Question

So, here’s my situation.

I’m using PyDev in Eclipse, Python interpreter version 2.7.2 in Windows.

I’m using the built in multiprocessing library in an attempt to fork off a bunch of processes to parallelize a very compute-intensive loop. The tutorials I’ve looked at say to use,

if __name__ == "__main__":

to prevent it from spawning off near-infinite processes and bringing my system to its knees, essentially.

The problem is, I am calling this from a module, not my main script; as such, nothing after it EVER gets executed. No chance for parallelism at all. Of course, if I remove it, I get the infiniprocess spam that kills the machine executing the code.

For reference’s sake, here’s the relevant code:

from tribe import DataCache
from tribe import WorldThread
from tribe import Actor
from time import sleep
import multiprocessing

class World:
def __init__(self,numThreads,numActors,tickRate):
    print "Initalizing world..."
    self.cache = DataCache.DataCache()
    self.numThreads = numThreads
    self.numActors = numActors
    self.tickRate = tickRate
    self.actors = []
    self.processes = []
    for i in range(numActors):
        self.actors.append(Actor.Actor("test.xml",self.cache))
    print "Actors loaded."
def start_world(self):
    print "Starting world"
    run_world = True;
    while run_world:
        self.world_tick()
        sleep(2)

def world_tick(self):
        if __name__ == '__main__':
            print "World tick"
            actor_chunk = len(self.actors)/self.numThreads
            if len(self.processes)==0:
                for _ in range(self.numThreads):
                    new_process = multiprocessing.Process(WorldThread.WorldProcess.work, args=(_, self.actors[_*actor_chunk,(_+1)*actor_chunk]))

And the class it is calling:

class WorldProcess():
def __init__(self):
    print "World process initilized."
    ''' Really, I'm not sure what kind of setup we'll be doing here yet. '''
def work(self, process_number, actors):
    print "World process" + str(process_number) + " running."
    for actor in actors:
        actor.tick()
    print "World process" + str(process_number) + " completed."

Am I correct in my assessment that the whole if name == “main”: check only works if you have it in the executable script itself? If so, how do you safely fork off processes from within modules? If not, why isn’t it working here?

Answer 1

Adding this as an answer, since it was in the comments:

if __name__ == "__main__" is something you do at the root level of a script that is going to be an entry point. Its a way to only do things if the script is being executed directly.

If you have a script that is the entry point, you do the name == main. And in a module you want to multiprocess, you just loop and start your processes the same way you loop and start threads.