Note when the verbose flag on the object is set to true, the StdOutCallbackHandler will be invoked even without being explicitly passed in.
from langchain.callbacks import StdOutCallbackHandler
from langchain.chains import LLMChain
from langchain.llms import OpenAI
from langchain.prompts import PromptTemplate
handler = StdOutCallbackHandler()
llm = OpenAI()
prompt = PromptTemplate.from_template("1 + {number} = ")
# First, let's explicitly set the StdOutCallbackHandler in `callbacks`
chain = LLMChain(llm=llm, prompt=prompt, callbacks=[handler])
chain.run(number=2)
# Then, let's use the `verbose` flag to achieve the same result
chain = LLMChain(llm=llm, prompt=prompt, verbose=True)
chain.run(number=2)
# Finally, let's use the request `callbacks` to achieve the same result
chain = LLMChain(llm=llm, prompt=prompt)
chain.run(number=2, callbacks=[handler])
> Entering new LLMChain chain...
Prompt after formatting:
1 + 2 = 
> Finished chain.
> Entering new LLMChain chain...
Prompt after formatting:
1 + 2 = 
> Finished chain.
> Entering new LLMChain chain...
Prompt after formatting:
1 + 2 = 
> Finished chain.
'\n\n3'
Creating a custom handler#
You can create a custom handler to set on the object as well. In the example below, we’ll implement streaming with a custom handler.
from langchain.callbacks.base import BaseCallbackHandler
from langchain.chat_models import ChatOpenAI
from langchain.schema import HumanMessage
class MyCustomHandler(BaseCallbackHandler):
rtdocs_stable/api.python.langchain.com/en/stable/modules/callbacks/getting_started.html