Python threading: can I sleep on two threading.Event()s simultaneously?

Question

If I have two threading.Event() objects, and wish to sleep until either one of them is set, is there an efficient way to do that in python? Clearly I could do

Answer 1

Here is a non-polling non-excessive thread solution: modify the existing Events to fire a callback whenever they change, and handle setting a new event in that callback:

import threading

def or_set(self):
    self._set()
    self.changed()

def or_clear(self):
    self._clear()
    self.changed()

def orify(e, changed_callback):
    e._set = e.set
    e._clear = e.clear
    e.changed = changed_callback
    e.set = lambda: or_set(e)
    e.clear = lambda: or_clear(e)

def OrEvent(*events):
    or_event = threading.Event()
    def changed():
        bools = [e.is_set() for e in events]
        if any(bools):
            or_event.set()
        else:
            or_event.clear()
    for e in events:
        orify(e, changed)
    changed()
    return or_event

Sample usage:

def wait_on(name, e):
    print "Waiting on %s..." % (name,)
    e.wait()
    print "%s fired!" % (name,)

def test():
    import time

    e1 = threading.Event()
    e2 = threading.Event()

    or_e = OrEvent(e1, e2)

    threading.Thread(target=wait_on, args=('e1', e1)).start()
    time.sleep(0.05)
    threading.Thread(target=wait_on, args=('e2', e2)).start()
    time.sleep(0.05)
    threading.Thread(target=wait_on, args=('or_e', or_e)).start()
    time.sleep(0.05)

    print "Firing e1 in 2 seconds..."
    time.sleep(2)
    e1.set()
    time.sleep(0.05)

    print "Firing e2 in 2 seconds..."
    time.sleep(2)
    e2.set()
    time.sleep(0.05)

The result of which was:

Waiting on e1...
Waiting on e2...
Waiting on or_e...
Firing e1 in 2 seconds...
e1 fired!or_e fired!

Firing e2 in 2 seconds...
e2 fired!

This should be thread-safe. Any comments are welcome.

EDIT: Oh and here is your wait_for_either function, though the way I wrote the code, it's best to make and pass around an or_event. Note that the or_event shouldn't be set or cleared manually.

def wait_for_either(e1, e2):
    OrEvent(e1, e2).wait()