部署LangChain+StreamLit PDF问答应用触发PydanticUserError的解决方法

我用LangChain、OpenAI（嵌入与LLM）、Chroma（向量嵌入存储）和StreamLit构建了PDF文档问答应用，尝试通过StreamLit云服务部署托管。安装完所有必要库和包后，触发如下错误：

PydanticUserError: If you use `@root_validator` with pre=False (the default) you MUST specify `skip_on_failure=True`. Note that `@root_validator` is deprecated and should be replaced with `@model_validator`. For further information visit https://errors.pydantic.dev/2.0.2/u/root-validator-pre-skip

附带回溯信息：

Traceback:
    File "/opt/render/project/src/.venv/lib/python3.7/site-packages/streamlit/runtime/scriptrunner/script_runner.py", line 552, in _run_script
    exec(code, module.__dict__)
File "/opt/render/project/src/app.py", line 3, in <module>
    from langchain.document_loaders import PyMuPDFLoader, PyPDFLoader
File "/opt/render/project/src/.venv/lib/python3.7/site-packages/langchain/__init__.py", line 8, in <module>
    from langchain.agents import MRKLChain, ReActChain, SelfAskWithSearchChain
File "/opt/render/project/src/.venv/lib/python3.7/site-packages/langchain/agents/__init__.py", line 2, in <module>
    from langchain.agents.agent import Agent
File "/opt/render/project/src/.venv/lib/python3.7/site-packages/langchain/agents/agent.py", line 10, in <module>
    from langchain.chains.base import Chain
File "/opt/render/project/src/.venv/lib/python3.7/site-packages/langchain/chains/__init__.py", line 2, in <module>
    from langchain.chains.conversation.base import ConversationChain
File "/opt/render/project/src/.venv/lib/python3.7/site-packages/langchain/chains/conversation/base.py", line 7, in <module>
    from langchain.chains.conversation.memory import ConversationBufferMemory
File "/opt/render/project/src/.venv/lib/python3.7/site-packages/langchain/chains/conversation/memory.py", line 7, in <module>
    from langchain.chains.conversation.prompt import SUMMARY_PROMPT
File "/opt/render/project/src/.venv/lib/python3.7/site-packages/langchain/chains/conversation/prompt.py", line 2, in <module>
    from langchain.prompts.prompt import PromptTemplate
File "/opt/render/project/src/.venv/lib/python3.7/site-packages/langchain/prompts/__init__.py", line 2, in <module>
    from langchain.prompts.base import BasePromptTemplate
File "/opt/render/project/src/.venv/lib/python3.7/site-packages/langchain/prompts/base.py", line 35, in <module>
    class BasePromptTemplate(BaseModel, ABC):
File "/opt/render/project/src/.venv/lib/python3.7/site-packages/langchain/prompts/base.py", line 41, in BasePromptTemplate
    @root_validator()
File "/opt/render/project/src/.venv/lib/python3.7/site-packages/pydantic/deprecated/class_validators.py", line 231, in root_validator
    code='root-validator-pre-skip',

问题原因 #

这个错误是LangChain版本与Pydantic v2.x不兼容导致的。LangChain旧版本使用了Pydantic v1.x的@root_validator语法，而Pydantic v2.x对该语法做了严格限制，同时标记@root_validator为废弃，要求替换为@model_validator。

解决方法 #

• 方法1：固定Pydantic为v1.x版本
  在requirements.txt中指定兼容的Pydantic版本：

  pydantic==1.10.12
  

  然后重新安装依赖：

  pip install -r requirements.txt
  

• 方法2：升级LangChain到兼容Pydantic v2.x的版本
  如果想保留Pydantic v2.x，需要升级LangChain到支持该版本的最新版：

  pip install --upgrade langchain
  

  注意：升级后可能需要检查代码中是否有其他依赖兼容问题，部分旧版LangChain的API在新版本中可能有变更。

• 方法3：使用LangChain的兼容版本分支
  对于一些特定场景，可以安装LangChain的pydantic-v2兼容分支：

  pip install langchain[pydantic-v2]
  

内容的提问来源于stack exchange，提问作者Shuhul Handoo