<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _artifacts(self):
 """Retrieve the artifacts json object"""

        if '_artifacts' not in self._memo:
            json = _get_url(self._artifacts_url).json()
            self._memo['_artifacts'] = json['artifacts']
        return self._memo['_artifacts']