CODEWITHSUNDEEP
pythonmultiprocessing
hemanths
hemanths

Reputation: 61

What is wrong with this Python Multiprocessing Code?

I am trying to create some multiprocessing code for my project. I have created a snippet of the things that I want to do. However its not working as per my expectations. Can you please let me know what is wrong with this.

from multiprocessing import Process, Pipe
import time

class A:
      def __init__(self,rpipe,spipe):
          print "In the function fun()"

      def run(self):
           print"in run method"
           time.sleep(5)
           message = rpipe.recv()
           message = str(message).swapcase()
           spipe.send(message)

 workers = []
 my_pipe_1 = Pipe(False)
 my_pipe_2 = Pipe(False)
 proc_handle = Process(target = A, args=(my_pipe_1[0], my_pipe_2[1],))
 workers.append(proc_handle)
 proc_handle.run()
 my_pipe_1[1].send("hello")
 message = my_pipe_2[0].recv()
 print message
 print "Back in the main function now"

The trace back displayed when i press ctrl-c:

 ^CTraceback (most recent call last):
 File "sim.py", line 22, in <module>
 message = my_pipe_2[0].recv()
 KeyboardInterrupt

When I run this above code, the main process does not continue after calling "proc_handle.run". Why is this?

Upvotes: 0

Views: 255

Answers (2)

dano
dano

Reputation: 94961

You've misunderstood how to use Process. You're creating a Process object, and passing it a class as target, but target is meant to be passed a callable (usually a function) that Process.run then executes. So in your case it's just instantiating A inside Process.run, and that's it.

You should instead make your A class a Process subclass, and just instantiate it directly:

#!/usr/bin/python

from multiprocessing import Process, Pipe
import time

class A(Process):
      def __init__(self,rpipe,spipe):
          print "In the function fun()"
          super(A, self).__init__()
          self.rpipe = rpipe
          self.spipe = spipe

      def run(self):
           print"in run method"
           time.sleep(5)
           message = self.rpipe.recv()
           message = str(message).swapcase()
           self.spipe.send(message)

if __name__ == "__main__":    
    workers = []
    my_pipe_1 = Pipe(False)
    my_pipe_2 = Pipe(False)
    proc_handle = A(my_pipe_1[0], my_pipe_2[1])
    workers.append(proc_handle)
    proc_handle.start()
    my_pipe_1[1].send("hello")
    message = my_pipe_2[0].recv()
    print message
    print "Back in the main function now"

mgilson was right, though. You should call start(), not run(), to make A.run execute in a child process.

With these changes, the program works fine for me:

dan@dantop:~> ./mult.py 
In the function fun()
in run method
HELLO
Back in the main function now

Upvotes: 1

mgilson
mgilson

Reputation: 310187

Taking a stab at this one, I think it's because you're calling proc_handle.run() instead of proc_handle.start().

The former is the activity that the process is going to do -- the latter actually arranges for run to be called on a separate process. In other words, you're never forking the process, so there's no other process for my_pipe_1[1] to communicate with so it hangs.

Upvotes: 0

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