The sleep function is not very precise, this basically checks how bad it is on your system.

sleep_check.py

import time

from datetime import datetime

def check_sleep(amount):
    start = datetime.now()
    time.sleep(amount)
    end = datetime.now()
    delta = end-start
    return delta.seconds + delta.microseconds/1000000.

if __name__ == '__main__':
	error = sum(abs(check_sleep(0.050)-0.050) for i in xrange(100))*10
	print "Average error is %0.3fms" % error