BUG: cherry-pick fix from 9cf45ce72c and add test from #61675

pandas/core/frame.py

@@ -4075,8 +4075,10 @@ def __getitem__(self, key):
         key = lib.item_from_zerodim(key)
         key = com.apply_if_callable(key, self)
 
-        if is_hashable(key) and not is_iterator(key):
+        if is_hashable(key) and not is_iterator(key) and not isinstance(key, slice):
             # is_iterator to exclude generator e.g. test_getitem_listlike
+            # As of Python 3.12, slice is hashable which breaks MultiIndex (GH#57500)
+
             # shortcut if the key is in columns
             is_mi = isinstance(self.columns, MultiIndex)
             # GH#45316 Return view if key is not duplicated

pandas/tests/frame/indexing/test_indexing.py

@@ -538,6 +538,16 @@ def test_loc_setitem_boolean_mask_allfalse(self):
         result.loc[result.b.isna(), "a"] = result.a.copy()
         tm.assert_frame_equal(result, df)
 
+    def test_getitem_slice_empty(self):
+        df = DataFrame([[1]], columns=MultiIndex.from_product([["A"], ["a"]]))
+        result = df[:]
+
+        expected = DataFrame([[1]], columns=MultiIndex.from_product([["A"], ["a"]]))
+
+        tm.assert_frame_equal(result, expected)
+        # Ensure df[:] returns a view of df, not the same object
+        assert result is not df
+
     def test_getitem_fancy_slice_integers_step(self):
         df = DataFrame(np.random.default_rng(2).standard_normal((10, 5)))
 

pandas/tests/frame/methods/test_join.py

@@ -574,3 +574,41 @@ def test_frame_join_tzaware(self):
 
         tm.assert_index_equal(result.index, expected)
         assert result.index.tz.zone == "US/Central"
+
+    def test_frame_join_categorical_index(self):
+        # GH 61675
+        cat_data = pd.Categorical(
+            [15, 16, 17, 18],
+            categories=pd.Series(list(range(3, 24)), dtype="Int64"),
+            ordered=True,
+        )
+        values1 = "a b c d".split()
+        values2 = "xyzzy foo bar ...".split()
+        df1 = DataFrame({"hr": cat_data, "values1": values1}).set_index("hr")
+        df2 = DataFrame({"hr": cat_data, "values2": values2}).set_index("hr")
+        df1.columns = pd.CategoricalIndex([4], dtype=cat_data.dtype, name="other_hr")
+        df2.columns = pd.CategoricalIndex([3], dtype=cat_data.dtype, name="other_hr")
+
+        df_joined_1 = (
+            df1.reset_index(level="hr")
+            .merge(df2.reset_index(level="hr"), on="hr")
+            .set_index("hr")
+        )
+        expected1 = DataFrame(
+            {"hr": cat_data, "values1": values1, "values2": values2}
+        ).set_index("hr")
+        expected1.columns = Index([4, 3], dtype="object", name="other_hr")
+
+        tm.assert_frame_equal(df_joined_1, expected1)
+
+        df_joined_2 = df1.join(df2)
+        expected2 = DataFrame(
+            {"hr": cat_data, "values1": values1, "values2": values2}
+        ).set_index("hr")
+        expected2.columns = pd.CategoricalIndex(
+            [4, 3], dtype=cat_data.dtype, name="other_hr"
+        )
+
+        tm.assert_frame_equal(df_joined_2, expected2)
+
+        assert df_joined_1.equals(df_joined_2)