If you want to trigger the standard signal handling logic, then raise(SIGINT) is also an option. On unix it probably won't help because of issue 21895, but using raise() here + fixing 21895 by using pthread_kill in the c level signal handler would together give a pretty simple and robust way to pretend that the user hit control-C.

But... interrupt_main isn't documented to raise a SIGINT, it's documented to raise KeyboardInterrupt. These are very different, e.g. if the user has installed a custom handler or even set SIGINT to SIG_IGN or masked it. (In the later two cases, using pthread_kill to send a signal to the main thread *won't* interrupt any ongoing syscall.) This is *much* more difficult to make work correctly. And actually what IDLE wants is a fake control-C anyway!

So I'd suggest either redefining the semantics of interrupt_main to be "acts as if SIGINT was received" or else abandoning interrupt_main to its slightly-buggy semantics and adding a new function that does raise(SIGINT) and switching IDLE to use this new function. And fix issue 21895 in either case.