docs: def call(self, jsondata):
          """
          Calls jsonrpc service's method and returns its return value in a JSON
          string or None if there is none.
  
          Arguments:
          jsondata -- remote method call in jsonrpc format
          """
          result = yield self.call_py(jsondata)
          if result is None:
              defer.returnValue(None)
          else:
              defer.returnValue(json.dumps(result))