Use 'd in memo' lookup instead of a sentinel

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

Author: tonghuaroot

Lib/copy.py

@@ -59,8 +59,6 @@ class Error(Exception):
 
 __all__ = ["Error", "copy", "deepcopy", "replace"]
 
-_MEMO_MISS = object()
-
 def copy(x):
     """Shallow copy operation on arbitrary Python objects.
 
@@ -124,9 +122,8 @@ def deepcopy(x, memo=None):
     if memo is None:
         memo = {}
     else:
-        y = memo.get(d, _MEMO_MISS)
-        if y is not _MEMO_MISS:
-            return y
+        if d in memo:
+            return memo[d]
 
     copier = _deepcopy_dispatch.get(cls)
     if copier is not None:

Lib/test/test_copy.py

@@ -946,7 +946,6 @@ 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])

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

@@ -1,3 +1,4 @@
-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.
+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.