Raise a clear error when fenn.yaml is empty or not a mapping

fenn/parser.py

@@ -45,6 +45,12 @@ def _config_missing(self) -> None:
             self._config_file,
         )
 
+    def _config_invalid(self, detail: str) -> None:
+        message = f"Configuration file {self._config_file} is invalid: {detail}"
+        logger.exception(message)
+
+        raise ValueError(message)
+
     def load_configuration(self) -> Any:
         """Loads the YAML configuration into the _args dictionary."""
 
@@ -53,8 +59,17 @@ def load_configuration(self) -> Any:
 
         # File exists → load YAML
         with open(self._config_file) as f:
-            self._args = yaml.safe_load(f)
-            self._args["project"] = self._config_file.stem
+            loaded = yaml.safe_load(f)
+
+        if loaded is None:
+            self._config_invalid("the file is empty")
+        if not isinstance(loaded, dict):
+            self._config_invalid(
+                f"the top level must be a mapping, got {type(loaded).__name__}"
+            )
+
+        self._args = loaded
+        self._args["project"] = self._config_file.stem
 
         return self._args
 

tests/unit/test_parser.py

@@ -0,0 +1,43 @@
+"""Tests for `fenn.parser.Parser.load_configuration`."""
+
+import pytest
+
+from fenn.parser import Parser
+
+
+@pytest.fixture(autouse=True)
+def _reset_parser():
+    Parser.reset()
+    yield
+    Parser.reset()
+
+
+def test_load_configuration_reads_a_mapping(tmp_path):
+    cfg = tmp_path / "fenn.yaml"
+    cfg.write_text("logger:\n  dir: logs\n")
+
+    args = Parser(cfg).load_configuration()
+
+    assert args["logger"] == {"dir": "logs"}
+    assert args["project"] == "fenn"
+
+
+def test_load_configuration_missing_file_raises_file_not_found(tmp_path):
+    with pytest.raises(FileNotFoundError):
+        Parser(tmp_path / "missing.yaml").load_configuration()
+
+
+def test_load_configuration_empty_file_raises_value_error(tmp_path):
+    cfg = tmp_path / "fenn.yaml"
+    cfg.write_text("")
+
+    with pytest.raises(ValueError, match="empty"):
+        Parser(cfg).load_configuration()
+
+
+def test_load_configuration_non_mapping_raises_value_error(tmp_path):
+    cfg = tmp_path / "fenn.yaml"
+    cfg.write_text("- just\n- a\n- list\n")
+
+    with pytest.raises(ValueError, match="mapping"):
+        Parser(cfg).load_configuration()