BUG: Indexing on nullable column (#44551)

Author: jbrockmendel

doc/source/whatsnew/v1.4.0.rst

@@ -621,6 +621,8 @@ Indexing
 - Bug in :meth:`Series.__setitem__` with a boolean mask indexer setting a listlike value of length 1 incorrectly broadcasting that value (:issue:`44265`)
 - Bug in :meth:`DataFrame.loc.__setitem__` and :meth:`DataFrame.iloc.__setitem__` with mixed dtypes sometimes failing to operate in-place (:issue:`44345`)
 - Bug in :meth:`DataFrame.loc.__getitem__` incorrectly raising ``KeyError`` when selecting a single column with a boolean key (:issue:`44322`).
+- Bug in indexing on columns with ``loc`` or ``iloc`` using a slice with a negative step with ``ExtensionDtype`` columns incorrectly raising (:issue:`44551`)
+-
 
 Missing
 ^^^^^^^

pandas/core/internals/blocks.py

@@ -1566,7 +1566,9 @@ def _slice(self, slicer) -> ExtensionArray:
                 )
             # GH#32959 only full-slicers along fake-dim0 are valid
             # TODO(EA2D): won't be necessary with 2D EAs
-            new_locs = self._mgr_locs[first]
+            # range(1) instead of self._mgr_locs to avoid exception on [::-1]
+            #  see test_iloc_getitem_slice_negative_step_ea_block
+            new_locs = range(1)[first]
             if len(new_locs):
                 # effectively slice(None)
                 slicer = slicer[1]

pandas/tests/indexing/test_iloc.py

@@ -1146,6 +1146,18 @@ def test_loc_setitem_boolean_list(self, rhs_func, indexing_func):
         expected = DataFrame({"a": [5, 1, 10]})
         tm.assert_frame_equal(df, expected)
 
+    def test_iloc_getitem_slice_negative_step_ea_block(self):
+        # GH#44551
+        df = DataFrame({"A": [1, 2, 3]}, dtype="Int64")
+
+        res = df.iloc[:, ::-1]
+        tm.assert_frame_equal(res, df)
+
+        df["B"] = "foo"
+        res = df.iloc[:, ::-1]
+        expected = DataFrame({"B": df["B"], "A": df["A"]})
+        tm.assert_frame_equal(res, expected)
+
 
 class TestILocErrors:
     # NB: this test should work for _any_ Series we can pass as