[Python-Dev] PyThreadState_SetAsyncExc bug?

[Tim Peters]

[tomer filiba]
> while working on a library for raising exceptions in the context
> of another thread, i've come across a bug in PyThreadState_SetAsyncExc.
> if i raise an instance, sys.exc_info() confuses the exception value for
> the exception type, and the exception value is set None. if i raise the
> type itself, the interpreter creates an instance internally, but then i can't
> pass arguments to the exception.

That appears to be the way it was designed; i.e., AFAICT, it's working
as intended.  This follows from the code in ceval.c that raises the
exception:

				if (tstate->async_exc != NULL) {
					x = tstate->async_exc;
					tstate->async_exc = NULL;
					PyErr_SetNone(x);
					Py_DECREF(x);
					why = WHY_EXCEPTION;
					goto on_error;
				}

PyErr_SetNone(x) there gives no possibility that setting an /instance/
could work as you hope -- `x` has to be an exception type, and
tstate->async_exc is simply the `exc` argument that was passed to
PyThreadState_SetAsyncExc().