> Using handle_accept() in my code and remembering to call listen() in my
> asyncore.dispatcher server's constructor did the trick.
>
> I am not sure if we still have a bug here though, since if the subclass
> doesn't define  a proper handle_accept() we get into the select() loop and
> 100% CPU utilization after the initial client connection.

No, it's not a bug.
The attached test case was for Python 3: Python 2 doesn't have
handle_accepted(), and since the default implementation of
handle_accept() doesn't nothing, the handler is called in a loop,
because the socket is effectively always ready for accept.