TypeError: can't pickle _thread.lock objects

Question

Trying to run two different functions at the same time with shared queue and get an error...how can I run two functions at the same time with a shared queue? This is Python version 3.6 on Windows 7.

from multiprocessing import Process
from queue import Queue
import logging

def main():
    x = DataGenerator()
    try:
        x.run()
    except Exception as e:
        logging.exception("message")


class DataGenerator:

    def __init__(self):
        logging.basicConfig(filename='testing.log', level=logging.INFO)

    def run(self):
        logging.info("Running Generator")
        queue = Queue()
        Process(target=self.package, args=(queue,)).start()
        logging.info("Process started to generate data")
        Process(target=self.send, args=(queue,)).start()
        logging.info("Process started to send data.")

    def package(self, queue): 
        while True:
            for i in range(16):
                datagram = bytearray()
                datagram.append(i)
                queue.put(datagram)

    def send(self, queue):
        byte_array = bytearray()
        while True:
            size_of__queue = queue.qsize()
            logging.info(" queue size %s", size_of_queue)
            if size_of_queue > 7:
                for i in range(1, 8):
                    packet = queue.get()
                    byte_array.append(packet)
                logging.info("Sending datagram ")
                print(str(datagram))
                byte_array(0)

if __name__ == "__main__":
    main()

The logs indicate an error, I tried running console as administrator and I get the same message...

INFO:root:Running Generator
ERROR:root:message
Traceback (most recent call last):
  File "test.py", line 8, in main
    x.run()
  File "test.py", line 20, in run
    Process(target=self.package, args=(queue,)).start()
  File "C:\ProgramData\Miniconda3\lib\multiprocessing\process.py", line 105, in start
    self._popen = self._Popen(self)
  File "C:\ProgramData\Miniconda3\lib\multiprocessing\context.py", line 223, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "C:\ProgramData\Miniconda3\lib\multiprocessing\context.py", line 322, in _Popen
    return Popen(process_obj)
  File "C:\ProgramData\Miniconda3\lib\multiprocessing\popen_spawn_win32.py", line 65, in __init__
    reduction.dump(process_obj, to_child)
  File "C:\ProgramData\Miniconda3\lib\multiprocessing\reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
TypeError: can't pickle _thread.lock objects

Answer 1

I had the same problem with Pool() in Python 3.6.3.

Error received: TypeError: can't pickle _thread.RLock objects

Let's say we want to add some number num_to_add to each element of some list num_list in parallel. The code is schematically like this:

class DataGenerator:
    def __init__(self, num_list, num_to_add)
        self.num_list = num_list # e.g. [4,2,5,7]
        self.num_to_add = num_to_add # e.g. 1 

        self.run()

    def run(self):
        new_num_list = Manager().list()

        pool = Pool(processes=50)
        results = [pool.apply_async(run_parallel, (num, new_num_list)) 
                      for num in num_list]
        roots = [r.get() for r in results]
        pool.close()
        pool.terminate()
        pool.join()

    def run_parallel(self, num, shared_new_num_list):
        new_num = num + self.num_to_add # uses class parameter
        shared_new_num_list.append(new_num)

The problem here is that self in function run_parallel() can't be pickled as it is a class instance. Moving this parallelized function run_parallel() out of the class helped. But it's not the best solution as this function probably needs to use class parameters like self.num_to_add and then you have to pass it as an argument.

Solution:

def run_parallel(num, shared_new_num_list, to_add): # to_add is passed as an argument
    new_num = num + to_add
    shared_new_num_list.append(new_num)

class DataGenerator:
    def __init__(self, num_list, num_to_add)
        self.num_list = num_list # e.g. [4,2,5,7]
        self.num_to_add = num_to_add # e.g. 1

        self.run()

    def run(self):
        new_num_list = Manager().list()

        pool = Pool(processes=50)
        results = [pool.apply_async(run_parallel, (num, new_num_list, self.num_to_add)) # num_to_add is passed as an argument
                      for num in num_list]
        roots = [r.get() for r in results]
        pool.close()
        pool.terminate()
        pool.join()

Other suggestions above didn't help me.

Answer 2

You need to change from queue import Queue to from multiprocessing import Queue.

The root reason is the former Queue is designed for threading module Queue while the latter is for multiprocessing.Process module.

Answer 3

multiprocessing.Pool - PicklingError: Can't pickle <type 'thread.lock'>: attribute lookup thread.lock failed

Move the queue to self instead of as an argument to your functions package and send