Added Python logging tracer (#14190)

This PR creates a logging handler and adds a simple unit test of it

Supercedes https://github.com/langchain-ai/langchain/pull/12862

---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

libs/langchain/langchain/callbacks/tracers/__init__.py

@@ -7,12 +7,14 @@ from langchain_core.tracers.stdout import (
     FunctionCallbackHandler,
 )
 
+from langchain.callbacks.tracers.logging import LoggingCallbackHandler
 from langchain.callbacks.tracers.wandb import WandbTracer
 
 __all__ = [
+    "ConsoleCallbackHandler",
+    "FunctionCallbackHandler",
+    "LoggingCallbackHandler",
     "LangChainTracer",
     "LangChainTracerV1",
-    "FunctionCallbackHandler",
-    "ConsoleCallbackHandler",
     "WandbTracer",
 ]

libs/langchain/langchain/callbacks/tracers/logging.py

@@ -0,0 +1,46 @@
+__all__ = ["LoggingCallbackHandler"]
+
+import logging
+from typing import Any, Optional
+from uuid import UUID
+
+from langchain_core.exceptions import TracerException
+from langchain_core.tracers.stdout import FunctionCallbackHandler
+from langchain_core.utils.input import get_bolded_text, get_colored_text
+
+
+class LoggingCallbackHandler(FunctionCallbackHandler):
+    """Tracer that logs via the input Logger."""
+
+    name: str = "logging_callback_handler"
+
+    def __init__(
+        self,
+        logger: logging.Logger,
+        log_level: int = logging.INFO,
+        extra: Optional[dict] = None,
+        **kwargs: Any,
+    ) -> None:
+        log_method = getattr(logger, logging.getLevelName(level=log_level).lower())
+
+        def callback(text: str) -> None:
+            log_method(text, extra=extra)
+
+        super().__init__(function=callback, **kwargs)
+
+    def on_text(
+        self,
+        text: str,
+        *,
+        run_id: UUID,
+        parent_run_id: Optional[UUID] = None,  # noqa: ARG002
+        **kwargs: Any,  # noqa: ARG002
+    ) -> None:
+        try:
+            crumbs_str = f"[{self.get_breadcrumbs(run=self._get_run(run_id=run_id))}] "
+        except TracerException:
+            crumbs_str = ""
+        self.function_callback(
+            f'{get_colored_text("[text]", color="blue")}'
+            f' {get_bolded_text(f"{crumbs_str}New text:")}\n{text}'
+        )

libs/langchain/tests/unit_tests/callbacks/tracers/test_logging.py

@@ -0,0 +1,37 @@
+import logging
+import sys
+import uuid
+
+import pytest
+
+from langchain.callbacks.tracers import LoggingCallbackHandler
+
+
+def test_logging(
+    caplog: pytest.LogCaptureFixture, capsys: pytest.CaptureFixture[str]
+) -> None:
+    # Set up a Logger and a handler so we can check the Logger's handlers work too
+    logger = logging.getLogger("test_logging")
+    logger.setLevel(logging.INFO)
+    logger.addHandler(logging.StreamHandler(sys.stdout))
+
+    handler = LoggingCallbackHandler(logger, extra={"test": "test_extra"})
+    handler.on_text("test", run_id=uuid.uuid4())
+
+    # Assert logging actually took place
+    assert len(caplog.record_tuples) == 1
+    record = caplog.records[0]
+    assert record.name == logger.name
+    assert record.levelno == logging.INFO
+    assert (
+        record.msg == "\x1b[36;1m\x1b[1;3m[text]\x1b[0m \x1b[1mNew text:\x1b[0m\ntest"
+    )
+    # Check the extra shows up
+    assert record.test == "test_extra"  # type: ignore[attr-defined]
+
+    # Assert log handlers worked
+    cap_result = capsys.readouterr()
+    assert (
+        cap_result.out
+        == "\x1b[36;1m\x1b[1;3m[text]\x1b[0m \x1b[1mNew text:\x1b[0m\ntest\n"
+    )