A couple of examples about how to attack the fact that _PyType_Lookup()

returns a borrowed ref.  Many of the calls are open to attack.

Lib/test/crashers/borrowed_ref_1.py

@@ -0,0 +1,29 @@
+"""
+_PyType_Lookup() returns a borrowed reference.
+This attacks the call in dictobject.c.
+"""
+
+class A(object):
+    pass
+
+class B(object):
+    def __del__(self):
+        print 'hi'
+        del D.__missing__
+
+class D(dict):
+    class __missing__:
+        def __init__(self, *args):
+            pass
+
+
+d = D()
+a = A()
+a.cycle = a
+a.other = B()
+del a
+
+prev = None
+while 1:
+    d[5]
+    prev = (prev,)