How to work with a multiprocessing.Manager() Array in python

Question

I'm launching a worker process using Python's multiprocessing. I need to be able to update an array in the subprocess that can be seen in the parent process. I use multiprocessing.Manager() to accomplish that.

a = multiprocessing.Manager().Array('f', [0,0])
a
<ArrayProxy object, typeid 'Array' at 0x7f4e2b4eeda0>

a.value
Traceback (most recent call last):
  File "<blender_console>", line 1, in <module>
AttributeError: 'ArrayProxy' object has no attribute 'value'

When I create a Value('f', 0.0) I can operate on it this way. When I look at dir(a) I don't see any obvious functions or attributes to call, and the documentation in python is really fuzzy about this point.

Answer 1

If I understand you correctly, you need to update multiple values inside a Array atomically.

In this case, I suggest you:

a.acquire()  # Using its internal lock object
a[0] = ...   # Do what ever you want
a[1] = ...
a.release()

Answer 2

a is a proxy object

<ArrayProxy object, typeid 'Array' at 0x7f4e2b4eeda0>

A proxy is an object which refers to a shared object which lives (presumably) in a different process. The shared object is said to be the referent of the proxy.

The relationship between proxy object and the real object as following shown, in multiprocessing/managers.py

SyncManager.register('Value', Value, ValueProxy)
SyncManager.register('Array', Array, ArrayProxy)

And we see the supported operators of ArrayProxy

ArrayProxy = MakeProxyType('ArrayProxy', (
    '__len__', '__getitem__', '__setitem__', '__getslice__', '__setslice__'
    ))                  # XXX __getslice__ and __setslice__ unneeded in Py3.0

The Array is actually array.array(...)

Value(typecode, value) is different from Array

Create an object with a writable value attribute and return a proxy for it.

class ValueProxy(BaseProxy):
    _exposed_ = ('get', 'set')
    def get(self):
        return self._callmethod('get')
    def set(self, value):
        return self._callmethod('set', (value,))
    value = property(get, set)