Adapting Existing LLM Projects to use LangChain

Refactoring OpenAI calls into something more powerful with LangChain

Photo by JJ Ying on Unsplash

Congratulations, you have a working LLM proof-of-concept that you are proud of and ready to show off to the world! Maybe you’ve utilized the OpenAI library directly or perhaps you’re using a different foundation model and HuggingFace transformers. Either way, you worked hard and are looking for the next step. That could mean code refactoring, adding support for multiple foundation models, or adding more advanced functionality such as agents or vector db. This is where LangChain comes in.

LangChain is an open source framework for working with LLMs and developing applications on top of them. It has versions in both Python and JavaScript and supports multiple foundation models and LLM providers. It also has a number of utility functions for handling common tasks such as document chunking or interacting with vector dbs. Convinced it’s something worth checking out? Have a look at this excellent article which is a great place to start understanding what is possible:

Getting Started with LangChain: A Beginner’s Guide to Building LLM-Powered Applications

This article will be not be focused on greenfield development and instead on refactoring an existing app. It will also assume some base understanding of LangChain, but there will be links to relevant documentation. Specifically, I will be looking at refactoring a project that I wrote called AdventureGPT, an autonomous agent for playing the 1977 Colossal Cave Adventure text-based adventure game. If you are interested in learning more about that project, check out my previous article about it:

AdventureGPT: Using LLM Backed Agents to Play Text-Based Adventure Games

There were several areas I was most interested in refactoring:

Utilizing Chains instead of direct OpenAI callsReplacing bespoke document utilities for the LangChain Document/Data handling

Each of these will be addressed in turn.

Let’s begin with what a chain is. A chain is a method of combining several prompt manipulation techniques together into a single unit that results in a single foundation model call. Once one has a working chain, chains can be combined and used together to achieve more complex tasks.

LangChain offers a few different types of chains, this article focuses on LLMChain and ConversationChain. LLMChain is the simplest type of chain, combining a prompt templates with the LLM objects LangChain supports. ConversationChains offer an experience tailored conversational workflows such as chatbots. One of the major features of ConversationChain is its ability to include memory and store past parts of the conversation effortlessly into the prompt.

Prompt templates are one of the most powerful features of LangChain, allowing you to include variables inside the prompts. When manually completing this task, one might us f-strings combined with string concatenation and careful use of custom __repr__ methods to take your data and insert it into a prompt for a foundation model. With the prompt template, you can format a string by escaping variables with brackets. That’s all you have to do.

Depending on the type of chain you are creating, some variables are set for you by default, such as the the conversational history or user input. This can take a fair amount of complexity. In a traditional conversational prompt, there are messages from the system, the the AI assistant, and user or human. When writing prompts by hand, you use labels like “System”, “Human”, and “AI” to label each of these messages for the prompt. LangChain can take care of this for you, allowing you to use the ChatPromptTemplate method from_messages allow you to specify each message as a list of objects, allowing for a higher level of abstraction and automatic history inclusion and formatting.

All this power comes at the cost of complexity. Rather than simply adapting the prompts with text, one needs to read the extensive documentation and possible extend the existing code to fit a specific use case. For example, conversational prompts tend to only include the user’s input and conversation history as variables. However, I wanted to include additional game context in my prompt for my PlayerAgent, which was responsible for interacting with the game world. After adding the additional variables to my prompt, I was greeted with the following error:

Got unexpected prompt input variables. The prompt expects [‘completed_tasks’, ‘input’, ‘history’, ‘objective’], but got [‘history’] as inputs from memory, and input as the normal input key. (type=value_error)

I did some digging and found an existing GitHub issue describing the exact issue I was having, but with no clear resolution. Unperturbed, I looked at the source code for the ConversationChain class and saw that there was a specific method used to validate that only the expected variables were passed in as input. I made a new class subclassing the original class and overrode the method. With my CustomConversationChain class in hand, I then needed to specify which variable should be utilized by the ConversationalMemoryBuffer for the user’s (or in my case, the game’s) input since there were multiple input variables. This was simple enough via the input_key instance variable and Bob’s your uncle, everything worked.

Once I finished converting my OpenAI calls to chains, it was time to address the way I was handling document ingestion. As part of my game loop, I accepted a path to a walkthrough text which would then be converted into game tasks to be completed by the PlayerAgent. When I first added this feature, I simply passed the whole walkthrough to the prompt and hoped for the best. As I found more sophisticated walkthoughs, that was no longer possible as the length of the walkthroughs exceeded the context window OpenAI allowed for ChatGPT. Therefore, I cut the text into chunks of 500 tokens and ran the prompt for converting walkthroughs into game tasks multiple times.

When I said I chunked the text by around 500 tokens, I did this very crudely, relying on Python’s string’s split method to tokenize the text (a very rough approximation that doesn’t match how most LLM’s tokenize text) and then turned the array of tokens back into a string via the join method, again from the String class. While this works, LangChain offers better solutions.

LangChain is able to split text in a number of different ways. The most relevant for most people is by token, as it preserves the flow the document. There is an entire page of documentation here dedicated to the different methods of splitting texts by token. A number of NLP libraries are supported for tokenization, but I chose the LLM native solution tiktoken, which is the first method described. It was only a few lines of code to easily and much more effectively split the text while preserving whitespace.

This is only scratching the surface of the document preparation that LangChain is capable of. It is also capable of converting the text chunks into an appropriate text embedding for storage in a vector database and later retrieval and inclusion in the prompt. I plan on doing this in the future of my project, including a relevant chunk of a supplied walthrough to the PlayerAgent.

LangChain is a powerful open source framework that offers a range of features and utility functions for working with LLMs and developing applications on top of them. Whether you’re using the OpenAI library or a different foundation model, LangChain provides support for multiple foundation models and LLM providers, making it a versatile choice for your projects.

While LangChain may introduce some complexity compared to raw prompt management, it offers numerous benefits and simplifies the interaction with LLMs. It standardizes the process and provides helpful tools to enhance your prompts and maximize the potential of your chosen LLM.

If you’re interested in seeing how LangChain can be implemented in a real project, you can check out the updated code base for AdventureGPT, which utilizes LangChain for refactoring and improving the existing app.

GitHub – oaguy1/AdventureGPT at langchain

Overall, LangChain is a valuable resource for developers working with LLMs, providing a comprehensive framework and a wide range of functionalities to enhance LLM-powered applications. Explore LangChain and unlock the full potential of your LLM projects!

Adapting Existing LLM Projects to use LangChain was originally published in Towards Data Science on Medium, where people are continuing the conversation by highlighting and responding to this story.

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