Uniform valid suffixes and clarify exceptions (#9463)

**Description**:
- Uniformed the current valid suffixes (file formats) for loading agents
from hubs and files (to better handle future additions);
 - Clarified exception messages (also in unit test).

libs/langchain/langchain/agents/loading.py

@@ -97,8 +97,9 @@ def load_agent(
     Returns:
         An agent executor.
     """
+    valid_suffixes = {"json", "yaml"}
     if hub_result := try_load_from_hub(
-        path, _load_agent_from_file, "agents", {"json", "yaml"}
+        path, _load_agent_from_file, "agents", valid_suffixes
     ):
         return hub_result
     else:
@@ -109,19 +110,20 @@ def _load_agent_from_file(
     file: Union[str, Path], **kwargs: Any
 ) -> Union[BaseSingleActionAgent, BaseMultiActionAgent]:
     """Load agent from file."""
+    valid_suffixes = {"json", "yaml"}
     # Convert file to Path object.
     if isinstance(file, str):
         file_path = Path(file)
     else:
         file_path = file
     # Load from either json or yaml.
-    if file_path.suffix == ".json":
+    if file_path.suffix[1:] == "json":
         with open(file_path) as f:
             config = json.load(f)
-    elif file_path.suffix == ".yaml":
+    elif file_path.suffix[1:] == "yaml":
         with open(file_path, "r") as f:
             config = yaml.safe_load(f)
     else:
-        raise ValueError("File type must be json or yaml")
+        raise ValueError(f"Unsupported file type, must be one of {valid_suffixes}.")
     # Load the agent from the config now.
     return load_agent_from_config(config, **kwargs)

libs/langchain/langchain/utilities/loading.py

@@ -35,7 +35,7 @@ def try_load_from_hub(
     if remote_path.parts[0] != valid_prefix:
         return None
     if remote_path.suffix[1:] not in valid_suffixes:
-        raise ValueError("Unsupported file type.")
+        raise ValueError(f"Unsupported file type, must be one of {valid_suffixes}.")
 
     # Using Path with URLs is not recommended, because on Windows
     # the backslash is used as the path separator, which can cause issues

libs/langchain/tests/unit_tests/utilities/test_loading.py

@@ -48,7 +48,9 @@ def test_invalid_suffix() -> None:
     loader = Mock()
     valid_suffixes = {"json"}
 
-    with pytest.raises(ValueError, match="Unsupported file type."):
+    with pytest.raises(
+        ValueError, match=f"Unsupported file type, must be one of {valid_suffixes}."
+    ):
         try_load_from_hub(path, loader, "chains", valid_suffixes)
 
     loader.assert_not_called()