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gh-154594: Fix deepcopy memo lookup when __deepcopy__ returns None #154595

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Merged
eendebakpt merged 5 commits into from
Sep 17, 2026
Merged

gh-154594: Fix deepcopy memo lookup when __deepcopy__ returns None #154595

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17c0eb9
gh-154594: Fix deepcopy memo lookup when __deepcopy__ returns None
tonghuaroot Jul 24, 2026
abc037e
Use 'd in memo' lookup instead of a sentinel
tonghuaroot Jul 30, 2026
37b18f0
Apply review suggestions: shorten NEWS, use class attribute in test
tonghuaroot Jul 30, 2026
a39f884
Merge branch 'main' into fix-deepcopy-none-sentinel
eendebakpt Aug 4, 2026
6857381
Merge branch 'main' into fix-deepcopy-none-sentinel
eendebakpt Sep 10, 2026
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Apply review suggestions: shorten NEWS, use class attribute in test
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@tonghuaroot
tonghuaroot committed Jul 30, 2026
commit 37b18f042762213da2fe5380e66625bca03eab20
7 changes: 3 additions & 4 deletions Lib/test/test_copy.py
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Original file line number Diff line number Diff line change
Expand Up @@ -941,15 +941,14 @@ def m(self):

def test_deepcopy_memo_none_result(self):
# Objects whose deepcopy result is None must still be memoized.
call_count = 0
class C:
call_count = 0
def __deepcopy__(self, memo):
nonlocal call_count
call_count += 1
C.call_count += 1
return None
obj = C()
copy.deepcopy([obj, obj, obj])
self.assertEqual(call_count, 1)
self.assertEqual(C.call_count, 1)


class TestReplace(unittest.TestCase):
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4 changes: 1 addition & 3 deletions Misc/NEWS.d/next/Library/2026-07-24-00-02-00.gh-issue-154594.Xk7mQ2.rst
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Original file line number Diff line number Diff line change
@@ -1,4 +1,2 @@
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.
still memoized. Patch by tonghuaroot.
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