core[patch]: On Chain Start Fix for Chain Class (#26593)

- **Issue:** #26588

---------

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

libs/core/langchain_core/callbacks/stdout.py

@@ -32,8 +32,14 @@ class StdOutCallbackHandler(BaseCallbackHandler):
             inputs (Dict[str, Any]): The inputs to the chain.
             **kwargs (Any): Additional keyword arguments.
         """
-        class_name = serialized.get("name", serialized.get("id", ["<unknown>"])[-1])
+        if "name" in kwargs:
-        print(f"\n\n\033[1m> Entering new {class_name} chain...\033[0m")  # noqa: T201
+            name = kwargs["name"]
+        else:
+            if serialized:
+                name = serialized.get("name", serialized.get("id", ["<unknown>"])[-1])
+            else:
+                name = "<unknown>"
+        print(f"\n\n\033[1m> Entering new {name} chain...\033[0m")  # noqa: T201
 
     def on_chain_end(self, outputs: dict[str, Any], **kwargs: Any) -> None:
         """Print out that we finished a chain.

libs/langchain/tests/unit_tests/callbacks/test_stdout.py

@@ -0,0 +1,44 @@
+from typing import Any, Dict, List, Optional
+
+import pytest
+
+from langchain.callbacks import StdOutCallbackHandler
+from langchain.chains.base import CallbackManagerForChainRun, Chain
+
+
+class FakeChain(Chain):
+    """Fake chain class for testing purposes."""
+
+    be_correct: bool = True
+    the_input_keys: List[str] = ["foo"]
+    the_output_keys: List[str] = ["bar"]
+
+    @property
+    def input_keys(self) -> List[str]:
+        """Input keys."""
+        return self.the_input_keys
+
+    @property
+    def output_keys(self) -> List[str]:
+        """Output key of bar."""
+        return self.the_output_keys
+
+    def _call(
+        self,
+        inputs: Dict[str, str],
+        run_manager: Optional[CallbackManagerForChainRun] = None,
+    ) -> Dict[str, str]:
+        return {"bar": "bar"}
+
+
+def test_stdoutcallback(capsys: pytest.CaptureFixture) -> Any:
+    """Test the stdout callback handler."""
+    chain_test = FakeChain(callbacks=[StdOutCallbackHandler(color="red")])
+    chain_test.invoke({"foo": "bar"})
+    # Capture the output
+    captured = capsys.readouterr()
+    # Assert the output is as expected
+    assert captured.out == (
+        "\n\n\x1b[1m> Entering new FakeChain "
+        "chain...\x1b[0m\n\n\x1b[1m> Finished chain.\x1b[0m\n"
+    )