If it helps, notice that the SetEvent(sigint_event) call used to wake up the main thread on windows is also performed unconditionally and after the call to Py_AddPendingEvent. From the point of view of twisted/tornado/trio, this is exactly the same as the write to the wakeup fd -- the only reason we use the wakeup fd instead of the sigint_event is that it's more convenient to wait on an fd than on an event object.