Allow custom .pmgrc.yaml location via new PMG_CONFIG_FILE env var (#3949)

Author: janosh

src/pymatgen/core/__init__.py

@@ -37,9 +37,12 @@
 def _load_pmg_settings() -> dict[str, Any]:
     settings: dict[str, Any] = {}
 
+    # PMG_CONFIG_FILE takes precedence over default settings location
+    settings_file = os.getenv("PMG_CONFIG_FILE") or SETTINGS_FILE
+
     # Load .pmgrc.yaml file
     yaml = YAML()
-    for file_path in (SETTINGS_FILE, OLD_SETTINGS_FILE):
+    for file_path in (settings_file, OLD_SETTINGS_FILE):
         try:
             with open(file_path, encoding="utf-8") as yml_file:
                 settings = yaml.load(yml_file) or {}

tests/core/test_settings.py

@@ -48,3 +48,34 @@ def test_load_settings(tmp_path: Path, monkeypatch: MonkeyPatch) -> None:
     # should return empty dict if file is invalid
     settings_file.write_text("---")
     assert _load_pmg_settings() == {}
+
+
+def test_env_var_pmg_config_file(tmp_path: Path, monkeypatch: MonkeyPatch) -> None:
+    custom_config_file = tmp_path / "custom_config.yaml"
+    custom_config_file.write_text("PMG_CUSTOM_SETTING: custom_value")
+
+    with monkeypatch.context() as ctx:
+        ctx.setenv("PMG_CONFIG_FILE", str(custom_config_file))
+        settings = _load_pmg_settings()
+        assert "PMG_CUSTOM_SETTING" in settings
+        assert settings["PMG_CUSTOM_SETTING"] == "custom_value"
+
+    # Test that PMG_CONFIG_FILE takes precedence over the default location
+    settings_file = tmp_path / ".pmgrc.yaml"
+    monkeypatch.setattr("pymatgen.core.SETTINGS_FILE", settings_file)
+    settings_file.write_text("PMG_DEFAULT_SETTING: default_value")
+    custom_config_file.write_text("PMG_CUSTOM_SETTING: custom_value")
+
+    with monkeypatch.context() as ctx:
+        ctx.setenv("PMG_CONFIG_FILE", str(custom_config_file))
+        settings = _load_pmg_settings()
+        assert "PMG_CUSTOM_SETTING" in settings
+        assert "PMG_DEFAULT_SETTING" not in settings
+        assert settings["PMG_CUSTOM_SETTING"] == "custom_value"
+
+    # Test that env vars still take precedence over the values specified in PMG_CONFIG_FILE
+    with monkeypatch.context() as ctx:
+        ctx.setenv("PMG_CONFIG_FILE", str(custom_config_file))
+        ctx.setenv("PMG_CUSTOM_SETTING", "env_value")
+        settings = _load_pmg_settings()
+        assert settings["PMG_CUSTOM_SETTING"] == "env_value"