DEP: warn that __array_finalize__ = None is deprecated

3 files changed, 27 insertions, 9 deletions

diff --git a/numpy/core/src/multiarray/ctors.c b/numpy/core/src/multiarray/ctors.c
index 2ce648998..25eb91977 100644
--- a/numpy/core/src/multiarray/ctors.c
+++ b/numpy/core/src/multiarray/ctors.c
@@ -891,9 +891,24 @@ PyArray_NewFromDescr_int(
         if (func == NULL) {
             goto fail;
         }
-        else if (func == ndarray_array_finalize || func == Py_None) {
+        else if (func == ndarray_array_finalize) {
             Py_DECREF(func);
         }
+        else if (func == Py_None) {
+            Py_DECREF(func);
+            /*
+             * 2022-01-08, NumPy 1.23; when deprecation period is over, remove this
+             * whole stanza so one gets a "NoneType object is not callable" TypeError.
+             */
+            if (DEPRECATE(
+                    "Setting __array_finalize__ = None to indicate no finalization"
+                    "should be done is deprecated.  Instead, just inherit from "
+                    "ndarray or, if that is not possible, explicitly set to "
+                    "ndarray.__array_function__; this will raise a TypeError "
+                    "in the future. (Deprecated since NumPy 1.23)") < 0) {
+                goto fail;
+            }
+        }
         else {
             if (PyCapsule_CheckExact(func)) {
                 /* A C-function is stored here */
diff --git a/numpy/core/tests/test_deprecations.py b/numpy/core/tests/test_deprecations.py
index 76486f755..d2a69d4cf 100644
--- a/numpy/core/tests/test_deprecations.py
+++ b/numpy/core/tests/test_deprecations.py
@@ -1239,3 +1239,14 @@ class TestMemEventHook(_DeprecationTestCase):
         with pytest.warns(DeprecationWarning,
                           match='PyDataMem_SetEventHook is deprecated'):
             ma_tests.test_pydatamem_seteventhook_end()
+
+
+class TestArrayFinalizeNone(_DeprecationTestCase):
+    message = "Setting __array_finalize__ = None"
+
+    def test_use_none_is_deprecated(self):
+        # Deprecated way that ndarray itself showed nothing needs finalizing.
+        class NoFinalize(np.ndarray):
+            __array_finalize__ = None
+
+        self.assert_deprecated(lambda: np.array(1).view(NoFinalize))
diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
index 8f4851036..708e82910 100644
--- a/numpy/core/tests/test_multiarray.py
+++ b/numpy/core/tests/test_multiarray.py
@@ -9021,14 +9021,6 @@ class TestArrayFinalize:
         a = np.array(1).view(SuperFinalize)
         assert_(a.saved_result is None)
 
-    def test_can_use_none(self):
-        # For backward compatibility, to show nothing needs finalizing.
-        class NoFinalize(np.ndarray):
-            __array_finalize__ = None
-
-        a = np.array(1).view(NoFinalize)
-        assert isinstance(a, NoFinalize)
-
 
 def test_orderconverter_with_nonASCII_unicode_ordering():
     # gh-7475