gh-154594: Fix deepcopy memo lookup when __deepcopy__ returns None

Author: tonghuaroot

Lib/copy.py

@@ -59,6 +59,8 @@ class Error(Exception):
 
 __all__ = ["Error", "copy", "deepcopy", "replace"]
 
+_MEMO_MISS = object()
+
 def copy(x):
     """Shallow copy operation on arbitrary Python objects.
 
@@ -122,8 +124,8 @@ def deepcopy(x, memo=None):
     if memo is None:
         memo = {}
     else:
-        y = memo.get(d, None)
-        if y is not None:
+        y = memo.get(d, _MEMO_MISS)
+        if y is not _MEMO_MISS:
             return y
 
     copier = _deepcopy_dispatch.get(cls)

Lib/test/test_copy.py

@@ -939,6 +939,19 @@ 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
+                memo[id(self)] = None
+                return None
+        obj = C()
+        copy.deepcopy([obj, obj, obj])
+        self.assertEqual(call_count, 1)
+
 
 class TestReplace(unittest.TestCase):
 

Misc/NEWS.d/next/Library/2026-07-24-00-02-00.gh-issue-154594.Xk7mQ2.rst

@@ -0,0 +1,3 @@
+Fix :func:`copy.deepcopy` memo lookup using ``None`` as the miss sentinel,
+which prevented memoization of objects whose deep copy result is ``None``.
+Use a private sentinel object instead. Patch by tonghuaroot.