def exist_node(self,address):
        if address in self.nodes:
            return True
        return False
Prevents direct instantiation of PolyModel.
dataset/ETHPy150Open UniversalDevicesInc/Polyglot/polyglot/nodeserver_api.py/SimpleNodeServer.exist_node--crossover with--dataset/ETHPy150Open AppScale/appscale/AppServer/google/appengine/ext/db/polymodel.py/PolyModel.__new__