Example of using multiprocessing module for seperating input and output from a main pygame display loop. The code creates a window that displays any key that is typed (escape quits) and writes data to a file called "output.txt" containing some timing data

multiprocessingPygameIO.py

#import used modules
import pygame
import sys
import time
import multiprocessing

#initialize pygame
pygame.init()

#create a window
screen = pygame.display.set_mode((400,400))

#initialize a font
defaultFontName = pygame.font.get_default_font()
feedbackFont = pygame.font.Font(defaultFontName, 10)

#create a multiprocessing Queue object for sending messages to an input process
queueToInputProcess = multiprocessing.Queue()

#create a multiprocessing Queue object for receiving messages from an input process
queueFromInputProcess = multiprocessing.Queue()

#define a function to run continuously in a seperate process that monitors the pygame event queue for keypress events
def inputProcessLoop(queueToInputProcess,queueFromInputProcess):
	done = False
	while not done:
		now = time.time()
		pygame.event.pump()
		for event in pygame.event.get() :
			if event.type == pygame.KEYDOWN :
				response = event.unicode
				response_time = now
				queueFromInputProcess.put([response,response_time])
		if not queueToInputProcess.empty():
			from_queue = queueToInputProcess.get()
			if from_queue == 'quit':
				done = True


#start up the input detector in a separate process
inputProcess = multiprocessing.Process(target=inputProcessLoop, args=(queueToInputProcess,queueFromInputProcess,))
inputProcess.start()


#create a multiprocessing Queue object for sending messages to an output process
queueToOutputProcess = multiprocessing.Queue()

#define a function to run continuously in a seperate process that monitors the output queue for messages and writes data as necessary
def outputProcessLoop(queueToOutputProcess):
	outfile = open('outfile.txt','w')
	done = False
	while not done:
		if not queueToOutputProcess.empty():
			from_queue = queueToOutputProcess.get()
			if from_queue == 'quit':
				outfile.close()
				done = True
			else:
				outfile.write(from_queue+'\n')


#start up the output process in a separate process
outputProcess = multiprocessing.Process(target=outputProcessLoop, args=(queueToOutputProcess,))
outputProcess.start()


#start the diaplay loop
done = False
updateDisplay = False
while not done:
	if not queueFromInputProcess.empty():
		from_queue = queueFromInputProcess.get()
		if from_queue[0]=='\x1b':
			queueToInputProcess.put('quit')
			queueToOutputProcess.put('quit')
			inputProcess.join()
			outputProcess.join()
			pygame.quit()
			sys.exit()
		else:
			toDisplay = from_queue[0]
			updateDisplay = True
	if updateDisplay:
		updateDisplay = False
		thisRender = feedbackFont.render(toDisplay, True, (255,255,255))
		x = screen.get_width()/2-thisRender.get_width()/2
		y = screen.get_height()/2-thisRender.get_height()/2
		screen.fill((0,0,0))
		screen.blit(thisRender,(x,y))
		pygame.display.flip()
		flipLatency = str(time.time()-from_queue[1])
		queueToOutputProcess.put(flipLatency)

I've seen big benefits with forking off writing data to disk when there is a large amount of data to write (eg. continuously polled mouse position). There are likely smaller benefits to forking input detection as pygame (via SDL) already uses an event queue. However, I like that when you put input detection in a separate queue you can immediately assign input a timestamp so that if you are doing time-consuming computations in your main stimulus presentation loop, you may see slightly more accurate timing.

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On Sat, Nov 12, 2011 at 12:32 PM, Sebastiaan Mathot ***@***.*** wrote: Zhiguo forwarded your email discussion to me. This looks very interesting, and it works fine for me on Ubuntu 11.10 (although it crashes when closing). Any idea whether it would significantly improve performance in a real-life setting? (I'm thinking that perhaps SDL already does something similar in its event loop?) --- Reply to this email directly or view it on GitHub: https://gist.github.com/1360283