<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def list_permissions(self, file_id):
 """Retrieve a list of permissions for a file. :param file_id: a spreadsheet ID (aka file ID.) :type file_id: str """

        url = '{0}/{1}/permissions'.format(DRIVE_FILES_API_V2_URL, file_id)

        r = self.request('get', url)

        return r.json()['items']