I've subclassed a Python dict, roughly like this: class JSONDict(dict): @property def __json__(self): return json.dumps(self) @property def __path__(self): return os.path.join(settings.BASE_PATH, self.entity, 'settings.json') def __init__(self, entity, *args, **kwargs): dict.__init__(self) self.entity = entity try: os.utime(self.__path__, None) except OSError: open(self.__path__, 'a').close() self.__file__ = open(self.__path__, 'r+') self.__to_python__() def __enter__(self): return self def __exit__(self, exc_type, exc_value, traceback): self.__file__.close() def __to_json__(self): self.__file__.seek(0) self.__file__.write(self.__json__) self.__file__.flush() self.__file__.truncate() def __to_python__(self): if os.path.getsize(self.__path__): self.__file__.seek(0) dict.clear(self) dict.update(self, json.load(self.__file__)) else: self.clear() def __getitem__(self, key): self.__to_python__() return dict.__getitem__(self, key) def __setitem__(self, key, value): dict.__setitem__(self, key, value) self.__to_json__() def update(self, data): dict.update(self, data) self.__to_json__() def clear(self): dict.clear(self) self.update(__default__) self['nar'] = self.nar I can use this quite fine, e.g.: >>> with JSONDict('entity-id') as json_info: ... json_info['setting_a'] = 'foo' However, there's an interesting problem when it comes to lists. When I modify the list directly, e.g json_info['setting_b'].append('bar'), the JSON file doesn't get updated. I was certain that overriding __setitem__ and __getitem__ would cover all item access, but clearly that's not the case. What should I override to get this work? UPDATE: Having read the first answers, I felt that this question wasn't formulated in a right way. The right question would be, how to effectively observe dict attribute changes in order to be able to update the JSON accordingly? Should it be done via proxy/descriptor objects, metaclasses, or is there a more generic way? Continue reading...
