feat: support index item assign in Series

bigframes/core/indexes/base.py

@@ -174,6 +174,11 @@ def dtypes(self) -> pandas.Series:
             index=typing.cast(typing.Tuple, self._block.index.names),
         )
 
+    def __setitem__(self, key, value) -> None:
+        """Index objects are immutable. Use Index constructor to create
+        modified Index."""
+        raise TypeError("Index does not support mutable operations")
+
     @property
     def size(self) -> int:
         return self.shape[0]

tests/system/small/test_index.py

@@ -499,3 +499,29 @@ def test_index_item_with_empty(session):
 
     with pytest.raises(ValueError, match=re.escape(expected_message)):
         bf_idx_empty.item()
+
+
+@pytest.mark.parametrize(
+    ("key", "value"),
+    [
+        (0, "string_value"),
+        (1, 42),
+        ("label", None),
+        (-1, 3.14),
+    ],
+)
+def test_index_setitem_different_types(scalars_dfs, key, value):
+    """Tests that custom Index setitem raises TypeError."""
+    scalars_df, _ = scalars_dfs
+    index = scalars_df.index
+
+    with pytest.raises(TypeError, match="Index does not support mutable operations"):
+        index[key] = value
+
+
+def test_custom_index_setitem_error():
+    """Tests that custom Index setitem raises TypeError."""
+    custom_index = bpd.Index([1, 2, 3, 4, 5], name="custom")
+
+    with pytest.raises(TypeError, match="Index does not support mutable operations"):
+        custom_index[2] = 999