FileCallbackHandler (#5589)

# like
[StdoutCallbackHandler](https://github.com/hwchase17/langchain/blob/master/langchain/callbacks/stdout.py),
but writes to a file

When running experiments I have found myself wanting to log the outputs
of my chains in a more lightweight way than using WandB tracing. This PR
contributes a callback handler that writes to file what
`StdoutCallbackHandler` would print.

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## Example Notebook

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See the included `filecallbackhandler.ipynb` notebook for usage. Would
it be better to include this notebook under `modules/callbacks` or under
`integrations/`?

![image](https://github.com/hwchase17/langchain/assets/6439365/c624de0e-343f-4eab-a55b-8808a887489f)


## Who can review?

Community members can review the PR once tests pass. Tag
maintainers/contributors who might be interested:

@agola11

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docs/modules/callbacks/filecallbackhandler.ipynb

@@ -0,0 +1,175 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "id": "63b87b91",
+   "metadata": {},
+   "source": [
+    "# Logging to file\n",
+    "This example shows how to print logs to file. It shows how to use the `FileCallbackHandler`, which does the same thing as [`StdOutCallbackHandler`](https://python.langchain.com/en/latest/modules/callbacks/getting_started.html#using-an-existing-handler), but instead writes the output to file. It also uses the `loguru` library to log other outputs that are not captured by the handler."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "id": "6cb156cc",
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "\n",
+      "\n",
+      "\u001b[1m> Entering new LLMChain chain...\u001b[0m\n",
+      "Prompt after formatting:\n",
+      "\u001b[32;1m\u001b[1;3m1 + 2 = \u001b[0m\n"
+     ]
+    },
+    {
+     "name": "stderr",
+     "output_type": "stream",
+     "text": [
+      "\u001b[32m2023-06-01 18:36:38.929\u001b[0m | \u001b[1mINFO    \u001b[0m | \u001b[36m__main__\u001b[0m:\u001b[36m<module>\u001b[0m:\u001b[36m20\u001b[0m - \u001b[1m\n",
+      "\n",
+      "3\u001b[0m\n"
+     ]
+    },
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "\n",
+      "\u001b[1m> Finished chain.\u001b[0m\n"
+     ]
+    }
+   ],
+   "source": [
+    "from loguru import logger\n",
+    "\n",
+    "from langchain.callbacks import FileCallbackHandler\n",
+    "from langchain.chains import LLMChain\n",
+    "from langchain.llms import OpenAI\n",
+    "from langchain.prompts import PromptTemplate\n",
+    "\n",
+    "logfile = 'output.log'\n",
+    "\n",
+    "logger.add(logfile, colorize=True, enqueue=True)\n",
+    "handler = FileCallbackHandler(logfile)\n",
+    "\n",
+    "llm = OpenAI()\n",
+    "prompt = PromptTemplate.from_template(\"1 + {number} = \")\n",
+    "\n",
+    "# this chain will both print to stdout (because verbose=True) and write to 'output.log'\n",
+    "# if verbose=False, the FileCallbackHandler will still write to 'output.log'\n",
+    "chain = LLMChain(llm=llm, prompt=prompt, callbacks=[handler], verbose=True)\n",
+    "answer = chain.run(number=2)\n",
+    "logger.info(answer)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "9c50d54f",
+   "metadata": {},
+   "source": [
+    "Now we can open the file `output.log` to see that the output has been captured."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "id": "aa32dc0a",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "!pip install ansi2html > /dev/null"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "id": "4af00719",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/html": [
+       "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\" \"http://www.w3.org/TR/html4/loose.dtd\">\n",
+       "<html>\n",
+       "<head>\n",
+       "<meta http-equiv=\"Content-Type\" content=\"text/html; charset=utf-8\">\n",
+       "<title></title>\n",
+       "<style type=\"text/css\">\n",
+       ".ansi2html-content { display: inline; white-space: pre-wrap; word-wrap: break-word; }\n",
+       ".body_foreground { color: #AAAAAA; }\n",
+       ".body_background { background-color: #000000; }\n",
+       ".inv_foreground { color: #000000; }\n",
+       ".inv_background { background-color: #AAAAAA; }\n",
+       ".ansi1 { font-weight: bold; }\n",
+       ".ansi3 { font-style: italic; }\n",
+       ".ansi32 { color: #00aa00; }\n",
+       ".ansi36 { color: #00aaaa; }\n",
+       "</style>\n",
+       "</head>\n",
+       "<body class=\"body_foreground body_background\" style=\"font-size: normal;\" >\n",
+       "<pre class=\"ansi2html-content\">\n",
+       "\n",
+       "\n",
+       "<span class=\"ansi1\">&gt; Entering new LLMChain chain...</span>\n",
+       "Prompt after formatting:\n",
+       "<span class=\"ansi1 ansi32\"></span><span class=\"ansi1 ansi3 ansi32\">1 + 2 = </span>\n",
+       "\n",
+       "<span class=\"ansi1\">&gt; Finished chain.</span>\n",
+       "<span class=\"ansi32\">2023-06-01 18:36:38.929</span> | <span class=\"ansi1\">INFO    </span> | <span class=\"ansi36\">__main__</span>:<span class=\"ansi36\">&lt;module&gt;</span>:<span class=\"ansi36\">20</span> - <span class=\"ansi1\">\n",
+       "\n",
+       "3</span>\n",
+       "\n",
+       "</pre>\n",
+       "</body>\n",
+       "\n",
+       "</html>\n"
+      ],
+      "text/plain": [
+       "<IPython.core.display.HTML object>"
+      ]
+     },
+     "metadata": {},
+     "output_type": "display_data"
+    }
+   ],
+   "source": [
+    "from IPython.display import display, HTML\n",
+    "from ansi2html import Ansi2HTMLConverter\n",
+    "\n",
+    "with open('output.log', 'r') as f:\n",
+    "    content = f.read()\n",
+    "\n",
+    "conv = Ansi2HTMLConverter()\n",
+    "html = conv.convert(content, full=True)\n",
+    "\n",
+    "display(HTML(html))"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3 (ipykernel)",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.9.16"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}

langchain/callbacks/__init__.py

@@ -4,6 +4,7 @@ from langchain.callbacks.aim_callback import AimCallbackHandler
 from langchain.callbacks.argilla_callback import ArgillaCallbackHandler
 from langchain.callbacks.clearml_callback import ClearMLCallbackHandler
 from langchain.callbacks.comet_ml_callback import CometCallbackHandler
+from langchain.callbacks.file import FileCallbackHandler
 from langchain.callbacks.human import HumanApprovalCallbackHandler
 from langchain.callbacks.manager import (
     get_openai_callback,
@@ -21,6 +22,7 @@ __all__ = [
     "ArgillaCallbackHandler",
     "OpenAICallbackHandler",
     "StdOutCallbackHandler",
+    "FileCallbackHandler",
     "AimCallbackHandler",
     "WandbCallbackHandler",
     "MlflowCallbackHandler",

langchain/callbacks/file.py

@@ -0,0 +1,75 @@
+"""Callback Handler that writes to a file."""
+from typing import Any, Dict, Optional, TextIO, cast
+
+from langchain.callbacks.base import BaseCallbackHandler
+from langchain.input import print_text
+from langchain.schema import AgentAction, AgentFinish
+
+
+class FileCallbackHandler(BaseCallbackHandler):
+    """Callback Handler that writes to a file."""
+
+    def __init__(
+        self, filename: str, mode: str = "a", color: Optional[str] = None
+    ) -> None:
+        """Initialize callback handler."""
+        self.file = cast(TextIO, open(filename, mode))
+        self.color = color
+
+    def __del__(self) -> None:
+        """Destructor to cleanup when done."""
+        self.file.close()
+
+    def on_chain_start(
+        self, serialized: Dict[str, Any], inputs: Dict[str, Any], **kwargs: Any
+    ) -> None:
+        """Print out that we are entering a chain."""
+        class_name = serialized["name"]
+        print_text(
+            f"\n\n\033[1m> Entering new {class_name} chain...\033[0m",
+            end="\n",
+            file=self.file,
+        )
+
+    def on_chain_end(self, outputs: Dict[str, Any], **kwargs: Any) -> None:
+        """Print out that we finished a chain."""
+        print_text("\n\033[1m> Finished chain.\033[0m", end="\n", file=self.file)
+
+    def on_agent_action(
+        self, action: AgentAction, color: Optional[str] = None, **kwargs: Any
+    ) -> Any:
+        """Run on agent action."""
+        print_text(action.log, color=color if color else self.color, file=self.file)
+
+    def on_tool_end(
+        self,
+        output: str,
+        color: Optional[str] = None,
+        observation_prefix: Optional[str] = None,
+        llm_prefix: Optional[str] = None,
+        **kwargs: Any,
+    ) -> None:
+        """If not the final action, print out observation."""
+        if observation_prefix is not None:
+            print_text(f"\n{observation_prefix}", file=self.file)
+        print_text(output, color=color if color else self.color, file=self.file)
+        if llm_prefix is not None:
+            print_text(f"\n{llm_prefix}", file=self.file)
+
+    def on_text(
+        self,
+        text: str,
+        color: Optional[str] = None,
+        end: str = "",
+        **kwargs: Any,
+    ) -> None:
+        """Run when agent ends."""
+        print_text(text, color=color if color else self.color, end=end, file=self.file)
+
+    def on_agent_finish(
+        self, finish: AgentFinish, color: Optional[str] = None, **kwargs: Any
+    ) -> None:
+        """Run on agent end."""
+        print_text(
+            finish.log, color=color if self.color else color, end="\n", file=self.file
+        )

langchain/input.py

@@ -1,5 +1,5 @@
 """Handle chained inputs."""
-from typing import Dict, List, Optional
+from typing import Dict, List, Optional, TextIO
 
 _TEXT_COLOR_MAPPING = {
     "blue": "36;1",
@@ -32,10 +32,11 @@ def get_bolded_text(text: str) -> str:
     return f"\033[1m{text}\033[0m"
 
 
-def print_text(text: str, color: Optional[str] = None, end: str = "") -> None:
+def print_text(
+    text: str, color: Optional[str] = None, end: str = "", file: Optional[TextIO] = None
+) -> None:
     """Print text with highlighting and no end characters."""
-    if color is None:
-        text_to_print = text
-    else:
-        text_to_print = get_colored_text(text, color)
-    print(text_to_print, end=end)
+    text_to_print = get_colored_text(text, color) if color else text
+    print(text_to_print, end=end, file=file)
+    if file:
+        file.flush()  # ensure all printed content are written to file