> traceback.print_last() depends on the existence of sys.last_type
Yes, that is exactly as documented:

>>> help(traceback.print_last)
Help on function print_last in module traceback:
print_last(limit=None, file=None)
    This is a shorthand for 'print_exception(sys.last_type,
    sys.last_value, sys.last_traceback, limit, file)'.

Furthermore, in the doc:
http://docs.python.org/dev/library/sys.html#sys.last_type
    "... they are set when an exception is not handled ..."
In your script, you *are* handling the exception. 
Your script really should use traceback.print_exc() instead.
BTW, your patch basically makes print_last() identical to print_exc()...