Validate output table file errors

src/telemetry_window_demo/io.py

@@ -93,7 +93,7 @@ def load_events(path: str | Path) -> pd.DataFrame:
 
 def load_feature_table(path: str | Path) -> pd.DataFrame:
     table_path = Path(path)
-    frame = pd.read_csv(table_path)
+    frame = _read_csv_table(table_path, table_name="feature table")
     _require_columns(frame, FEATURE_TABLE_REQUIRED_COLUMNS, source=str(table_path))
     _parse_datetime_columns(
         frame,
@@ -105,7 +105,7 @@ def load_feature_table(path: str | Path) -> pd.DataFrame:
 
 def load_alert_table(path: str | Path) -> pd.DataFrame:
     table_path = Path(path)
-    frame = pd.read_csv(table_path)
+    frame = _read_csv_table(table_path, table_name="alert table")
     _require_columns(frame, ALERT_TABLE_REQUIRED_COLUMNS, source=str(table_path))
     _parse_datetime_columns(
         frame,
@@ -115,6 +115,20 @@ def load_alert_table(path: str | Path) -> pd.DataFrame:
     return frame
 
 
+def _read_csv_table(table_path: Path, *, table_name: str) -> pd.DataFrame:
+    if not table_path.exists():
+        display_name = table_name[:1].upper() + table_name[1:]
+        raise FileNotFoundError(f"{display_name} not found: {table_path}")
+    try:
+        return pd.read_csv(table_path)
+    except (
+        pd.errors.EmptyDataError,
+        pd.errors.ParserError,
+        UnicodeDecodeError,
+    ) as exc:
+        raise ValueError(f"Invalid {table_name} CSV in {table_path}: {exc}") from exc
+
+
 def _require_columns(
     frame: pd.DataFrame,
     required_columns: tuple[str, ...],

tests/test_cli_errors.py

@@ -66,3 +66,21 @@ def test_main_reports_bad_plot_feature_table_without_traceback(tmp_path, capsys)
     assert stderr.startswith("error: ")
     assert "event_count" in stderr
     assert "Traceback" not in stderr
+
+
+def test_main_reports_missing_default_alert_table_without_traceback(tmp_path, capsys) -> None:
+    features_path = tmp_path / "features.csv"
+    features_path.write_text(
+        "window_start,window_end,event_count,error_rate\n"
+        "2026-03-10T10:00:00Z,2026-03-10T10:01:00Z,10,0.25\n",
+        encoding="utf-8",
+    )
+
+    with pytest.raises(SystemExit) as excinfo:
+        main(["plot", "--features", str(features_path)])
+
+    assert excinfo.value.code == 1
+    stderr = capsys.readouterr().err
+    assert stderr.startswith("error: ")
+    assert "Alert table not found" in stderr
+    assert "Traceback" not in stderr

tests/test_io.py

@@ -176,6 +176,13 @@ def test_load_feature_table_requires_plot_columns(tmp_path) -> None:
     assert "event_count" in message
 
 
+def test_load_feature_table_reports_missing_file(tmp_path) -> None:
+    path = tmp_path / "missing-features.csv"
+
+    with pytest.raises(FileNotFoundError, match="Feature table not found"):
+        load_feature_table(path)
+
+
 def test_load_feature_table_rejects_invalid_window_timestamp(tmp_path) -> None:
     path = tmp_path / "features.csv"
     path.write_text(
@@ -192,6 +199,19 @@ def test_load_feature_table_rejects_invalid_window_timestamp(tmp_path) -> None:
     assert "window_start" in message
 
 
+def test_load_alert_table_rejects_invalid_csv(tmp_path) -> None:
+    path = tmp_path / "alerts.csv"
+    path.write_text(
+        'alert_time,window_start,window_end,rule_name,severity\n'
+        '"2026-03-10T10:01:00Z,2026-03-10T10:00:00Z,'
+        '2026-03-10T10:01:00Z,high_error_rate,medium\n',
+        encoding="utf-8",
+    )
+
+    with pytest.raises(ValueError, match="Invalid alert table CSV"):
+        load_alert_table(path)
+
+
 def test_load_alert_table_requires_plot_columns(tmp_path) -> None:
     path = tmp_path / "alerts.csv"
     path.write_text(