diff -r aa214fefbdc1 Doc/ACKS.txt
--- a/Doc/ACKS.txt	Sat Jan 28 20:33:52 2012 -0500
+++ b/Doc/ACKS.txt	Tue Jan 31 22:46:32 2012 -0800
@@ -62,6 +62,7 @@ docs@python.org), and we'll be glad to c
    * Stefan Franke
    * Jim Fulton
    * Peter Funk
+   * Ethan Furman
    * Lele Gaifax
    * Matthew Gallagher
    * Gabriel Genellina
diff -r aa214fefbdc1 Doc/library/exceptions.rst
--- a/Doc/library/exceptions.rst	Sat Jan 28 20:33:52 2012 -0500
+++ b/Doc/library/exceptions.rst	Tue Jan 31 22:46:32 2012 -0800
@@ -34,6 +34,23 @@ programmers are encouraged to at least d
 defining exceptions is available in the Python Tutorial under
 :ref:`tut-userexceptions`.
 
+When raising (or re-raising) an exception in an :keyword:`except` clause
+:attr:`__context__` is automatically set to the last exception caught; if the
+new exception is not handled the traceback that is eventually displayed will
+include the originating exception(s) and the final exception.  
+
+This implicit exception chain can be made explicit by using :keyword:`from`
+with :keyword:`raise`.  The single argument to :keyword:`from` must be an
+exception or :keyword:`None`, and it will bet set as :attr:`__cause__` on the
+raised exception.  If :attr:`__cause__` is an exception it will be displayed
+instead of :attr:`__context__`; if :attr:`__cause__` is None, 
+:attr:`__context__` will not be displayed by the default exception handling
+code.
+
+In either case, the default exception handling code will not display
+any of the remaining links in the :attr:`__context__` chain if
+:attr:`__cause__` has been set.
+
 
 Base classes
 ------------
diff -r aa214fefbdc1 Lib/test/test_exceptions.py
--- a/Lib/test/test_exceptions.py	Sat Jan 28 20:33:52 2012 -0500
+++ b/Lib/test/test_exceptions.py	Tue Jan 31 22:46:32 2012 -0800
@@ -388,18 +388,18 @@ class ExceptionTests(unittest.TestCase):
     def testChainingAttrs(self):
         e = Exception()
         self.assertEqual(e.__context__, None)
-        self.assertEqual(e.__cause__, None)
+        self.assertEqual(e.__cause__, False)
 
         e = TypeError()
         self.assertEqual(e.__context__, None)
-        self.assertEqual(e.__cause__, None)
+        self.assertEqual(e.__cause__, False)
 
         class MyException(EnvironmentError):
             pass
 
         e = MyException()
         self.assertEqual(e.__context__, None)
-        self.assertEqual(e.__cause__, None)
+        self.assertEqual(e.__cause__, False)
 
     def testKeywordArgs(self):
         # test that builtin exception don't take keyword args,
diff -r aa214fefbdc1 Lib/test/test_raise.py
--- a/Lib/test/test_raise.py	Sat Jan 28 20:33:52 2012 -0500
+++ b/Lib/test/test_raise.py	Tue Jan 31 22:46:32 2012 -0800
@@ -77,6 +77,16 @@ class TestRaise(unittest.TestCase):
                 nested_reraise()
         self.assertRaises(TypeError, reraise)
 
+    def test_raise_from_None(self):
+        try:
+            try:
+                raise TypeError("foo")
+            except:
+                raise ValueError() from None
+        except ValueError as e:
+            self.assertTrue(isinstance(e.__context__, TypeError))
+            self.assertIsNone(e.__cause__)
+
     def test_with_reraise1(self):
         def reraise():
             try:
diff -r aa214fefbdc1 Lib/traceback.py
--- a/Lib/traceback.py	Sat Jan 28 20:33:52 2012 -0500
+++ b/Lib/traceback.py	Tue Jan 31 22:46:32 2012 -0800
@@ -120,8 +120,8 @@ def _iter_chain(exc, custom_tb=None, see
     seen.add(exc)
     its = []
     cause = exc.__cause__
-    if cause is not None and cause not in seen:
-        its.append(_iter_chain(cause, None, seen))
+    if cause is not False and cause is not None and cause not in seen:
+        its.append(_iter_chain(cause, False, seen))
         its.append([(_cause_message, None)])
     else:
         context = exc.__context__
diff -r aa214fefbdc1 Objects/exceptions.c
--- a/Objects/exceptions.c	Sat Jan 28 20:33:52 2012 -0500
+++ b/Objects/exceptions.c	Tue Jan 31 22:46:32 2012 -0800
@@ -293,7 +293,7 @@ static PyObject *
 BaseException_get_cause(PyObject *self) {
     PyObject *res = PyException_GetCause(self);
     if (res) return res;  /* new reference already returned above */
-    Py_RETURN_NONE;
+    Py_RETURN_FALSE;
 }
 
 static int
diff -r aa214fefbdc1 Python/ceval.c
--- a/Python/ceval.c	Sat Jan 28 20:33:52 2012 -0500
+++ b/Python/ceval.c	Tue Jan 31 22:46:32 2012 -0800
@@ -3576,6 +3576,11 @@ do_raise(PyObject *exc, PyObject *cause)
         else if (PyExceptionInstance_Check(cause)) {
             fixed_cause = cause;
         }
+        else if (cause == Py_None) {
+            fixed_cause = Py_None;
+            Py_INCREF(Py_None);
+            Py_DECREF(cause);
+        }
         else {
             PyErr_SetString(PyExc_TypeError,
                             "exception causes must derive from "
diff -r aa214fefbdc1 Python/pythonrun.c
--- a/Python/pythonrun.c	Sat Jan 28 20:33:52 2012 -0500
+++ b/Python/pythonrun.c	Tue Jan 31 22:46:32 2012 -0800
@@ -1691,7 +1691,11 @@ print_exception_recursive(PyObject *f, P
         else if (PyExceptionInstance_Check(value)) {
             cause = PyException_GetCause(value);
             context = PyException_GetContext(value);
-            if (cause) {
+            if (cause && cause == Py_None) {
+                /* print neither cause nor context */
+                ;
+            }
+            else if (cause) {
                 res = PySet_Contains(seen, cause);
                 if (res == -1)
                     PyErr_Clear();