io_wait.py

class ReadWait(SystemCall):
    def __init__(self, file):
        self.file = file
        
    def handle(self):
        fd = self.file.fileno()
        self.sched.waitforread(self.task, fd)

class Scheduler(object):        

    # Add task and file descriptor to the list
    # to be checked on polling
    def waitforread(self, task, fd):
       self.read_waiting[fd] = task
    
    # select.select is an OS API to determine
    # when a file descriptor is "ready" 
    def iopoll(self,timeout):
        if self.read_waiting or self.write_waiting:
           read_fds, write_fds, _x = select.select(
             self.read_waiting,
             # ...
           )
           for fd in read_fds: self.schedule(self.read_waiting.pop(fd))
           # ...
   
    # Models polling as a regular task - whenever
    # it gets run by the scheduler it counts as a 
    # poll
    def iotask(self):
        while True:
            if self.ready.empty():
                self.iopoll(None)
            else:
                self.iopoll(0)
            yield