BUG: Correct shuffling of objects in 1-d array likes

While introducing the buffer fixed the in-place problem years ago,
running valgrind (and masked arrays) pointed out to me that without
the additional `...` NumPy will unpack and repack objects leading
to slightly incorrect results.

MAINT: Warn about shuffle bug instead of fixing it in old random API

1 files changed, 25 insertions, 0 deletions

diff --git a/numpy/random/tests/test_random.py b/numpy/random/tests/test_random.py
index a0c72b419..2b3b65c19 100644
--- a/numpy/random/tests/test_random.py
+++ b/numpy/random/tests/test_random.py
@@ -522,6 +522,31 @@ class TestRandomDist:
             random.shuffle(values)
         assert "test_random" in rec[0].filename
 
+    @pytest.mark.parametrize("random",
+        [np.random, np.random.RandomState(), np.random.default_rng()])
+    @pytest.mark.parametrize("use_array_like", [True, False])
+    def test_shuffle_no_object_unpacking(self, random, use_array_like):
+        class MyArr(np.ndarray):
+            pass
+
+        items = [None, np.array([3]), np.float64(3), np.array(10), np.float64(7)]
+        arr = np.array(items, dtype=object)
+        item_ids = {id(i) for i in items}
+        if use_array_like:
+            arr = arr.view(MyArr)
+
+        # The array was created fine, and did not modify any objects:
+        assert all(id(i) in item_ids for i in arr)
+
+        if use_array_like and not isinstance(random, np.random.Generator):
+            # The old API gives incorrect results, but warns about it.
+            with pytest.warns(UserWarning,
+                    match="Shuffling a one dimensional array.*"):
+                random.shuffle(arr)
+        else:
+            random.shuffle(arr)
+            assert all(id(i) in item_ids for i in arr)
+
     def test_shuffle_memoryview(self):
         # gh-18273
         # allow graceful handling of memoryviews