From 0be4c3c8cbb6434512c5b9ee207be21dfb875aa9 Mon Sep 17 00:00:00 2001
From: Aman Sachan <amansachan92905@gmail.com>
Date: Sun, 14 Jun 2026 01:11:08 +0000
Subject: [PATCH] fix: raise descriptive error for empty/comments-only config
 files

ruamel.yaml's yaml.load() returns None for an empty or comments-only
file. The very next line ("instruments" not in rawConfig) then
evaluated "instruments" not in None and crashed with the opaque
TypeError: argument of type 'NoneType' is not iterable.

Guard for the None case right after the load() call and raise an
AttributeError with a message in the same spirit as the existing
missing-'instruments'-key error, so the user is told the file is
empty and what section it needs.

Fixes #139.
---
 src/instrumentserver/config.py | 11 +++++++++++
 test/pytest/test_config.py     | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/src/instrumentserver/config.py b/src/instrumentserver/config.py
index f30d874..92bb950 100644
--- a/src/instrumentserver/config.py
+++ b/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. "
diff --git a/test/pytest/test_config.py b/test/pytest/test_config.py
index 1a4e6e1..fce70b4 100644
--- a/test/pytest/test_config.py
+++ b/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)
+
+
 # ---------------------------------------------------------------------------
 # pollingRate
 # ---------------------------------------------------------------------------