Fixed linting issues and improved CLI error messages.

This update fixes long lines to follow the 88-character limit and formats the code properly. It also improves error messages in the serve command to make them clearer. The changes ensure the code passes linting checks and works smoothly with ruff and make lint

Author: gauravyad86

libs/cli/langchain_cli/cli.py

@@ -10,30 +10,32 @@
 from langchain_cli.namespaces.migrate import main as migrate_namespace
 from langchain_cli.utils.packages import get_langserve_export, get_package_root
 
+TEMPLATE_CMD = "template"
+APP_CMD = "app"
+INTEGRATION_CMD = "integration"
+
 app = typer.Typer(no_args_is_help=True, add_completion=False)
 app.add_typer(
-    template_namespace.package_cli, name="template", help=template_namespace.__doc__
+    template_namespace.package_cli, name=TEMPLATE_CMD, help=template_namespace.__doc__
 )
-app.add_typer(app_namespace.app_cli, name="app", help=app_namespace.__doc__)
+app.add_typer(app_namespace.app_cli, name=APP_CMD, help=app_namespace.__doc__)
 app.add_typer(
     integration_namespace.integration_cli,
-    name="integration",
+    name=INTEGRATION_CMD,
     help=integration_namespace.__doc__,
 )
 
 app.command(
     name="migrate",
     context_settings={
-        # Let Grit handle the arguments
         "allow_extra_args": True,
         "ignore_unknown_options": True,
     },
-)(
-    migrate_namespace.migrate,
-)
+)(migrate_namespace.migrate)
 
 
 def version_callback(show_version: bool) -> None:
+    """Displays the CLI version and exits."""
     if show_version:
         typer.echo(f"langchain-cli {__version__}")
         raise typer.Exit()
@@ -49,34 +51,41 @@ def main(
         callback=version_callback,
         is_eager=True,
     ),
-):
+) -> None:
+    """CLI entry point."""
     pass
 
 
 @app.command()
 def serve(
     *,
     port: Annotated[
-        Optional[int], typer.Option(help="The port to run the server on")
+        Optional[int],
+        typer.Option(help="The port to run the server on"),
     ] = None,
     host: Annotated[
-        Optional[str], typer.Option(help="The host to run the server on")
+        Optional[str],
+        typer.Option(help="The host to run the server on"),
     ] = None,
 ) -> None:
-    """
-    Start the LangServe app, whether it's a template or an app.
-    """
-
-    # see if is a template
+    """Start the LangServe app, either as a template or an app."""
     try:
         project_dir = get_package_root()
         pyproject = project_dir / "pyproject.toml"
         get_langserve_export(pyproject)
-    except KeyError:
-        # not a template
+    except KeyError as e:
+        typer.secho(
+            f"Error: {e}. This project does not seem to be a template.",
+            fg=typer.colors.RED,
+        )
         app_namespace.serve(port=port, host=host)
+    except FileNotFoundError:
+        typer.secho(
+            "Error: `pyproject.toml` not found. "
+            "Ensure you are in a valid project directory.",
+            fg=typer.colors.RED,
+        )
     else:
-        # is a template
         template_namespace.serve(port=port, host=host)