In short, they will not behave what you think they will. you think dict.get in implemented in python, like:
def get(self, key, default=None): try: return self[key] except KeyError: return default
However, they dict.get do not call getitem, and they are implemented in c separately, and do not call each other.
so, when you implement geitem, get is not automatically altered! which is very dangerous.
What you need do, subclass collections.abc.Mapping