BUG: 1D slices over extension types turn into N-dimensional slices over ExtensionArrays

doc/source/whatsnew/v1.3.2.rst

@@ -29,7 +29,7 @@ Fixed regressions
 
 Bug fixes
 ~~~~~~~~~
--
+- 1D slices over extension types turn into N-dimensional slices over ExtensionArrays (:issue:`42430`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/internals/blocks.py

@@ -1552,12 +1552,10 @@ def _slice(self, slicer):
     def getitem_block_index(self, slicer: slice) -> ExtensionBlock:
         """
         Perform __getitem__-like specialized to slicing along index.
-
-        Assumes self.ndim == 2
         """
-        # error: Invalid index type "Tuple[ellipsis, slice]" for
-        # "Union[ndarray, ExtensionArray]"; expected type "Union[int, slice, ndarray]"
-        new_values = self.values[..., slicer]  # type: ignore[index]
+        # GH#42787 in principle this is equivalent to values[..., slicer], but we don't
+        # require subclasses of ExtensionArray to support that form (for now).
+        new_values = self.values[slicer]
         return type(self)(new_values, self._mgr_locs, ndim=self.ndim)
 
     def fillna(

pandas/tests/extension/base/getitem.py

@@ -425,3 +425,23 @@ def test_item(self, data):
 
         with pytest.raises(ValueError, match=msg):
             s.item()
+
+    def test_ellipsis_index(self):
+        # GH42430 1D slices over extension types turn into N-dimensional slices over
+        #  ExtensionArrays
+        class CapturingStringArray(pd.arrays.StringArray):
+            """Extend StringArray to capture arguments to __getitem__"""
+
+            def __getitem__(self, item):
+                self.last_item_arg = item
+                return super().__getitem__(item)
+
+        df = pd.DataFrame(
+            {"col1": CapturingStringArray(np.array(["hello", "world"], dtype=object))}
+        )
+        _ = df.iloc[:1]
+
+        # String comparison because there's no native way to compare slices.
+        # Before the fix for GH42430, last_item_arg would get set to the 2D slice
+        # (Ellipsis, slice(None, 1, None))
+        self.assert_equal(str(df["col1"].array.last_item_arg), "slice(None, 1, None)")