gh-123745: Improve performance of deepcopy by setting keep alive key in the memo

Lib/copy.py

@@ -121,6 +121,7 @@ def deepcopy(x, memo=None):
     d = id(x)
     if memo is None:
         memo = {}
+        memo[id(memo)] = []
     else:
         y = memo.get(d, None)
         if y is not None:
@@ -159,7 +160,7 @@ def deepcopy(x, memo=None):
     # If is its own copy, don't memoize.
     if y is not x:
         memo[d] = y
-        _keep_alive(x, memo) # Make sure x lives at least as long as d
+        memo[id(memo)].append(x) # Make sure x lives at least as long as d
     return y
 
 _atomic_types = frozenset({types.NoneType, types.EllipsisType, types.NotImplementedType,
@@ -209,22 +210,6 @@ def _deepcopy_method(x, memo): # Copy instance methods
 
 del d
 
-def _keep_alive(x, memo):
-    """Keeps a reference to the object x in the memo.
-
-    Because we remember objects by their id, we have
-    to assure that possibly temporary objects are kept
-    alive by referencing them.
-    We store a reference at the id of the memo, which should
-    normally not be used unless someone tries to deepcopy
-    the memo itself...
-    """
-    try:
-        memo[id(memo)].append(x)
-    except KeyError:
-        # aha, this is the first one :-)
-        memo[id(memo)]=[x]
-
 def _reconstruct(x, memo, func, args,
                  state=None, listiter=None, dictiter=None,
                  *, deepcopy=deepcopy):

Lib/test/test_copy.py

@@ -435,19 +435,22 @@ def test_deepcopy_reflexive_dict(self):
 
     def test_deepcopy_keepalive(self):
         memo = {}
+        memo[id(memo)] = []
         x = []
         y = copy.deepcopy(x, memo)
         self.assertIs(memo[id(memo)][0], x)
 
     def test_deepcopy_dont_memo_immutable(self):
         memo = {}
+        memo[id(memo)] = []
         x = [1, 2, 3, 4]
         y = copy.deepcopy(x, memo)
         self.assertEqual(y, x)
         # There's the entry for the new list, and the keep alive.
         self.assertEqual(len(memo), 2)
 
         memo = {}
+        memo[id(memo)] = []
         x = [(1, 2)]
         y = copy.deepcopy(x, memo)
         self.assertEqual(y, x)

Misc/NEWS.d/next/Library/2025-05-23-21-29-17.gh-issue-123746.dGpBeY.rst

@@ -0,0 +1 @@
+Improve performance :meth:`copy.deepcopy`  setting keep alive key on the memo construction.