core[patch]: Improve some error messages and add another test for checking RunnableWithMessageHistory (#25209)

Also add more useful error messages.

---------

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

libs/core/langchain_core/_api/deprecation.py

@@ -214,7 +214,7 @@ def deprecated(
             if not _obj_type:
                 _obj_type = "attribute"
             if not _name:
-                raise ValueError()
+                raise ValueError(f"Field {obj} must have a name to be deprecated.")
             old_doc = obj.description
 
             def finalize(wrapper: Callable[..., Any], new_doc: str) -> T:

libs/core/langchain_core/prompts/chat.py

@@ -551,13 +551,13 @@ class _StringImageMessagePromptTemplate(BaseMessagePromptTemplate):
                             input_variables=input_variables, template=img_template
                         )
                     else:
-                        raise ValueError()
+                        raise ValueError(f"Invalid image template: {tmpl}")
                     prompt.append(img_template_obj)
                 else:
-                    raise ValueError()
+                    raise ValueError(f"Invalid template: {tmpl}")
             return cls(prompt=prompt, **kwargs)
         else:
-            raise ValueError()
+            raise ValueError(f"Invalid template: {template}")
 
     @classmethod
     def from_template_file(

libs/core/langchain_core/runnables/history.py

@@ -433,7 +433,9 @@ class RunnableWithMessageHistory(RunnableBindingBase):
             # This occurs for chat models - since we batch inputs
             if isinstance(input_val[0], list):
                 if len(input_val) != 1:
-                    raise ValueError()
+                    raise ValueError(
+                        f"Expected a single list of messages. Got {input_val}."
+                    )
                 return input_val[0]
             return list(input_val)
         else:

libs/core/tests/unit_tests/runnables/test_runnable_events_v2.py

@@ -1889,6 +1889,25 @@ async def test_runnable_with_message_history() -> None:
         input_messages_key="question",
         history_messages_key="history",
     )
+
+    # patch with_message_history._get_output_messages to listen for errors
+    # so we can raise them in this main thread
+    raised_errors = []
+
+    def collect_errors(fn):  # type: ignore
+        nonlocal raised_errors
+
+        def _get_output_messages(*args, **kwargs):  # type: ignore
+            try:
+                return fn(*args, **kwargs)
+            except Exception as e:
+                raised_errors.append(e)
+                raise e
+
+        return _get_output_messages
+
+    old_ref = with_message_history._get_output_messages
+    with_message_history.__dict__["_get_output_messages"] = collect_errors(old_ref)
     await with_message_history.with_config(
         {"configurable": {"session_id": "session-123"}}
     ).ainvoke({"question": "hello"})
@@ -1911,6 +1930,7 @@ async def test_runnable_with_message_history() -> None:
             AIMessage(content="world", id="ai4"),
         ]
     }
+    assert not raised_errors
 
 
 EXPECTED_EVENTS = [