Missing attribute after using multiprocessing Queue

[smuecke]

When using multiprocessing's Queue class to share PCBO instances between processes, the attribute _degree is missing after extraction. A minimal example:

import multiprocessing as mp
from qubovert import PCBO

def produce(queue):
    pcbo = PCBO({(0,1): -5})
    queue.put(pcbo)

def consume(queue):
    pcbo = queue.get() # AttributeError occurs here
    print(pcbo)

if __name__ == '__main__':
    queue = mp.Queue()

    process1 = mp.Process(target=produce, args=(queue,))
    process2 = mp.Process(target=consume, args=(queue,))

    process1.start()
    process2.start()

    process1.join()
    process2.join()

Executing this produces the following error:

Process Process-2:
Traceback (most recent call last):
  File "/usr/lib/python3.8/multiprocessing/process.py", line 315, in _bootstrap
    self.run()
  File "/usr/lib/python3.8/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "test.py", line 9, in consume
    pcbo = queue.get()
  File "/usr/lib/python3.8/multiprocessing/queues.py", line 116, in get
    return _ForkingPickler.loads(res)
  File "/home/muecke/.local/lib/python3.8/site-packages/qubovert/utils/_bo_parentclass.py", line 219, in __setitem__
    super().__setitem__(key, value)
  File "/home/muecke/.local/lib/python3.8/site-packages/qubovert/utils/_pubomatrix.py", line 384, in __setitem__
    self._degree = max(self._degree, len(k))
AttributeError: 'PCBO' object has no attribute '_degree'

OS: Linux (Manjaro, kernel 5.7.9-1-MANJARO)
Python version 3.8.3, qubovert version 1.2.3

Thanks in advance!

[jtiosue]

Hi, thanks for raising the issue! I'm afraid I'm not very familiar with the multiprocessing package or Queue object. Have you been able to reproduce this error in another way?

[smuecke]

This seems to be an issue with serialization, as I was able to reproduce it with pickle:

import pickle
from qubovert import PCBO

pcbo = PCBO({(0, 1): -5}) # error happens only when instance is not empty
data = pickle.dumps(pcbo)
pcbo_ = pickle.loads(data) # AttributeError occurs here

The clean solution will probably be to implement the __getstate__() and __setstate__() methods for all instances of PCBO, QUBO etc., which store and restore the object states manually. I think attributes like _degree starting with an underscore are problematic for serialization, because Python renames such attributes in subclasses.
Another solution might be to just rename attributes starting with _ to something without underscore.

Best regards ;)