Python multiprocessing and an imported module

Question

I have two processing running that access an imported module like that:

import foo

def bar():
  while True:
    foo.a = True

def baz():
  while True:
    print foo.a

p1 = Process(target=bar)
p2 = Process(target=baz)
p1.start()
p2.start()

It seems that each process have their own instance of module foo, bar() changes value to True, but in baz() it's False. Any workaround?

Answer 1

Unlike threads, separate processes do not share memory. There are ways, however, to share data between separate processes. One way is to use a mp.Value:

foo.py:

import multiprocessing as mp
a = mp.Value('b', False)

then the script

import time
import foo
import multiprocessing as mp

def bar():
    foo.a.value = True

def baz():
    for i in range(10**5):
        print foo.a.value

if __name__ == '__main__':
    p2 = mp.Process(target=baz)
    p2.start()
    time.sleep(0.5)
    p1 = mp.Process(target=bar)
    p1.start()

yields

0
0
0
...
1
1
1
...

---

For sharing a boolean value, an mp.Event is perhaps a better option:

foo.py:

import multiprocessing as mp
a = mp.Event()

script.py:

def bar():
    foo.a.set()

def baz():
    for i in range(10**5):
        print foo.a.is_set()

yields

False
False
False
...
True
True
True
...