Hooking exceptions outside of call stack

[Warren Stringer]

Here is what I would like to do:

#------------------------------------------------------------
a = Tr3()              # implements domain specific language
a.b = 1                # this works, Tr3 overrides __getattr__
a.__dict__['b'] = 2    # just so you know that b is local
a[b] = 3               # I want to resolve locally, but get:

Traceback (most recent call last): ...
    exec cmd in globals, locals ...
NameError: global name 'b' is not defined
#------------------------------------------------------------

So far, I've tried capturing exceptions in __getattr__, __setitem__, and
__setattr__, with no luck. I do NOT want to put `a[b]=3` inside a
try...except block because I want to enable a command line `>>>a[b]=3` 

Is there a way to hook a NameError exception outside of the call stack?
Perhaps, inside Tr3.__init__ ? Is there a way of putting back unhandled
NameError exceptions, so that they unwind the stack normally?

This is intended for production code.

Many thanks!

Warren