fix: Add graceful fallback when command is non-existent

Author: jaisw7

src/nanocli/config.py

@@ -254,7 +254,23 @@ def compile_config(
     # Apply overrides (tree rewrite)
     if overrides:
         override_cfg = parse_overrides(overrides)
-        cfg = OmegaConf.merge(cfg, override_cfg)
+        try:
+            cfg = OmegaConf.merge(cfg, override_cfg)
+        except Exception as e:
+            # Extract the key from OmegaConf error message
+            error_msg = str(e)
+            if "Key" in error_msg and "not in" in error_msg:
+                # Parse: Key 'typer' not in 'ModelConfig'
+                import re
+
+                match = re.search(r"Key '(\w+)' not in '(\w+)'", error_msg)
+                if match:
+                    key, cls = match.groups()
+                    raise ConfigError(
+                        f"Invalid config key '{key}' in '{cls}'. Check for typos in your overrides."
+                    ) from None
+            # Re-raise with friendlier message
+            raise ConfigError(f"Config error: {error_msg}") from None
 
     # Convert to typed object if schema provided
     if schema is not None:

src/nanocli/core.py

@@ -18,6 +18,7 @@
 from rich.panel import Panel
 
 from nanocli.config import (
+    ConfigError,
     compile_config,
     get_schema_structure,
     load_yaml,
@@ -202,7 +203,13 @@ def _execute(self, args: list[str]) -> None:
 
                 # Compile config
                 if schema:
-                    config = compile_config(base=base, overrides=overrides, schema=schema)
+                    try:
+                        config = compile_config(base=base, overrides=overrides, schema=schema)
+                    except ConfigError as e:
+                        console.print(f"[red]Error:[/red] {e}")
+                        console.print()
+                        current._show_command_help(console, part, schema)
+                        sys.exit(1)
                 else:
                     config = None
 
@@ -525,7 +532,13 @@ def run(
     base = load_yaml(cfg_file) if cfg_file else None
 
     # Compile config
-    config = compile_config(base=base, overrides=overrides, schema=schema)
+    try:
+        config = compile_config(base=base, overrides=overrides, schema=schema)
+    except ConfigError as e:
+        console.print(f"[red]Error:[/red] {e}")
+        console.print()
+        _show_run_help(console, schema)
+        sys.exit(1)
 
     # Handle print
     if flags["print"] or flags["print_global"]:

tests/test_core.py

@@ -662,3 +662,62 @@ def download(cfg: SimpleConfig):
             sys.stdout = old_stdout
 
         assert "train:" in output or "data:" in output
+
+
+class TestErrorHandling:
+    """Tests for graceful error handling."""
+
+    def test_invalid_override_key_in_run(self):
+        """Test that invalid config keys show helpful error."""
+        from nanocli.config import ConfigError, compile_config
+
+        with pytest.raises(ConfigError, match="Invalid config key"):
+            compile_config(schema=SimpleConfig, overrides=["invalid_key=value"])
+
+    def test_invalid_nested_key(self):
+        """Test that invalid nested keys are caught."""
+        from nanocli.config import ConfigError, compile_config
+
+        with pytest.raises(ConfigError, match="Invalid config key"):
+            compile_config(schema=ParentConfig, overrides=["child.invalid=value"])
+
+    def test_run_with_invalid_key_exits(self):
+        """Test that run() exits gracefully with invalid key."""
+        with pytest.raises(SystemExit):
+            run(SimpleConfig, args=["invalid_key=value"])
+
+    def test_nanocli_command_with_invalid_key(self):
+        """Test NanoCLI command with invalid key shows error and help."""
+        app = NanoCLI()
+
+        @app.command()
+        def train(cfg: SimpleConfig):
+            pass
+
+        with pytest.raises(SystemExit):
+            old_stdout = sys.stdout
+            sys.stdout = io.StringIO()
+            try:
+                app(["train", "invalid_key=value"])
+            finally:
+                sys.stdout = old_stdout
+
+
+class TestConfigErrorMessages:
+    """Tests for config error message formatting."""
+
+    def test_parse_overrides_no_equals(self):
+        """Test that missing = raises ConfigError."""
+        from nanocli.config import ConfigError, parse_overrides
+
+        with pytest.raises(ConfigError, match="Invalid override"):
+            parse_overrides(["no_equals_sign"])
+
+    def test_config_error_message_has_key(self):
+        """Test that error message contains the invalid key."""
+        from nanocli.config import ConfigError, compile_config
+
+        try:
+            compile_config(schema=SimpleConfig, overrides=["typo_key=value"])
+        except ConfigError as e:
+            assert "typo_key" in str(e)