Change to 'before' validator to ensure list of str internally

nomenclature/code.py

@@ -243,13 +243,14 @@ class RegionCode(Code):
     """
 
     hierarchy: str = None
-    countries: Optional[Union[List[str], str]] = None
+    countries: Optional[List[str]] = None
     iso3_codes: Optional[Union[List[str], str]] = None
 
-    @field_validator("countries")
+    @field_validator("countries", mode="before")
     def check_countries(cls, v: List[str], info: ValidationInfo) -> List[str]:
         """Verifies that each country name is defined in `nomenclature.countries`."""
-        if invalid_country_names := set(to_list(v)) - set(countries.names):
+        v = to_list(v)
+        if invalid_country_names := set(v) - set(countries.names):
             raise ValueError(
                 f"Region '{info.data['name']}' uses non-standard country name(s): "
                 + ", ".join(invalid_country_names)

tests/data/codelist/region_codelist/countries_attribute_str/region.yaml

@@ -2,4 +2,4 @@
   - World
 - countries:
   - Some region:
-      countries: Austria
+      countries: Austria

tests/test_codelist.py

@@ -178,7 +178,7 @@ def test_region_codelist_str_country_name():
         "region",
         MODULE_TEST_DATA_DIR / "region_codelist" / "countries_attribute_str",
     )
-    assert code["Some region"].countries == "Austria"
+    assert code["Some region"].countries == ["Austria"]
 
 
 def test_norway_as_str():