gistfile1.py

class PersistentDict(object):
    def __init__(self, store_file):
        self._store_file = store_file
        self._is_modified = False
        self.reload()

    def reload(self):
        if self._is_modified:
            raise Exception('data was modified and not persisted')

        with open(self._store_file, 'r') as fp:
            self._data = json.loads(fp.read())

    def persist(self):
        with open(self._store_file, 'w') as fp:
            fp.write(json.dumps(self._data))

        self._is_modified = False

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

    def __setitem__(self, key, value):
        self._is_modified = True
        self._data[key] = value

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