How to write a custom LLM wrapper

This notebook goes over how to create a custom LLM wrapper, in case you want to use your own LLM or a different wrapper than one that is supported in LangChain.

There is only one required thing that a custom LLM needs to implement:

1. A _call method that takes in a string, some optional stop words, and returns a string

There is a second optional thing it can implement:

1. An _identifying_params property that is used to help with printing of this class. Should return a dictionary.

Let’s implement a very simple custom LLM that just returns the first N characters of the input.

from typing import Any, List, Mapping, Optional

from langchain.callbacks.manager import CallbackManagerForLLMRun
from langchain.llms.base import LLM

class CustomLLM(LLM):
    
    n: int
        
    @property
    def _llm_type(self) -> str:
        return "custom"
    
    def _call(
        self,
        prompt: str,
        stop: Optional[List[str]] = None,
        run_manager: Optional[CallbackManagerForLLMRun] = None,
    ) -> str:
        if stop is not None:
            raise ValueError("stop kwargs are not permitted.")
        return prompt[:self.n]
    
    @property
    def _identifying_params(self) -> Mapping[str, Any]:
        """Get the identifying parameters."""
        return {"n": self.n}

We can now use this as an any other LLM.

llm = CustomLLM(n=10)

llm("This is a foobar thing")

'This is a '

We can also print the LLM and see its custom print.

print(llm)

CustomLLM
Params: {'n': 10}