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):
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