multiprocessing在类中使用问题记录

问题：当在类中使用multiprocessing.Pool会报pickling error错误.

原因：pool方法都使用了queue.Queue将task传递给工作进程。multiprocessing必须将数据序列化以在进程间传递。方法只有在模块的顶层时才能被序列化，跟类绑定的方法不能被序列化，就会出现上面的异常。

解决：

方法1：用线程代替进程

# coding: utf8
from multiprocessing.pool import ThreadPool as Pool
 
 
class MyTask(object):
    def task(self, x):
        return x*x
 
    def run(self):
        pool = Pool(3)
 
        a = [1, 2, 3]
        ret = pool.map(self.task, a)
        print ret
 
 
if __name__ == '__main__':
    t = MyTask()
    t.run()

方法2：可以使用copy_reg来规避上面的异常.

# coding: utf8
import multiprocessing
import types
import copy_reg
 
 
def _pickle_method(m):
    if m.im_self is None:
        return getattr, (m.im_class, m.im_func.func_name)
    else:
        return getattr, (m.im_self, m.im_func.func_name)
 
 
copy_reg.pickle(types.MethodType, _pickle_method)
 
 
class MyTask(object):
    def __init__(self):
        self.__result = []
 
    def task(self, x):
        return x * x
 
    def result_collector(self, result):
        self.__result.append(result)
 
    def run(self):
        pool = multiprocessing.Pool(processes=3)
 
        a = [1, 2, 3]
        ret = pool.map(self.task, a)
        print ret
 
 
if __name__ == '__main__':
    t = MyTask()
    t.run()

方法3：换库(dill或者pathos)
dill 或pathos.multiprocesssing ：use pathos.multiprocesssing, instead of multiprocessing. pathos.multiprocessing is a fork of multiprocessing that uses dill. dill can serialize almost anything in python, so you are able to send a lot more around in parallel.