Not sure I understand when you say turn off the alarm.
If everything is working well then you will not be able to lock the car with the key in the ignition.
If it was possible to lock one or the other in the car you would have in your hand the one you used to lock the car with. You could then use it to open the car.
If the car automatically relocked because you did not open the door it would not be possible to have locked the key in the car because you needed it outside the car to lock it, and opening the door to put the key in the car would cancel the auto locking feature, you could throw the key through an open window of the locked car but that would be just plain dumb.