Question

I have a Manager (main thread), that creates other Threads to handle various operations.
I would like my Manager to be notified when a Thread it created ends (when run() method execution is finished).

I know I could do it by checking the status of all my threads with the Thread.isActive() method, but polling sucks, so I wanted to have notifications.

I was thinking of giving a callback method to the Threads, and call this function at the end of the run() method:

class Manager():
    ...
    MyThread(self.on_thread_finished).start() # How do I pass the callback

    def on_thread_finished(self, data):
        pass
    ...

class MyThread(Thread):
    ...
    def run(self):
        ....
        self.callback(data) # How do I call the callback?
    ...

Thanks!

Answer 1

The thread can’t call the manager unless it has a reference to the manager. The easiest way for that to happen is for the manager to give it to the thread at instantiation.

class Manager(object):
    def new_thread(self):
        return MyThread(parent=self)
    def on_thread_finished(self, thread, data):
        print thread, data

class MyThread(Thread):

    def __init__(self, parent=None):
        self.parent = parent
        super(MyThread, self).__init__()

    def run(self):
        # ...
        self.parent and self.parent.on_thread_finished(self, 42)

mgr    = Manager()
thread = mgr.new_thread()
thread.start()

If you want to be able to assign an arbitrary function or method as a callback, rather than storing a reference to the manager object, this becomes a bit problematic because of method wrappers and such. It’s hard to design the callback so it gets a reference to both the manager and the thread, which is what you will want. I worked on that for a while and did not come up with anything I’d consider useful or elegant.