Recipe9.8.Multitasking Cooperatively Without Threads

Recipe 9.8. Multitasking Cooperatively Without Threads

Credit: Brian Bush, Troy Melhase, David Beach, Martin Miller

Problem

You have a task that seems suited to multithreading, but you don't want to incur the overhead that real thread-switching would impose.

Solution

Generators were designed to simplify iteration, but they're also quite suitable as a basis for cooperative multitasking, also known as microthreading :

import signal
# credit: original idea was based on an article by David Mertz
# http://gnosis.cx/publish/programming/charming_python_b7.txt
# some example 'microthread' generators
def empty(name):
    """ This is an empty task for demonstration purposes. """
    while True:
        print "<empty process>", name
        yield None
def terminating(name, maxn):
    """ This is a counting task for demonstration purposes. """
    for i in xrange(maxn):
        print "Here %s, %s out of %s" % (name, i, maxn)
        yield None
    print "Done with %s, bailing out after %s times" % (name, maxn)
def delay(duration=0.8):
    """ Do nothing at all for 'duration' seconds. """
    import time
    while True:
        print "<sleep %d>" % duration
        time.sleep(duration)
        yield None
class GenericScheduler(object):
    def _ _init_ _(self, threads, stop_asap=False):
        signal.signal(signal.SIGINT, self.shutdownHandler)
        self.shutdownRequest = False
        self.threads = threads
        self.stop_asap = stop_asap
    def shutdownHandler(self, n, frame):
        """ Initiate a request to shutdown cleanly on SIGINT."""
        print "Request to shut down."
        self.shutdownRequest = True
    def schedule(self):
        def noop( ):
            while True: yield None
        n = len(self.threads)
        while True:
            for i, thread in enumerate(self.threads):
                try: thread.next( )
                except StopIteration:
                    if self.stop_asap: return
                    n -= 1
                    if n==0: return
                    self.threads[i] = noop( )
                if self.shutdownRequest:
                    return
if _ _name_ _== "_ _main_ _":
    s = GenericScheduler([ empty('boo'), delay( ), empty('foo'),
                           terminating('fie', 5), delay(0.5),
                        ], stop_asap=True)
    s.schedule( )
    s = GenericScheduler([ empty('boo'), delay( ), empty('foo'),
                           terminating('fie', 5), delay(0.5),
                        ], stop_asap=False)
    s.schedule( )

Discussion

Microthreading (or cooperative multitasking) is an important technique. If you want to pursue it in earnest for complex uses, you should definitely look up the possibilities of Christian Tismer's Stackless , a Python version specialized for microthreading, at http://www.stackless.com/. However, you can get a taste of cooperative multitasking without straying from Python's core , by making creative use of generators, as shown in this recipe.

A simple approach to cooperative multitasking, such as the one presented in this recipe, is not suitable when your tasks must perform long-running work, particularly I/O tasks that may involve blocking system calls. For such applications, look into real threading, or, as a strong alternative, look into the event-driven approach offered by module asyncore in the Python Standard Library (on a simple scale) and by package Twisted at http://twistedmatrix.com/products/twisted (on a grandiose scale). But if your application has modest I/O needs, and you can slice up any computation your tasks perform into short chunks , each of which you can end with a yield , this recipe may be just what you're looking for.

See Also

David Mertz's site, chock-full of idiosyncratic, fascinating ideas, is at http://gnosis.cx/; Christian Tismer's Stackless Python , the best way to do cooperative multitasking in Python (and much else besides), is at http://www.stackless.com/; Twisted Matrix, the best way to do event-driven (asynchronous) programming, is at http://twistedmatrix.com/.