diff -r 52f68c95e025 Doc/ACKS.txt
--- a/Doc/ACKS.txt	Thu Jan 26 08:47:27 2012 -0600
+++ b/Doc/ACKS.txt	Thu Feb 02 13:58:45 2012 -0800
@@ -62,6 +62,7 @@
    * Stefan Franke
    * Jim Fulton
    * Peter Funk
+   * Ethan Furman
    * Lele Gaifax
    * Matthew Gallagher
    * Gabriel Genellina
diff -r 52f68c95e025 Doc/c-api/exceptions.rst
--- a/Doc/c-api/exceptions.rst	Thu Jan 26 08:47:27 2012 -0600
+++ b/Doc/c-api/exceptions.rst	Thu Feb 02 13:58:45 2012 -0800
@@ -421,17 +421,24 @@
 
 .. c:function:: PyObject* PyException_GetCause(PyObject *ex)
 
-   Return the cause (another exception instance set by ``raise ... from ...``)
-   associated with the exception as a new reference, as accessible from Python
-   through :attr:`__cause__`.  If there is no cause associated, this returns
-   *NULL*.
+   Return the cause (either an exception instance, or :keyword:`None`,
+   set by ``raise ... from ...``) associated with the exception as a new
+   reference, as accessible from Python through :attr:`__cause__`.  
+   
+   If there is no cause associated, this returns *NULL* (from Python
+   ``__cause__ is Ellipsis``).  If the cause is :keyword:`None`, the default
+   exception display routines stop showing the context chain.
 
 
 .. c:function:: void PyException_SetCause(PyObject *ex, PyObject *ctx)
 
    Set the cause associated with the exception to *ctx*.  Use *NULL* to clear
-   it.  There is no type check to make sure that *ctx* is an exception instance.
-   This steals a reference to *ctx*.
+   it.  There is no type check to make sure that *ctx* is either an exception
+   instance or :keyword:`None`.  This steals a reference to *ctx*.
+
+   If the cause is set to :keyword:`None` the default exception display
+   routines will not display this exception's context, and will not follow the
+   chain any further.
 
 
 .. _unicodeexceptions:
diff -r 52f68c95e025 Doc/library/exceptions.rst
--- a/Doc/library/exceptions.rst	Thu Jan 26 08:47:27 2012 -0600
+++ b/Doc/library/exceptions.rst	Thu Feb 02 13:58:45 2012 -0800
@@ -34,6 +34,24 @@
 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.  (Note:  the default value for :attr:`__context__` is :keyword:`None`,
+while the default value for :attr:`__cause__` is :class:`Ellipsis`.)
+
+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 52f68c95e025 Lib/test/test_exceptions.py
--- a/Lib/test/test_exceptions.py	Thu Jan 26 08:47:27 2012 -0600
+++ b/Lib/test/test_exceptions.py	Thu Feb 02 13:58:45 2012 -0800
@@ -387,19 +387,35 @@
 
     def testChainingAttrs(self):
         e = Exception()
-        self.assertEqual(e.__context__, None)
-        self.assertEqual(e.__cause__, None)
+        self.assertIsNone(e.__context__)
+        self.assertIs(e.__cause__, Ellipsis)
 
         e = TypeError()
-        self.assertEqual(e.__context__, None)
-        self.assertEqual(e.__cause__, None)
+        self.assertIsNone(e.__context__)
+        self.assertIs(e.__cause__, Ellipsis)
 
         class MyException(EnvironmentError):
             pass
 
         e = MyException()
-        self.assertEqual(e.__context__, None)
-        self.assertEqual(e.__cause__, None)
+        self.assertIsNone(e.__context__)
+        self.assertIs(e.__cause__, Ellipsis)
+
+        try:
+            raise Exception()
+        except Exception as exc:
+            e = exc
+
+        self.assertIsNone(e.__context__)
+        self.assertIs(e.__cause__, Ellipsis)
+
+        e.__context__ = NameError()
+        e.__cause__ = None
+        self.assertIsInstance(e.__context__, NameError)
+        self.assertIsNone(e.__cause__)
+
+        e.__cause__ = Ellipsis
+        self.assertIs(e.__cause__, Ellipsis)
 
     def testKeywordArgs(self):
         # test that builtin exception don't take keyword args,
diff -r 52f68c95e025 Lib/test/test_raise.py
--- a/Lib/test/test_raise.py	Thu Jan 26 08:47:27 2012 -0600
+++ b/Lib/test/test_raise.py	Thu Feb 02 13:58:45 2012 -0800
@@ -77,6 +77,16 @@
                 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 52f68c95e025 Lib/traceback.py
--- a/Lib/traceback.py	Thu Jan 26 08:47:27 2012 -0600
+++ b/Lib/traceback.py	Thu Feb 02 13:58:45 2012 -0800
@@ -120,8 +120,8 @@
     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 not in (Ellipsis, None) and cause not in seen:
+        its.append(_iter_chain(cause, False, seen))
         its.append([(_cause_message, None)])
     else:
         context = exc.__context__
diff -r 52f68c95e025 Objects/exceptions.c
--- a/Objects/exceptions.c	Thu Jan 26 08:47:27 2012 -0600
+++ b/Objects/exceptions.c	Thu Feb 02 13:58:45 2012 -0800
@@ -293,7 +293,8 @@
 BaseException_get_cause(PyObject *self) {
     PyObject *res = PyException_GetCause(self);
     if (res) return res;  /* new reference already returned above */
-    Py_RETURN_NONE;
+    Py_INCREF(Py_Ellipsis);
+    return Py_Ellipsis;
 }
 
 static int
@@ -301,8 +302,11 @@
     if (arg == NULL) {
         PyErr_SetString(PyExc_TypeError, "__cause__ may not be deleted");
         return -1;
+    } else if (arg == Py_Ellipsis) {
+        arg = NULL;
     } else if (arg == Py_None) {
-        arg = NULL;
+        /* let __cause__ be set to None */
+        ;
     } else if (!PyExceptionInstance_Check(arg)) {
         PyErr_SetString(PyExc_TypeError, "exception cause must be None "
                         "or derive from BaseException");
diff -r 52f68c95e025 Python/ceval.c
--- a/Python/ceval.c	Thu Jan 26 08:47:27 2012 -0600
+++ b/Python/ceval.c	Thu Feb 02 13:58:45 2012 -0800
@@ -3576,6 +3576,11 @@
         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 52f68c95e025 Python/pythonrun.c
--- a/Python/pythonrun.c	Thu Jan 26 08:47:27 2012 -0600
+++ b/Python/pythonrun.c	Thu Feb 02 13:58:45 2012 -0800
@@ -1691,7 +1691,11 @@
         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();