Backport PR #57139 on branch 2.2.x (BUG: Index(Series) makes array read only for object dtype) (#57143)

Backport PR #57139: BUG: Index(Series) makes array read only for object dtype

Co-authored-by: Patrick Hoefler <[email protected]>

Author: meeseeksmachine

doc/source/whatsnew/v2.2.1.rst

@@ -28,7 +28,7 @@ Fixed regressions
 
 Bug fixes
 ~~~~~~~~~
--
+- Fixed bug in :meth:`DataFrame.__getitem__` for empty :class:`DataFrame` with Copy-on-Write enabled (:issue:`57130`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_221.other:

pandas/_libs/ops.pyx

@@ -29,7 +29,7 @@ from pandas._libs.util cimport is_nan
 
 @cython.wraparound(False)
 @cython.boundscheck(False)
-def scalar_compare(object[:] values, object val, object op) -> ndarray:
+def scalar_compare(ndarray[object] values, object val, object op) -> ndarray:
     """
     Compare each element of `values` array with the scalar `val`, with
     the comparison operation described by `op`.

pandas/core/common.py

@@ -233,6 +233,8 @@ def asarray_tuplesafe(values: Iterable, dtype: NpDtype | None = None) -> ArrayLi
         values = list(values)
     elif isinstance(values, ABCIndex):
         return values._values
+    elif isinstance(values, ABCSeries):
+        return values._values
 
     if isinstance(values, list) and dtype in [np.object_, object]:
         return construct_1d_object_array_from_listlike(values)

pandas/tests/indexes/base_class/test_constructors.py

@@ -71,3 +71,10 @@ def test_inference_on_pandas_objects(self):
         with tm.assert_produces_warning(FutureWarning, match="Dtype inference"):
             result = Index(ser)
         assert result.dtype != np.object_
+
+    def test_constructor_not_read_only(self):
+        # GH#57130
+        ser = Series([1, 2], dtype=object)
+        with pd.option_context("mode.copy_on_write", True):
+            idx = Index(ser)
+            assert idx._values.flags.writeable

pandas/tests/indexes/test_common.py

@@ -500,3 +500,12 @@ def test_ndarray_compat_properties(index):
     # test for validity
     idx.nbytes
     idx.values.nbytes
+
+
+def test_compare_read_only_array():
+    # GH#57130
+    arr = np.array([], dtype=object)
+    arr.flags.writeable = False
+    idx = pd.Index(arr)
+    result = idx > 69
+    assert result.dtype == bool