Test for assignment to None are easily done in the compiler.

pep-0283.txt

@@ -75,6 +75,8 @@ Completed features for 2.3
     - Warn when an extension type's tp_compare returns anything except
       -1, 0 or 1.  http://www.python.org/sf/472523 
 
+    - Warn for assignment to None (in various forms).
+
 
 Planned features for 2.3
 
@@ -199,17 +201,15 @@ Features unlikely to make it into Python 2.3
       http://www.python.org/sf/558745
       Not much enthusiasm I believe.
 
-    - Making None a keyword.  Can't be done right away, but a warning
-      would be a first step.
-      http://mail.python.org/pipermail/python-dev/2002-April/023600.html
-      Ditto for 'as', which has been a pseudo-keyword long enough.
-
     - PEP 286  Enhanced Argument Tuples                     von Loewis
 
       I haven't had the time to review this thoroughly.  It seems a
       deep optimization hack (also makes better correctness guarantees
       though).
 
+    - Make 'as' a keyword.  It has been a pseudo-keyword long enough.
+      Too much effort to bother.
+
 
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