python singleton into multiprocessing

Question

How can I code to share the same instance of a \"singletonic\" class among processes?

Answer 1

Best is to designate one specific process as owning that instance and dedicated to it; any other process requiring access to that instance obtains it by sending messages to the owning process via a Queue (as supplied by the multiprocessing module) or other IPC mechanisms for message passing, and gets answers back via similar mechanisms.

Answer 2

I do not think you can share the instance between the processes, but you can have the instance access shared memory: http://docs.python.org/library/multiprocessing.html#sharing-state-between-processes to control it's state if that is really what you want to do.

However, as stated in other answers, it might be easier to achieve what you want with a Queue.

Answer 3

You could also use one of the multiprocessing.manager shareable data types

import multiprocessing as mp
manager = mp.Manager()
shared_list = manager.list()

def worker1(l):
    l.append(1)

def worker2(l):
    l.append(2)

process1 = mp.Process(target=worker1, args=[shared_list])
process2 = mp.Process(target=worker2, args=[shared_list])

process1.start()
process2.start()
process1.join()
process2.join()

print shared_list

output would look like:

[1, 2]

There are a variety of data structures on offer but you'll need the parent process to pass in the shared object to the child objects.

Answer 4

In Python 2.6 the multiprocessing module has a Value object used for sharing state between processes. They have a code sample that should give you an idea of how to share state in this manner, and you can use this approach when writing a singleton class.

Answer 5

The whole point of processes is to have different address spaces. If you want to share information between processes you must use some means of interprocess communication.