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

doc/source/whatsnew/v1.3.2.rst

@@ -26,7 +26,7 @@ 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 +1552,8 @@ 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]
+        new_values = self.array_values[slicer]
         return type(self)(new_values, self._mgr_locs, ndim=self.ndim)
 
     def fillna(

pandas/tests/arrays/test_array.py

@@ -317,6 +317,26 @@ def test_bounds_check():
         pd.array([-1, 2, 3], dtype="UInt16")
 
 
+def test_ellipsis_index():
+    # GH42430
+    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))
+    tm.assert_equal(str(df["col1"].array.last_item_arg), "slice(None, 1, None)")
+
+
 # ---------------------------------------------------------------------------
 # A couple dummy classes to ensure that Series and Indexes are unboxed before
 # getting to the EA classes.