Problem with switch_thrift_API.py file

[zrabbbit]

I'm trying to run a project using p4-utils. But when I call SimpleSwitchThriftAPI() in the connect_to_switches() function, I find that I always get an exception error at the load_json_config() function in the thrift_API.py. The connect_to_switches() function like this:
def connect_to_switches(self):
for p4switch in self.topo.get_p4switches():
thrift_port = self.topo.get_thrift_port(p4switch)
self.controllers[p4switch] = SimpleSwitchThriftAPI(thrift_port)