Multi: A helper class for waiting on multiple asynchronous things happening in parallel

multi.py

class Multi(object):

    def __init__(self, callback):
        self.callback = callback
        self.counter = 0
        self.fired = False
        self.started = False

    def enter(self):
        assert not self.started
        self.counter += 1

    def exit(self):
        self.counter -= 1
        if self.counter == 0 and self.started and not self.fired:
            self.callback()
            self.fired = True

    def start(self):
        self.started = True
        if self.counter <= 0:
            self.callback()
            self.fired = True

if __name__ == '__main__':
    def c():
        print 'done'

    m = Multi(c)

    for i in xrange(20):
        m.enter()

    m.start()

    for i in xrange(20):
        m.exit()

This is a class I end up defining on every async project I work on. It's the cleanest way I know of to have a callback fire when a set of operations finish, regardless of the order that the operations happen in. It really should be in functools.