How to pass and run a callback method in Python

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.

Answer 2

If you want the main thread to wait for children threads to finish execution, you are probably better off using some kind of synchronization mechanism. If simply being notified when one or more threads has finished executing, a Condition is enough:

import threading

class MyThread(threading.Thread):
    def __init__(self, condition):
        threading.Thread.__init__(self)
        self.condition = condition

    def run(self):
        print "%s done" % threading.current_thread()
        with self.condition:
            self.condition.notify()


condition = threading.Condition()
condition.acquire()

thread = MyThread(condition)
thread.start()

condition.wait()

However, using a Queue is probably better, as it makes handling multiple worker threads a bit easier.

Answer 3

Anything wrong with doing it this way?

from threading import Thread

class Manager():
    def Test(self):
        MyThread(self.on_thread_finished).start()

    def on_thread_finished(self, data):
        print "on_thread_finished:", data

class MyThread(Thread):
    def __init__(self, callback):
        Thread.__init__(self)
        self.callback = callback

    def run(self):
        data = "hello"
        self.callback(data)

m = Manager()
m.Test() # prints "on_thread_finished: hello"