fix: raise descriptive error for empty/comments-only config files (#139)

src/instrumentserver/config.py

@@ -47,6 +47,17 @@ def loadConfig(
     yaml = ruamel.yaml.YAML()
     rawConfig = yaml.load(configPath)
 
+    # ruamel.yaml returns ``None`` for an empty (or comments-only) config
+    # file. The membership test on the next line would then raise an opaque
+    # ``TypeError: argument of type 'NoneType' is not iterable``, so we
+    # surface a descriptive error that matches the style of the missing-key
+    # message below.
+    if rawConfig is None:
+        raise AttributeError(
+            f"The config file '{configPath}' is empty. "
+            "It needs at least an 'instruments:' section."
+        )
+
     if "instruments" not in rawConfig:
         raise AttributeError(
             "All configurations must be inside the 'instruments' field. "

test/pytest/test_config.py

@@ -177,6 +177,39 @@ def test_missing_instruments_key_raises(tmp_path):
         loadConfig(cfg)
 
 
+# ---------------------------------------------------------------------------
+# Error: empty / comments-only config file
+# ---------------------------------------------------------------------------
+
+
+def test_empty_config_raises(tmp_path):
+    """An empty config file must not crash with an opaque TypeError."""
+    cfg = _write_config(tmp_path, "")
+    with pytest.raises(AttributeError, match="empty"):
+        loadConfig(cfg)
+
+
+def test_comments_only_config_raises(tmp_path):
+    """A config file that contains only comments also parses to ``None``."""
+    cfg = _write_config(
+        tmp_path,
+        """\
+# this file is intentionally just comments
+# nothing else
+""",
+    )
+    with pytest.raises(AttributeError, match="empty"):
+        loadConfig(cfg)
+
+
+def test_empty_config_error_mentions_instruments(tmp_path):
+    """The empty-config error should guide the user toward the required key."""
+    cfg = _write_config(tmp_path, "")
+    with pytest.raises(AttributeError) as excinfo:
+        loadConfig(cfg)
+    assert "instruments" in str(excinfo.value)
+
+
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 # pollingRate
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