[Python-checkins] cpython: don't memoize objects that are their own copies (closes #12422)

[benjamin.peterson]

http://hg.python.org/cpython/rev/e24ad85e9608
changeset:   71018:e24ad85e9608
user:        Benjamin Peterson <benjamin at python.org>
date:        Mon Jun 27 16:22:46 2011 -0500
summary:
  don't memoize objects that are their own copies (closes #12422)

Patch mostly by Alex Gaynor.

files:
  Lib/copy.py           |  12 +++++++-----
  Lib/test/test_copy.py |  19 +++++++++++++++++--
  Misc/NEWS             |   3 +++
  3 files changed, 27 insertions(+), 7 deletions(-)


diff --git a/Lib/copy.py b/Lib/copy.py
--- a/Lib/copy.py
+++ b/Lib/copy.py
@@ -173,8 +173,10 @@
                                 "un(deep)copyable object of type %s" % cls)
                 y = _reconstruct(x, rv, 1, memo)
 
-    memo[d] = y
-    _keep_alive(x, memo) # Make sure x lives at least as long as d
+    # 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
 
 _deepcopy_dispatch = d = {}
@@ -214,9 +216,10 @@
     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 @@
             break
     else:
         y = x
-    memo[d] = y
     return y
 d[tuple] = _deepcopy_tuple
 
diff --git a/Lib/test/test_copy.py b/Lib/test/test_copy.py
--- a/Lib/test/test_copy.py
+++ b/Lib/test/test_copy.py
@@ -321,9 +321,24 @@
 
     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:
diff --git a/Misc/NEWS b/Misc/NEWS
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -200,6 +200,9 @@
 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

-- 
Repository URL: http://hg.python.org/cpython