Frame-by-frame processing with openMSX (prototype)

#!/usr/bin/env python

from subprocess import Popen, PIPE, STDOUT
import threading
import time
import os

class MSXClient:

    def __init__(self, rom_filename, machine='C-BIOS_MSX1'):
        # open emulator as a pipe
        self.pipe = Popen(['openmsx', '-control', 'stdio', '-machine', machine], stdout=PIPE, stdin=PIPE, stderr=STDOUT)
        # run stdout listener thread
        self.finished = False
        th = threading.Thread(target=self.stdout_listener)
        th.start()
        # send some init commands
        self.command("cart %s" % rom_filename)
        self.command("set power on")
        self.command("set renderer SDL")
        self.command("set scale_factor 1")
        self.command("set pause on")
        self.command("set throttle off")
        self.command("reverse stop")
        self.frame_num = 0
        self.afters = {}

    def command(self, c, need_print=True):
        """
        Send a command to openMSX's stdin, print if needed.

        """
        cmd = "<command>%s</command>\n" % c
        self.pipe.stdin.write(cmd)
        if need_print:
            print cmd[:-1]

    def readline(self, need_print=False):
        """
        Read a line from openMSX's stdout, print if needed.
        Also, detect if frame is complete (screen saved).

        """
        out = self.pipe.stdout.readline().strip()
        if out.find("Screen saved") >= 0 or out.find("frameisready") >= 0:
            self.paused = True
        if out.find("after#") >= 0:
            after = out.split(">")[1].split("<")[0]
            self.afters[after] = True
        if need_print:
            print out
        return out

    def reset(self):
        """
        Shortcut to 'reset' command.

        """
        self.command("reset")

    def shutdown(self):
        """
        Send 'exit' command to openMSX and wait until subprocess if finished.

        """
        self.command("exit")
        self.finished = True
        self.pipe.communicate()

    def stdout_listener(self):
        while not self.finished:
            self.readline()

    def process_frame(self):
        """
        A frame is in ./screen.png now, process it here.

        """
        pass

    def step(self):
        """
        Wait for the next frame and process it.

        """
        self.paused = False
        self.command("after frame \"set pause on; screenshot -raw ./screen.png\"")
        #self.command("after frame \"set pause on; message frameisready\"")
        self.command("set pause off")
        while not self.paused:
            time.sleep(0.001)
        for after in self.afters.keys():
            self.command("after cancel %s" % after)
            del self.afters[after]
        self.process_frame()

if __name__ == "__main__":
    client = MSXClient("./roms/area5.rom")
    for i in xrange(10000):
        client.step()
    client.shutdown()