BUG: ndarrays pickled by 1.16 cannot be loaded by 1.15.4 and lower

numpy/core/multiarray.py

@@ -40,6 +40,10 @@
     'tracemalloc_domain', 'typeinfo', 'unpackbits', 'unravel_index', 'vdot',
     'where', 'zeros']
 
+# For backward compatibility, make sure pickle imports these functions from here
+_reconstruct.__module__ = 'numpy.core.multiarray'
+scalar.__module__ = 'numpy.core.multiarray'
+
 
 arange.__module__ = 'numpy'
 array.__module__ = 'numpy'

numpy/core/tests/test_regression.py

@@ -2411,6 +2411,24 @@ def test_field_access_by_title(self):
             if HAS_REFCOUNT:
                 assert_(base <= sys.getrefcount(s))
 
+    @pytest.mark.parametrize('val', [
+        # arrays and scalars
+        np.ones((10, 10), dtype='int32'),
+        np.uint64(10),
+        ])
+    @pytest.mark.parametrize('protocol',
+        range(2, pickle.HIGHEST_PROTOCOL + 1)
+        )
+    def test_pickle_module(self, protocol, val):
+        # gh-12837
+        s = pickle.dumps(val, protocol)
+        assert b'_multiarray_umath' not in s
+        if protocol == 5 and len(val.shape) > 0:
+            # unpickling ndarray goes through _frombuffer for protocol 5
+            assert b'numpy.core.numeric' in s
+        else:
+            assert b'numpy.core.multiarray' in s
+
     def test_object_casting_errors(self):
         # gh-11993
         arr = np.array(['AAAAA', 18465886.0, 18465886.0], dtype=object)