class PubSub:
    def __init__(self):
        self.cbs = {}
    def _check_msg_type(self, msg_type):
        if msg_type not in self.cbs:
            self.cbs[msg_type] = []

    def subscribe(self, msg_type, cb):
        self._check_msg_type(msg_type)
        self.cbs[msg_type].append(cb)

    def publish(self, msg_type, body):
        self._check_msg_type(msg_type)
        print(f"publishing msg {msg_type}")
        for cb in self.cbs[msg_type]:
            cb(body)