From 86f41d5ee53a409c8e7f42fb7d44ef801cd4733b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?tonghuaroot=20=28=E7=AB=A5=E8=AF=9D=29?=
 <tonghuaroot@gmail.com>
Date: Fri, 18 Sep 2026 03:11:17 +0800
Subject: [PATCH] gh-154594: Fix deepcopy memo lookup when __deepcopy__ returns
 None (GH-154595)
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* gh-154594: Fix deepcopy memo lookup when __deepcopy__ returns None

* Use 'd in memo' lookup instead of a sentinel

Addresses review: clearer, no sentinel, and the test treats memo as opaque.

* Apply review suggestions: shorten NEWS, use class attribute in test

---------
(cherry picked from commit 5539c2a5437acc4f4719aabac375368e0d310bd9)

Co-authored-by: tonghuaroot (童话) <tonghuaroot@gmail.com>
Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>
---
 Lib/copy.py                                           |  5 ++---
 Lib/test/test_copy.py                                 | 11 +++++++++++
 .../2026-07-24-00-02-00.gh-issue-154594.Xk7mQ2.rst    |  2 ++
 3 files changed, 15 insertions(+), 3 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-07-24-00-02-00.gh-issue-154594.Xk7mQ2.rst

diff --git a/Lib/copy.py b/Lib/copy.py
index 6149301ad1389e2..f7fd680657b85a5 100644
--- 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
index 13b7ce29565deed..2a12a6737df1631 100644
--- a/Lib/test/test_copy.py
+++ b/Lib/test/test_copy.py
@@ -951,6 +951,17 @@ 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.
+        class C:
+            call_count = 0
+            def __deepcopy__(self, memo):
+                C.call_count += 1
+                return None
+        obj = C()
+        copy.deepcopy([obj, obj, obj])
+        self.assertEqual(C.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
new file mode 100644
index 000000000000000..5f96bba97aa19f4
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-24-00-02-00.gh-issue-154594.Xk7mQ2.rst
@@ -0,0 +1,2 @@
+Fix :func:`copy.deepcopy` so that an object whose deep copy is ``None`` is
+still memoized. Patch by tonghuaroot.