test_dict_subclass_get_method.py

# Unlike MutableMapping, dict has it's own implementation of get() that doesn't use __getitem__ underneath. This will cause your subclass to return None unexpectedly, unless you don't implement get() as well. Probably better to just subclass MutableMapping :-).

# example code:
def test_dict_subclass_get_method():
    import collections
    # WORKS: class Hello(collections.MutableMapping):
    class Hello(dict):
        def __init__(self, underlying):
            super(Hello, self).__init__()
            self._underlying = underlying

        def __len__(self):
            return len(self._underlying)

        def __iter__(self):
            return self._underlying.__iter__()

        # def get(self, key, default):
        #     return self._underlying.get(key, default)

        def __getitem__(self, key):
            return self._underlying[key]

        def __setitem__(self, key, value):
            self._underlying[key] = value

        def __delitem__(self, key):
            del self._underlying[key]

    underlying = {}
    hello = Hello(underlying)

    hello['world'] = 'foo'
    assert hello['world'] == 'foo'
    assert hello.get('world') == 'foo'