eendebakpt · Sep 17, 2026 · Jul 24, 2026 · Jul 30, 2026 · Jul 30, 2026 · Aug 4, 2026
diff --git a/Lib/copy.py b/Lib/copy.py
@@ -122,9 +122,8 @@ def deepcopy(x, memo=None):
     if memo is None:
         memo = {}
     else:
-        y = memo.get(d, None)
-        if y is not None:
-            return y
+        if d in memo:
+            return memo[d]
 
     copier = _deepcopy_dispatch.get(cls)
     if copier is not None:

diff --git a/Lib/test/test_copy.py b/Lib/test/test_copy.py
@@ -939,6 +939,18 @@ def m(self):
         self.assertIs(g.b.__self__, g)
         g.b()
 
+    def test_deepcopy_memo_none_result(self):
+        # Objects whose deepcopy result is None must still be memoized.
+        call_count = 0
+        class C:
+            def __deepcopy__(self, memo):
+                nonlocal call_count
+                call_count += 1
+                return None
+        obj = C()
+        copy.deepcopy([obj, obj, obj])
+        self.assertEqual(call_count, 1)
+
 
 class TestReplace(unittest.TestCase):
 

diff --git a/Misc/NEWS.d/next/Library/2026-07-24-00-02-00.gh-issue-154594.Xk7mQ2.rst b/Misc/NEWS.d/next/Library/2026-07-24-00-02-00.gh-issue-154594.Xk7mQ2.rst
@@ -0,0 +1,4 @@
+Fix :func:`copy.deepcopy` so that an object whose deep copy is ``None`` is
+still memoized. The memo miss was detected with ``None``, which is a valid
+result, so such objects were copied again on every reference. Patch by
+tonghuaroot.