How to update a global variable between threads of ThreadingTCPServer?

Question

I am trying to write a simple REST API client-server model. On the server side, I have

transaction = 0

    class MyHandler(http.server.BaseHTTPRequestHandler):
        global transaction     
   
        def __init__(self, *args, **kwargs):
            ....
        def update(self):
            # do something
            with self.lock:
                transaction = transaction + 1

if __init__ == '__main__':
...
    with socketserver.ThreadingTCPServer(('', PORT), MyHandler) as server:
...

This is a part of my code. When multiple client requests come in (for example, GET/POST), multiple threads should be created to handle requests. I need to update the transaction for each request, but such attempt cause failure. If I want to print it, also failed.

Also, if transaction has such issue, I think I also need to move lock outside of MyHandler.

I don't understand why I can't access and update global variable. I did it with my multithreading socket server. Does it have relation with http.server.BaseHTTPRequestHandler?