How to use a multiprocessing.Manager()?

Question

I have a concern about multiprocessing.Manager() in python. Here is the example:

import multiprocessing

def f(ns):
    ns.x *=10
    ns.y *= 10

if __name__ == '__main__':
    manager = multiprocessing.Manager()
    ns = manager.Namespace()
    ns.x = 1
    ns.y = 2

    print 'before', ns
    p = multiprocessing.Process(target=f, args=(ns,))
    p.start()
    p.join()
    print 'after', ns

and the output is:

before Namespace(x=1, y=2)
after Namespace(x=10, y=20)

Until now, it worked as I expected, then I modified the code like this:

import multiprocessing

def f(ns):
    ns.x.append(10)
    ns.y.append(10)

if __name__ == '__main__':
    manager = multiprocessing.Manager()
    ns = manager.Namespace()
    ns.x = []
    ns.y = []

    print 'before', ns
    p = multiprocessing.Process(target=f, args=(ns,))
    p.start()
    p.join()
    print 'after', ns

Now the output is:

before Namespace(x=[], y=[])
after Namespace(x=[], y=[])

It confuses me why the lists were not changed as I expected. Can anyone help me to figure out what happened?

Answer 1

Manager proxy objects are unable to propagate changes made to (unmanaged) mutable objects inside a container. So in other words, if you have a manager.list() object, any changes to the managed list itself are propagated to all the other processes. But if you have a normal Python list inside that list, any changes to the inner list are not propagated, because the manager has no way of detecting the change.

In order to propagate the changes, you have to use manager.list() objects for the nested lists too (requires Python 3.6 or newer), or you need to modify the manager.list() object directly (see the note on manager.list in Python 3.5 or older).

For example, consider the following code and its output:

import multiprocessing
import time

def f(ns, ls, di):
    ns.x += 1
    ns.y[0] += 1
    ns_z = ns.z
    ns_z[0] += 1
    ns.z = ns_z

    ls[0] += 1
    ls[1][0] += 1 # unmanaged, not assigned back
    ls_2 = ls[2]  # unmanaged...
    ls_2[0] += 1
    ls[2] = ls_2  # ... but assigned back
    ls[3][0] += 1 # managed, direct manipulation

    di[0] += 1
    di[1][0] += 1 # unmanaged, not assigned back
    di_2 = di[2]  # unmanaged...
    di_2[0] += 1
    di[2] = di_2  # ... but assigned back
    di[3][0] += 1 # managed, direct manipulation

if __name__ == '__main__':
    manager = multiprocessing.Manager()
    ns = manager.Namespace()
    ns.x = 1
    ns.y = [1]
    ns.z = [1]
    ls = manager.list([1, [1], [1], manager.list([1])])
    di = manager.dict({0: 1, 1: [1], 2: [1], 3: manager.list([1])})

    print('before', ns, ls, ls[2], di, di[2], sep='\n')
    p = multiprocessing.Process(target=f, args=(ns, ls, di))
    p.start()
    p.join()
    print('after', ns, ls, ls[2], di, di[2], sep='\n')

Output:

before
Namespace(x=1, y=[1], z=[1])
[1, [1], [1], <ListProxy object, typeid 'list' at 0x10b8c4630>]
[1]
{0: 1, 1: [1], 2: [1], 3: <ListProxy object, typeid 'list' at 0x10b8c4978>}
[1]
after
Namespace(x=2, y=[1], z=[2])
[2, [1], [2], <ListProxy object, typeid 'list' at 0x10b8c4630>]
[2]
{0: 2, 1: [1], 2: [2], 3: <ListProxy object, typeid 'list' at 0x10b8c4978>}
[2]

As you can see, when a new value is assigned directly to the managed container, it changes; when it is assigned to a mutable container within the managed container, it doesn't change; but if the mutable container is then reassigned to the managed container, it changes again. Using a nested managed container also works, detecting changes directly without having to assign back to the parent container.

Answer 2

ns is a NamespaceProxy instance. These objects have special __getattr__, __setattr__, and __delattr__ methods that allow values to be shared across processes. In order to take advantage of this mechanism when changing a value, you must trigger __setattr__.

ns.x.append(10)

causes ns.__getattr__ to be called to retrieve ns.x, but it does not cause ns.__setattr__ to be called.

To fix this, you must use ns.x = ....

def f(ns):   
    tmp = ns.x     # retrieve the shared value
    tmp.append(10)
    ns.x = tmp     # set the shared value