Address PR comments

Author: JDegner0129

publish_subscribe.py

@@ -17,10 +17,7 @@ def notify(self, msg):
 self.msg_queue.append(msg)
 
 defsubscribe(self, msg, subscriber):
-ifmsgnotinself.subscribers:
-self.subscribers[msg] = []
-
-self.subscribers[msg].append(subscriber)
+self.subscribers.setdefault(msg, []).append(subscriber)
 
 defunsubscribe(self, msg, subscriber):
 self.subscribers[msg].remove(subscriber)