Backport PR #42787: BUG: 1D slices over extension types turn into N-dimensional slices over ExtensionArrays (#42850)

Co-authored-by: Fred Reiss <[email protected]>

Author: simonjayhawkins

doc/source/whatsnew/v1.3.2.rst

@@ -30,7 +30,7 @@ Fixed regressions
 
 Bug fixes
 ~~~~~~~~~
--
+- 1D slices over extension types turn into N-dimensional slices over ExtensionArrays (:issue:`42430`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/internals/blocks.py

@@ -1557,6 +1557,16 @@ def _slice(self, slicer):
 
         return self.values[slicer]
 
+    @final
+    def getitem_block_index(self, slicer: slice) -> ExtensionBlock:
+        """
+        Perform __getitem__-like specialized to slicing along 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(
         self, value, limit=None, inplace: bool = False, downcast=None
     ) -> list[Block]:

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)")