who is at fault when pulling into a parking spot and other car opens door

Its hard to tell who is at fault from your description.  If someone is already parked, and opened their door, an argument could be made that you should have seen that and stopped in time.  An argument can also be made that they were careless in opening their door.  It would seem to me that if the other person flung the door open as you were pulling into the spot that's the same as opening it into traffic if you're parked on the street and they should have been more careful.