Example of unexpected results with mutable hashable objects

hashable_mutable.py

class MyDict(dict):
    def __hash__(self):
        return hash(frozenset(self))


key = MyDict(a=1)
mapping = {key: 'foo'}
key['b'] = 2
mapping[key]  # KeyError: {'a': 1, 'b': 2}
# Do you expect that variable which you have added to dict would raise KeyError?
# And this may happen in another function, if you have passed key somewhere else in between