Index: Python/pythonrun.c
===================================================================
--- Python/pythonrun.c	(revision 73857)
+++ Python/pythonrun.c	(working copy)
@@ -1685,8 +1685,33 @@
 
 	if (exitfunc) {
 		PyObject *res;
+		int should_warn = 1;
+
 		Py_INCREF(exitfunc);
 		PySys_SetObject("exitfunc", (PyObject *)NULL);
+		if (PyFunction_Check(exitfunc)) {
+			PyObject *atexit_str;
+			PyFunctionObject *f = (PyFunctionObject *)exitfunc;
+
+			atexit_str = PyString_FromString("atexit");
+			if (atexit_str) {
+				/* Don't warn when the exit function come from
+				   the atexit module, or when there was an
+				   error. */
+				should_warn = !PyObject_RichCompareBool(
+					atexit_str, f->func_module, Py_EQ);
+				Py_DECREF(atexit_str);
+			}
+			else {
+				PyErr_Clear();
+				should_warn = 0;
+			}
+		}
+		if (should_warn) {
+			PyErr_WarnPy3k("sys.exitfunc is not supported in 3.x; "
+				       "use the atexit module instead.", 1);
+			PyErr_Clear();
+		}
 		res = PyEval_CallObject(exitfunc, (PyObject *)NULL);
 		if (res == NULL) {
 			if (!PyErr_ExceptionMatches(PyExc_SystemExit)) {