Fix 7846: add_column and add_item erroneously(?) require new_fingerprint parameter (#7884)

* fix 7846

* Update tests/test_arrow_dataset.py

* make style

* Update arrow_dataset.py

* make style

---------

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

Author: sajmaru

src/datasets/arrow_dataset.py

@@ -6076,7 +6076,11 @@ def get_deletions_and_dataset_card() -> tuple[str, list[CommitOperationDelete],
     @transmit_format
     @fingerprint_transform(inplace=False)
     def add_column(
-        self, name: str, column: Union[list, np.ndarray], new_fingerprint: str, feature: Optional[FeatureType] = None
+        self,
+        name: str,
+        column: Union[list, np.ndarray],
+        new_fingerprint: Optional[str] = None,
+        feature: Optional[FeatureType] = None,
     ):
         """Add column to Dataset.
 
@@ -6333,7 +6337,7 @@ def add_elasticsearch_index(
 
     @transmit_format
     @fingerprint_transform(inplace=False)
-    def add_item(self, item: dict, new_fingerprint: str):
+    def add_item(self, item: dict, new_fingerprint: Optional[str] = None):
         """Add item to Dataset.
 
         <Added version="1.7"/>

tests/test_arrow_dataset.py

@@ -4783,3 +4783,13 @@ def test_from_polars_save_to_disk_and_load_from_disk_round_trip_with_large_list(
 def test_polars_round_trip():
     ds = Dataset.from_dict({"x": [[1, 2], [3, 4, 5]], "y": ["a", "b"]})
     assert isinstance(Dataset.from_polars(ds.to_polars()), Dataset)
+
+
+def test_add_column():
+    from datasets import Dataset
+
+    ds = Dataset.from_dict({"a": [1, 2]})
+    ds = ds.add_column("b", [3, 4])
+    assert "b" in ds.features
+    assert ds[0] == {"a": 1, "b": 3}
+    assert ds[1] == {"a": 2, "b": 4}