This could be done automatically, i.e. whenever you call PyErr_SetString() etc. the currently active exception will be normalized and put into a fourth global variable (exc_cause). In Python code whenever a exception handler raises a new exception the exception that is currently handled could be set as the cause for this exception automatically by the raise statement.
More globals of course defeat the purpose (one of the reasons for putting the traceback in the exception is to get rid of sys.exc_traceback). I also think that doing this automatically whenever PyErr_Set*() is called would cause a lot of spurious tracebacks. (Unless the idea is to skip the chaining if the original exception has an empty traceback; that will almost always be the case for exceptions raised at the C level.) I worry about backwards compatibility. --Guido van Rossum (home page: http://www.python.org/~guido/)