LangChain 1.0 Upgrade Migration (Python)

Overview

The first deploy after pip install -U langchain crashes on import with:

ImportError: cannot import name 'ChatOpenAI' from 'langchain.chat_models'

Fix the import, restart, and the next error lands:

ImportError: cannot import name 'LLMChain' from 'langchain.chains'
AttributeError: module 'langchain.agents' has no attribute 'initialize_agent'
AttributeError: 'ConversationBufferMemory' object has no attribute 'save_context'

LangChain 1.0 removed four entire public-API surfaces in one release: