code: def call(self, jsondata):
          
          result = yield self.call_py(jsondata)
          if result is None:
              defer.returnValue(None)
          else:
              defer.returnValue(json.dumps(result))
docs: 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