fix: raise error in FolderBasedBuilder when data_dir and data_files are missing (#7623)

* Update folder_based_builder.py

* Update folder_based_builder.py

* Update test_folder_based_builder.py

---------

Co-authored-by: Quentin Lhoest <[email protected]>

Author: ArjunJagdale

src/datasets/packaged_modules/folder_based_builder/folder_based_builder.py

@@ -58,6 +58,12 @@ class FolderBasedBuilder(datasets.GeneratorBasedBuilder):
     METADATA_FILENAMES: list[str] = ["metadata.csv", "metadata.jsonl", "metadata.parquet"]
 
     def _info(self):
+        if not self.config.data_dir and not self.config.data_files:
+            raise ValueError(
+                "Folder-based datasets require either `data_dir` or `data_files` to be specified. "
+                "Neither was provided."
+            )
+
         return datasets.DatasetInfo(features=self.config.features)
 
     def _split_generators(self, dl_manager):

tests/packaged_modules/test_folder_based_builder.py

@@ -285,7 +285,7 @@ def test_default_folder_builder_not_usable(data_files_with_labels_no_metadata, c
 # test that AutoFolder is extended for streaming when it's child class is instantiated:
 # see line 115 in src/datasets/streaming.py
 def test_streaming_patched():
-    _ = DummyFolderBasedBuilder()
+    _ = DummyFolderBasedBuilder(data_dir=".")
     module = importlib.import_module(FolderBasedBuilder.__module__)
     assert hasattr(module, "_patched_for_streaming")
     assert module._patched_for_streaming