i found 2 relative bugs in asyncore.dispatcher_with_send class:

one is in the asyncore.dispatcher_with_send.writable():
    def writable(self):
        return (not self.connected) or len(self.out_buffer)
why is a not connected connection writable? i think this is definitely a bug
my fix:
    def writable(self):
        return self.connected and len(self.out_buffer)

another bug is more obscure, i'm not sure is it a bug or something should be handled by user(programmer)

the bug is also in asyncore.dispatcher_with_send class, and maybe also in asyncore.dispatcher class. asyncore.dispatcher uses unblocking socket to handle network missions, when we use the connect() method of dispatcher to establish the socket, it will call socket.connect_ex() method to create the connection, however, socket.connect_ex() may return 10035(EWOULDBLOCK) as it is an unblocking socket indicates that the connection creating is not finished yet, if we call dispatcher.connect() immediately after .connect(), socket error 10057 may be raised, indicating that the socket is not established yet, then the asyncore main loop catches this exception, and calls handle_error(in my case i close the connection in handle_error so the connection which would be established with no problem breaks), i think there should be a connection state check in asyncore.dispatcher.send(), or at least in asyncore.dispatcher_with_send.send.

my fix for asyncore.dispatcher_with_send.send():
    def send(self, data):
        if self.debug:
            self.log_info('sending %s' % repr(data))
        self.out_buffer = self.out_buffer + data
        if self.connected:        # do not call send() if connection is
            self.initiate_send()  # not established yet, just put data 
                                  # in buffer

for the second bug, to reproduce it, just create a unblocking socket to a remote, long delay port with socket.connect_ex and call send immediately