code: def passwd_keys(self, username):
          
          keys = []
          if username == 'mysql':
              log("Bad username '%s'" % (username), level=WARNING)
  
          if username:
              # IMPORTANT: *newer* format must be returned first
              keys.append('mysql-%s.passwd' % (username))
              keys.append('%s.passwd' % (username))
          else:
              keys.append('mysql.passwd')
  
          for key in keys:
              yield key
docs: Generator to return keys used to store passwords in peer store.
  
          NOTE: we support both legacy and new format to support mysql
          charm prior to refactor. This is necessary to avoid LP 1451890.