2)
Traceback:

    response = qa({"question": pregunta}, return_only_outputs=True)
File "C:\Anaconda3\envs\iagen_3_10\lib\site-packages\langchain\chains\base.py", line 166, in __call__
    raise e
File "C:\Anaconda3\envs\iagen_3_10\lib\site-packages\langchain\chains\base.py", line 160, in __call__
    self._call(inputs, run_manager=run_manager)
File "C:\Anaconda3\envs\iagen_3_10\lib\site-packages\langchain\chains\qa_with_sources\base.py", line 132, in _call
    answer, sources = re.split(r"SOURCES:\s", answer)
```
This is due to LLM model generating subsequent question, answer and
sources, that is complement in a similar form as below:
```
<final_answer>
SOURCES: <sources>
QUESTION: <new_or_repeated_question>
FINAL ANSWER: <new_or_repeated_final_answer>
SOURCES: <new_or_repeated_sources>
```
It leads the following line
```
 re.split(r"SOURCES:\s", answer)
```
to return more than 2 elements and result in ValueError. The simple fix
is to split also with "QUESTION:\s" and take the first two elements:
```
answer, sources = re.split(r"SOURCES:\s|QUESTION:\s", answer)[:2]
```

Sometimes LLM might also generate some other texts, like alternative
answers in a form:
```
<final_answer_1>
SOURCES: <sources>

<final_answer_2>
SOURCES: <sources>

<final_answer_3>
SOURCES: <sources>
```
In such cases it is the best to split previously obtained sources with
new line:
```
sources = re.split(r"\n", sources.lstrip())[0]
```



---

## Issue:
Resolves #7184

---

## Maintainer:
@baskaryan

libs/langchain/langchain/chains/qa_with_sources/base.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 import inspect
 import re
 from abc import ABC, abstractmethod
-from typing import Any, Dict, List, Optional
+from typing import Any, Dict, List, Optional, Tuple
 
 from pydantic_v1 import Extra, root_validator
 
@@ -119,6 +119,15 @@ class BaseQAWithSourcesChain(Chain, ABC):
             values["combine_documents_chain"] = values.pop("combine_document_chain")
         return values
 
+    def _split_sources(self, answer: str) -> Tuple[str, str]:
+        """Split sources from answer."""
+        if re.search(r"SOURCES:\s", answer):
+            answer, sources = re.split(r"SOURCES:\s|QUESTION:\s", answer)[:2]
+            sources = re.split(r"\n", sources)[0]
+        else:
+            sources = ""
+        return answer, sources
+
     @abstractmethod
     def _get_docs(
         self,
@@ -145,10 +154,7 @@ class BaseQAWithSourcesChain(Chain, ABC):
         answer = self.combine_documents_chain.run(
             input_documents=docs, callbacks=_run_manager.get_child(), **inputs
         )
-        if re.search(r"SOURCES:\s", answer):
+        answer, sources = self._split_sources(answer)
-            answer, sources = re.split(r"SOURCES:\s", answer)
-        else:
-            sources = ""
         result: Dict[str, Any] = {
             self.answer_key: answer,
             self.sources_answer_key: sources,
@@ -182,10 +188,7 @@ class BaseQAWithSourcesChain(Chain, ABC):
         answer = await self.combine_documents_chain.arun(
             input_documents=docs, callbacks=_run_manager.get_child(), **inputs
         )
-        if re.search(r"SOURCES:\s", answer):
+        answer, sources = self._split_sources(answer)
-            answer, sources = re.split(r"SOURCES:\s", answer)
-        else:
-            sources = ""
         result: Dict[str, Any] = {
             self.answer_key: answer,
             self.sources_answer_key: sources,

libs/langchain/tests/unit_tests/chains/test_qa_with_sources.py

@@ -0,0 +1,71 @@
+import pytest
+
+from langchain.chains.qa_with_sources.base import QAWithSourcesChain
+from tests.unit_tests.llms.fake_llm import FakeLLM
+
+
+@pytest.mark.parametrize(
+    "text,answer,sources",
+    [
+        (
+            "This Agreement is governed by English law.\nSOURCES: 28-pl",
+            "This Agreement is governed by English law.\n",
+            "28-pl",
+        ),
+        (
+            "This Agreement is governed by English law.\n"
+            "SOURCES: 28-pl\n\n"
+            "QUESTION: Which state/country's law governs the interpretation of the "
+            "contract?\n"
+            "FINAL ANSWER: This Agreement is governed by English law.\n"
+            "SOURCES: 28-pl",
+            "This Agreement is governed by English law.\n",
+            "28-pl",
+        ),
+        (
+            "The president did not mention Michael Jackson in the provided content.\n"
+            "SOURCES: \n\n"
+            "Note: Since the content provided does not contain any information about "
+            "Michael Jackson, there are no sources to cite for this specific question.",
+            "The president did not mention Michael Jackson in the provided content.\n",
+            "",
+        ),
+        # The following text was generated by gpt-3.5-turbo
+        (
+            "To diagnose the problem, please answer the following questions and send "
+            "them in one message to IT:\nA1. Are you connected to the office network? "
+            "VPN will not work from the office network.\nA2. Are you sure about your "
+            "login/password?\nA3. Are you using any other VPN (e.g. from a client)?\n"
+            "A4. When was the last time you used the company VPN?\n"
+            "SOURCES: 1\n\n"
+            "ALTERNATIVE OPTION: Another option is to run the VPN in CLI, but keep in "
+            "mind that DNS settings may not work and there may be a need for manual "
+            "modification of the local resolver or /etc/hosts and/or ~/.ssh/config "
+            "files to be able to connect to machines in the company. With the "
+            "appropriate packages installed, the only thing needed to establish "
+            "a connection is to run the command:\nsudo openvpn --config config.ovpn"
+            "\n\nWe will be asked for a username and password - provide the login "
+            "details, the same ones that have been used so far for VPN connection, "
+            "connecting to the company's WiFi, or printers (in the Warsaw office)."
+            "\n\nFinally, just use the VPN connection.\n"
+            "SOURCES: 2\n\n"
+            "ALTERNATIVE OPTION (for Windows): Download the"
+            "OpenVPN client application version 2.6 or newer from the official "
+            "website: https://openvpn.net/community-downloads/\n"
+            "SOURCES: 3",
+            "To diagnose the problem, please answer the following questions and send "
+            "them in one message to IT:\nA1. Are you connected to the office network? "
+            "VPN will not work from the office network.\nA2. Are you sure about your "
+            "login/password?\nA3. Are you using any other VPN (e.g. from a client)?\n"
+            "A4. When was the last time you used the company VPN?\n",
+            "1",
+        ),
+    ],
+)
+def test_spliting_answer_into_answer_and_sources(
+    text: str, answer: str, sources: str
+) -> None:
+    qa_chain = QAWithSourcesChain.from_llm(FakeLLM())
+    generated_answer, generated_sources = qa_chain._split_sources(text)
+    assert generated_answer == answer
+    assert generated_sources == sources