The problem is that the other thread is blocked, waiting for new client connections. A common pattern for very simple server applications (like you write during exercises at universities) is:

2 threads:
 1. A server thread waiting for clients, maybe spawning even more threads for each connection.
 2. A small shell thread reading commands from stdin. When "quit" is read it closes the server socket, which shuts down the server (the server thread just notices that the socket was closed and shuts down properly).

This pattern we used all the time when writing exercises in Java. However, it seems to work quite differently in Python 3. I have to check if this problem still exists.