- When `colander.null` was unpickled, the reference created during

unpickling was *not* a reference to the singleton but rather a new instance
  of the ``colander._null`` class.  This was unintentional, because lots of
  code checks for ``if x is colander.null``, which will fail across pickling
  and unpickling.  Now the reference created when ``colander.null`` is
  pickled is unpickled as the singleton itself.

CHANGES.txt

@@ -4,6 +4,12 @@ Changes
 Next release
 ------------
 
+- When ``colander.null`` was unpickled, the reference created during
+  unpickling was *not* a reference to the singleton but rather a new instance
+  of the ``colander._null`` class.  This was unintentional, because lots of
+  code checks for ``if x is colander.null``, which will fail across pickling
+  and unpickling.  Now the reference created when ``colander.null`` is
+  pickled is unpickled as the singleton itself.
 
 0.9  (2010-11-28)
 -----------------

colander/__init__.py

@@ -18,6 +18,9 @@ class _null(object):
     def __repr__(self):
         return '<colander.null>'
 
+    def __reduce__(self):
+        return 'null' # when unpickled, refers to "null" below (singleton)
+
 null = _null()
 
 def interpolate(msgs):

colander/tests.py

@@ -1854,6 +1854,11 @@ class Test_null(unittest.TestCase):
         from colander import null
         self.assertEqual(repr(null), '<colander.null>')
 
+    def test_pickling(self):
+        from colander import null
+        import cPickle
+        self.failUnless(cPickle.loads(cPickle.dumps(null)) is null)
+
 class Dummy(object):
     pass