Allow countries attribute as string (#412)

* Allow countries attribute as string

* Add str countries attribute test

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

nomenclature/code.py

@@ -246,9 +246,10 @@ class RegionCode(Code):
     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`."""
+        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): "

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

@@ -0,0 +1,5 @@
+- common:
+  - World
+- countries:
+  - Some region:
+      countries: Austria

tests/test_codelist.py

@@ -172,6 +172,15 @@ def test_region_codelist_nonexisting_country_name():
         )
 
 
+def test_region_codelist_str_country_name():
+    """Check that country name as string is validated against `nomenclature.countries`"""
+    code = RegionCodeList.from_directory(
+        "region",
+        MODULE_TEST_DATA_DIR / "region_codelist" / "countries_attribute_str",
+    )
+    assert code["Some region"].countries == ["Austria"]
+
+
 def test_norway_as_str():
     """guard against casting of 'NO' to boolean `False` by PyYAML or pydantic"""
     region = RegionCodeList.from_directory(