Multi-threaded map()

mtmap.py

def mtmap(fn, iterable):
    """ A multi-threaded map(). Must be used with care - large
        iterables might cause a machine to crash from too many
        threads. If an exception is thrown in a thread, 
        the iteration stops and that exception is thrown. 
    """
    result      = {}
    threads     = []
    exception_q = Queue.Queue()

    def sort_dict(d):
        keys = d.keys()
        keys.sort()
        return [d[key] for key in keys]

    def exc(fn, i, item):
        try:
            result[i] = fn(item)
        except Exception as e:
            result[i] = None
            exception_q.put(e)

    for i, item in enumerate(iterable):
        t = threading.Thread(target=exc, args=(fn, i, item))
        threads.append(t)
        t.start()

    for t in threads:
        t.join()
        try:
            ex = exception_q.get(block=False)
            exception_q.task_done()
            raise ex
        except Queue.Empty:
            continue

    return sort_dict(result)