don't memoize objects that are their own copies (closes #12422)

Patch mostly by Alex Gaynor.

Lib/copy.py

@@ -173,6 +173,8 @@ def deepcopy(x, memo=None, _nil=[]):
                                 "un(deep)copyable object of type %s" % cls)
                 y = _reconstruct(x, rv, 1, memo)
 
+    # 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
     return y
@@ -214,9 +216,10 @@ def _deepcopy_tuple(x, memo):
     y = []
     for a in x:
         y.append(deepcopy(a, memo))
-    d = id(x)
+    # We're not going to put the tuple in the memo, but it's still important we
+    # check for it, in case the tuple contains recursive mutable structures.
     try:
-        return memo[d]
+        return memo[id(x)]
     except KeyError:
         pass
     for i in range(len(x)):
@@ -225,7 +228,6 @@ def _deepcopy_tuple(x, memo):
             break
     else:
         y = x
-    memo[d] = y
     return y
 d[tuple] = _deepcopy_tuple
 

Lib/test/test_copy.py

@@ -321,9 +321,24 @@ class TestCopy(unittest.TestCase):
 
     def test_deepcopy_keepalive(self):
         memo = {}
-        x = 42
+        x = []
         y = copy.deepcopy(x, memo)
-        self.assertTrue(memo[id(x)] is x)
+        self.assertIs(memo[id(memo)][0], x)
+
+    def test_deepcopy_dont_memo_immutable(self):
+        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 = {}
+        x = [(1, 2)]
+        y = copy.deepcopy(x, memo)
+        self.assertEqual(y, x)
+        # Tuples with immutable contents are immutable for deepcopy.
+        self.assertEqual(len(memo), 2)
 
     def test_deepcopy_inst_vanilla(self):
         class C:

Misc/NEWS

@@ -200,6 +200,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #12422: In the copy module, don't store objects that are their own copy
+  in the memo dict.
+
 - Issue #12303: Add sigwaitinfo() and sigtimedwait() to the signal module.
 
 - Issue #12404: Remove C89 incompatible code from mmap module. Patch by Akira