BUG: Fix cast_pointwise_result with all-NA values

Author: heoh

doc/source/whatsnew/v3.0.0.rst

@@ -941,6 +941,7 @@ Numeric
 
 Conversion
 ^^^^^^^^^^
+- Bug in :meth:`BaseMaskedArray._cast_pointwise_result` with all-NA values results returned ``object`` dtype instead of preserving the original dtype (:issue:`62344`)
 - Bug in :meth:`DataFrame.astype` not casting ``values`` for Arrow-based dictionary dtype correctly (:issue:`58479`)
 - Bug in :meth:`DataFrame.update` bool dtype being converted to object (:issue:`55509`)
 - Bug in :meth:`Series.astype` might modify read-only array inplace when casting to a string dtype (:issue:`57212`)

pandas/core/arrays/masked.py

@@ -149,6 +149,8 @@ def _from_sequence(cls, scalars, *, dtype=None, copy: bool = False) -> Self:
         return cls(values, mask)
 
     def _cast_pointwise_result(self, values) -> ArrayLike:
+        if isna(values).all():
+            return type(self)._from_sequence(values, dtype=self.dtype)
         values = np.asarray(values, dtype=object)
         result = lib.maybe_convert_objects(values, convert_to_nullable_dtype=True)
         lkind = self.dtype.kind

pandas/tests/extension/test_masked.py

@@ -360,3 +360,21 @@ def check_accumulate(self, ser: pd.Series, op_name: str, skipna: bool):
             )
         )
         tm.assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "arr, values",
+    [
+        (pd.array([True, False]), [pd.NA, pd.NA]),
+        (pd.array([1, 2]), [pd.NA, pd.NA]),
+    ],
+)
+def test_cast_pointwise_result_all_na_respects_dtype(arr, values):
+    """
+    GH#62344
+    Ensure that _cast_pointwise_result respects the original dtype
+    even when the result consists entirely of NA values.
+    """
+    result = arr._cast_pointwise_result(values)
+    assert result.dtype == arr.dtype
+    assert all(x is pd.NA for x in result)