Wait until one of several greenlets finished

Question

I have two functions that receives data from two different connections, and i should close both connections after getting result from one of them.

def first():
    gevent.sleep(randint(1, 100))  # i don't know how much time it will work
    return 'foo'

def second():
    gevent.sleep(randint(1, 100))  # i don't know how much time it will work
    return 'bar'

Then i spawn each function:

lst = [gevent.spawn(first), gevent.spawn(second)]

gevent.joinall blocks current greenlet until both two greenlets from lst are ready.

gevent.joinall(lst)  # wait much time
print lst[0].get(block=False)   # -> 'foo'
print lst[1].get(block=False)   # -> 'bar'

I want to wait until eiter first or second greenlet become ready:

i_want_such_function(lst)  # returns after few seconds
print lst[0].get(block=False)  # -> 'foo' because this greenlet is ready
print lst[1].get(block=False)  # -> raised Timeout because this greenlet is not ready

How can i do it?

Answer 1

You could use gevent.event.Event (or AsyncResult) and Greenlet's link() method, like this:

...
ready = gevent.event.Event()
ready.clear()

def callback():
    ready.set()

lst = [gevent.spawn(first), gevent.spawn(second)]
for g in lst:
    g.link(callback)

ready.wait()
...

Answer 2

callback receives gevent child process and you can get e.g. return value from it

...

cars = []

def _callback(job):
    cars.append(job.value)

for car in xml_all_cars:
    print "creating jobs"
    g_parse = Greenlet.spawn(myMainFunction)
    g_parse.start()
    g_parse.link(_callback)
    jobs.append(g_parse)

print "starting all jobs"
gevent.joinall(jobs)
print "jobs done"

return cars

cars will have list of all values that myMainFunction returns (for every car)