Index: Objects/obmalloc.c
===================================================================
--- Objects/obmalloc.c	(Revision 66572)
+++ Objects/obmalloc.c	(Arbeitskopie)
@@ -677,8 +677,8 @@
 /* This is only useful when running memory debuggers such as
  * Purify or Valgrind.  Uncomment to use.
  *
+#define Py_USING_MEMORY_DEBUGGER
  */
-#define Py_USING_MEMORY_DEBUGGER
 
 #ifdef Py_USING_MEMORY_DEBUGGER
 
Index: Misc/NEWS
===================================================================
--- Misc/NEWS	(Revision 66572)
+++ Misc/NEWS	(Arbeitskopie)
@@ -12,6 +12,8 @@
 Core and Builtins
 -----------------
 
+- Bug #3951: Py_USING_MEMORY_DEBUGGER should not be enabled by default.
+
 Library
 -------