use ellipsis as sentinel for deepcopy memo

Author: eendebakpt

Lib/copy.py

@@ -120,8 +120,7 @@ def deepcopy(x, memo=None):
 
     d = id(x)
     if memo is None:
-        memo = {}
-        memo[id(memo)] = []
+        memo = {...: []}
     else:
         y = memo.get(d, None)
         if y is not None:
@@ -160,7 +159,7 @@ def deepcopy(x, memo=None):
     # If is its own copy, don't memoize.
     if y is not x:
         memo[d] = y
-        memo[id(memo)].append(x) # Make sure x lives at least as long as d
+        memo[...].append(x) # Make sure x lives at least as long as d
     return y
 
 _atomic_types = frozenset({types.NoneType, types.EllipsisType, types.NotImplementedType,

Lib/test/test_copy.py

@@ -435,22 +435,22 @@ def test_deepcopy_reflexive_dict(self):
 
     def test_deepcopy_keepalive(self):
         memo = {}
-        memo[id(memo)] = []
+        memo[...] = []
         x = []
         y = copy.deepcopy(x, memo)
-        self.assertIs(memo[id(memo)][0], x)
+        self.assertIs(memo[...][0], x)
 
     def test_deepcopy_dont_memo_immutable(self):
         memo = {}
-        memo[id(memo)] = []
+        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)] = []
+        memo[...] = []
         x = [(1, 2)]
         y = copy.deepcopy(x, memo)
         self.assertEqual(y, x)