From 9ef8c074cbc9f60142062395c360e402e7da95a3 Mon Sep 17 00:00:00 2001
From: Janosh Riebesell <janosh.riebesell@gmail.com>
Date: Tue, 12 Sep 2023 12:36:39 +0200
Subject: [PATCH 1/2] on existing but empty atomate config file: issue warning,
 not ValidationError 'NoneType' object is not iterable

---
 src/atomate2/settings.py | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/src/atomate2/settings.py b/src/atomate2/settings.py
index f1af5a90d0..e75b128bd5 100644
--- a/src/atomate2/settings.py
+++ b/src/atomate2/settings.py
@@ -1,5 +1,6 @@
 """Settings for atomate2."""
 
+import warnings
 from pathlib import Path
 from typing import Literal, Optional, Tuple, Union
 
@@ -177,7 +178,17 @@ def load_default_settings(cls, values):
 
         new_values = {}
         if Path(config_file_path).expanduser().exists():
-            new_values.update(loadfn(Path(config_file_path).expanduser()))
-
-        new_values.update(values)
-        return new_values
+            if Path(config_file_path).stat().st_size == 0:
+                warnings.warn(
+                    f"Using atomate2 config file at {config_file_path} but it's empty",
+                    stacklevel=2,
+                )
+            else:
+                try:
+                    new_values.update(loadfn(config_file_path))
+                except ValueError:
+                    raise SyntaxError(
+                        f"atomate2 config file at {config_file_path} is unparsable"
+                    ) from None
+
+        return {**new_values, **values}

From 7d1687749e81d29c37ce2b84cbab2b6441f36bbd Mon Sep 17 00:00:00 2001
From: Janosh Riebesell <janosh.riebesell@gmail.com>
Date: Tue, 12 Sep 2023 12:38:18 +0200
Subject: [PATCH 2/2] add test_empty_and_invalid_config_file

---
 tests/common/test_settings.py | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)
 create mode 100644 tests/common/test_settings.py

diff --git a/tests/common/test_settings.py b/tests/common/test_settings.py
new file mode 100644
index 0000000000..a80e195377
--- /dev/null
+++ b/tests/common/test_settings.py
@@ -0,0 +1,26 @@
+import pytest
+
+
+def test_empty_and_invalid_config_file(clean_dir):
+    import os
+    from pathlib import Path
+
+    from atomate2.settings import Atomate2Settings
+
+    # set path to load settings from though ATOMATE2_CONFIG_FILE env variable
+    config_file_path = Path.cwd() / "test-atomate2-config.yaml"
+    os.environ["ATOMATE2_CONFIG_FILE"] = str(config_file_path)
+
+    # test warning if config file is empty
+    config_file_path.touch()
+    with pytest.warns(
+        UserWarning, match="Using atomate2 config file at .+ but it's empty"
+    ):
+        Atomate2Settings()
+
+    # test error if the file exists and contains invalid YAML
+    with open(config_file_path, "w") as file:
+        file.write("invalid yaml")
+
+    with pytest.raises(SyntaxError, match="atomate2 config file at"):
+        Atomate2Settings()