<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def json(self):
 """Returns de-JSONed data or None if it's a different content type."""

        if self._data_json:
            return self._data_json

        if not self.data or self.content_type != 'application/json':
            return None

        self._data_json = json.loads(self.data)
        return self._data_json