Fix missing info when loading some datasets from Parquet export (#6635)

* set default config when single config parquet export

* fix tests

* fix test

* Update src/datasets/utils/metadata.py

Co-authored-by: Albert Villanova del Moral <8515462+albertvillanova@users.noreply.github.com>

---------

Co-authored-by: Albert Villanova del Moral <8515462+albertvillanova@users.noreply.github.com>

src/datasets/utils/metadata.py

@@ -253,7 +253,7 @@ def to_dataset_card_data(self, dataset_card_data: DatasetCardData) -> None:
     def get_default_config_name(self) -> Optional[str]:
         default_config_name = None
         for config_name, metadata_config in self.items():
-            if config_name == "default" or metadata_config.get("default"):
+            if len(self) == 1 or config_name == "default" or metadata_config.get("default"):
                 if default_config_name is None:
                     default_config_name = config_name
                 else:

tests/test_load.py

@@ -523,7 +523,7 @@ def test_LocalDatasetModuleFactoryWithoutScript_with_single_config_in_metadata(s
         assert module_builder_configs[0].drop_labels is True  # parameter is passed from metadata
 
         # config named "default" is automatically considered to be a default config
-        assert module_factory_result.builder_configs_parameters.default_config_name is None
+        assert module_factory_result.builder_configs_parameters.default_config_name == "custom"
 
         # we don't pass config params to builder in builder_kwargs, they are stored in builder_configs directly
         assert "drop_labels" not in module_factory_result.builder_kwargs
@@ -695,7 +695,7 @@ def test_HubDatasetModuleFactoryWithoutScript_with_one_default_config_in_metadat
         assert module_builder_configs[0].drop_labels is True  # parameter is passed from metadata
 
         # config named "default" is automatically considered to be a default config
-        assert module_factory_result.builder_configs_parameters.default_config_name is None
+        assert module_factory_result.builder_configs_parameters.default_config_name == "custom"
 
         # we don't pass config params to builder in builder_kwargs, they are stored in builder_configs directly
         assert "drop_labels" not in module_factory_result.builder_kwargs

tests/test_metadata_util.py

@@ -224,7 +224,7 @@ def test_from_yaml_string(self):
 @pytest.mark.parametrize(
     "readme_content, expected_metadata_configs_dict, expected_default_config_name",
     [
-        (README_METADATA_SINGLE_CONFIG, EXPECTED_METADATA_SINGLE_CONFIG, None),
+        (README_METADATA_SINGLE_CONFIG, EXPECTED_METADATA_SINGLE_CONFIG, "custom"),
         (README_METADATA_TWO_CONFIGS_WITH_DEFAULT_FLAG, EXPECTED_METADATA_TWO_CONFIGS_DEFAULT_FLAG, "v2"),
         (README_METADATA_TWO_CONFIGS_WITH_DEFAULT_NAME, EXPECTED_METADATA_TWO_CONFIGS_DEFAULT_NAME, "default"),
     ],