Python multiprocessing Queues pickling error

Question

I have the following class:

class MDP(object):

    def __init__(self, level, state_var):
        self.state_var = state_var
        self.level = level
        ...
   
    def __repr__(self):
       return "level {} var {}".format(self.level, self.state_var)

    def __eq__(self, other):
        return self.level == other.level and self.state_var == other.state_var

    def __hash__(self):
        return hash((self.level,) + self.state_var)

    def __lt__(self, other):
        return self.state_var < other.state_var
    ...

I have another class for my GUI that looks like this:

class GUI:
    ...
    self.queue = multiprocessing.Queue()
    self.process = multiprocessing.Process(target=self.start, args=(self.queue,))
    self.process.start()

    def start(self, queue):
        ...
        pygame.init()
        ...
        while self.run:
            clock.tick(Consts.FPS)

            if not queue.empty():
                event = queue.get()

            self.container.render()
            pygame.display.update()

        queue.close()
        pygame.quit()
        sys.exit()
    
    def render_q_values(self, q_values):
        self.queue.put(Event(EventType.QVAL, q_values))

Event is simple and looks like this:

class Event():
    def __init__(self, kind, data):
        self.kind = kind
        self.data = data

When I call gui.render_q_values(q_values) where q_values is a dictionary where keys are MDP objects and values are integers, I get the following error on the line event = queue.get() :

event = queue.get()

File "/.../python3.6/multiprocessing/queues.py", line 113, in get
    return _ForkingPickler.loads(res)
File ".../mdp.py", line 50, in __eq__
    return self.level == other.level and self.state_var == other.state_var
AttributeError: 'MDP' object has no attribute 'level'

My MDP objects are hashable and all those attributes are initialized, I have them in sets and dictionaries, but when I try to use the multiprocessing module to put them in Queues, I get these kind of errors. The main problem seems to be that the multiprocessing module can't pickle my MDP objects, any ideas why?

Answer 1

This error actually had nothing to do with the multiprocessing module, and all to do with how I defined equality and hash functions in my MDP object: Can't pickle objects with recursive references