Added thread mutex to client.send_frame() because Server and Client may call it simultaneously

server.py

 import socket
 import logging
 from traceback import format_exc
-from threading import Thread
+from threading import Thread, Lock
 
 from websocket import WebSocket
 from exceptions import InvalidRequest
@@ -85,6 +85,11 @@ class Client(WebSocket):
         self.address = address
         self.send_lock = Lock()
 
+    def send_frame(self, frame):
+        self.send_lock.acquire()
+        WebSocket.send_frame(self, frame)
+        self.send_lock.release()
+
     def onopen(self):
         self.server.onopen(self)