langchain/docs/docs/how_to/callbacks_constructor.ipynb

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    "# How to propagate callbacks  constructor\n",
    "\n",
    ":::info Prerequisites\n",
    "\n",
    "This guide assumes familiarity with the following concepts:\n",
    "\n",
    "- [Callbacks](/docs/concepts/#callbacks)\n",
    "- [Custom callback handlers](/docs/how_to/custom_callbacks)\n",
    "\n",
    ":::\n",
    "\n",
    "Most LangChain modules allow you to pass `callbacks` directly into the constructor (i.e., initializer). In this case, the callbacks will only be called for that instance (and any nested runs).\n",
    "\n",
    ":::{.callout-warning}\n",
    "Constructor callbacks are scoped only to the object they are defined on. They are **not** inherited by children of the object. This can lead to confusing behavior,\n",
    "and it's generally better to pass callbacks as a run time argument.\n",
    ":::\n",
    "\n",
    "Here's an example:"
   ]
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    "# | output: false\n",
    "# | echo: false\n",
    "\n",
    "%pip install -qU langchain langchain_anthropic\n",
    "\n",
    "import getpass\n",
    "import os\n",
    "\n",
    "os.environ[\"ANTHROPIC_API_KEY\"] = getpass.getpass()"
   ]
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     "text": [
      "Chat model started\n",
      "Chat model ended, response: generations=[[ChatGeneration(text='1 + 2 = 3', message=AIMessage(content='1 + 2 = 3', response_metadata={'id': 'msg_01CdKsRmeS9WRb8BWnHDEHm7', 'model': 'claude-3-sonnet-20240229', 'stop_reason': 'end_turn', 'stop_sequence': None, 'usage': {'input_tokens': 16, 'output_tokens': 13}}, id='run-2d7fdf2a-7405-4e17-97c0-67e6b2a65305-0'))]] llm_output={'id': 'msg_01CdKsRmeS9WRb8BWnHDEHm7', 'model': 'claude-3-sonnet-20240229', 'stop_reason': 'end_turn', 'stop_sequence': None, 'usage': {'input_tokens': 16, 'output_tokens': 13}} run=None\n"
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       "AIMessage(content='1 + 2 = 3', response_metadata={'id': 'msg_01CdKsRmeS9WRb8BWnHDEHm7', 'model': 'claude-3-sonnet-20240229', 'stop_reason': 'end_turn', 'stop_sequence': None, 'usage': {'input_tokens': 16, 'output_tokens': 13}}, id='run-2d7fdf2a-7405-4e17-97c0-67e6b2a65305-0')"
      ]
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     "execution_count": 18,
     "metadata": {},
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   ],
   "source": [
    "from typing import Any, Dict, List\n",
    "\n",
    "from langchain_anthropic import ChatAnthropic\n",
    "from langchain_core.callbacks import BaseCallbackHandler\n",
    "from langchain_core.messages import BaseMessage\n",
    "from langchain_core.outputs import LLMResult\n",
    "from langchain_core.prompts import ChatPromptTemplate\n",
    "\n",
    "\n",
    "class LoggingHandler(BaseCallbackHandler):\n",
    "    def on_chat_model_start(\n",
    "        self, serialized: Dict[str, Any], messages: List[List[BaseMessage]], **kwargs\n",
    "    ) -> None:\n",
    "        print(\"Chat model started\")\n",
    "\n",
    "    def on_llm_end(self, response: LLMResult, **kwargs) -> None:\n",
    "        print(f\"Chat model ended, response: {response}\")\n",
    "\n",
    "    def on_chain_start(\n",
    "        self, serialized: Dict[str, Any], inputs: Dict[str, Any], **kwargs\n",
    "    ) -> None:\n",
    "        print(f\"Chain {serialized.get('name')} started\")\n",
    "\n",
    "    def on_chain_end(self, outputs: Dict[str, Any], **kwargs) -> None:\n",
    "        print(f\"Chain ended, outputs: {outputs}\")\n",
    "\n",
    "\n",
    "callbacks = [LoggingHandler()]\n",
    "llm = ChatAnthropic(model=\"claude-3-sonnet-20240229\", callbacks=callbacks)\n",
    "prompt = ChatPromptTemplate.from_template(\"What is 1 + {number}?\")\n",
    "\n",
    "chain = prompt | llm\n",
    "\n",
    "chain.invoke({\"number\": \"2\"})"
   ]
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "You can see that we only see events from the chat model run - no chain events from the prompt or broader chain.\n",
    "\n",
    "## Next steps\n",
    "\n",
    "You've now learned how to pass callbacks into a constructor.\n",
    "\n",
    "Next, check out the other how-to guides in this section, such as how to [pass callbacks at runtime](/docs/how_to/callbacks_runtime)."
   ]
  }
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