I am trying to understand how multiprocess pools work. In the following programing I created a pool of 4 processes.
And I call apply_async
with a callback function that should update a list called result_list
import Queue
from multiprocessing import Process
from multiprocessing import Pool
result_list = []
def foo_pool(q): #Function for each process
print "foo_pool"
if(q.qsize() > 0):
number = q.get()
return number * 2
def log_result(result):
# This is called whenever foo_pool(i) returns a result.
# result_list is modified only by the main process, not the pool workers.
result_list.append(result)
if __name__ == "__main__":
q = Queue.Queue()
for i in range(4):
q.put(i + 1) #Put 1..4 in the queue
p = Pool(4)
p.apply_async(foo_pool, args = (q, ), callback = log_result)
I realize I don't need to use a queue here. But I am testing this for another program which requires me to use a queue. When I run the program, the function foo_pool is not being called. The print statement print "foo_pool"
does not execute. Why is this?
Roughly speaking, apply_async only schedule async task, but not run it. You need to call p.close()
and p.join()
to trigger execution or r = p.apply_async()
and r.get()
.
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