python进程池multiprocessing.Pool运行错误：The "freeze_support()" line can be omitted if the program is not g

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本文介绍了一个使用Python的multiprocessing模块创建进程池时遇到的典型错误，并提供了正确的代码实现方式。错误源于未在主模块中正确使用进程启动方法，通过加入if __name__ == '__main__':判断并调用multiprocessing.freeze_support()可以解决此问题。

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测试代码如下：

# -*- coding: utf-8 -*-

import multiprocessing
import time


def func(msg):
    print('msg: ', msg)
    time.sleep(1)
    print('----')

pool = multiprocessing.Pool(processes=4)
for i in range(10):
    msg = 'hello world %d' % i
    pool.apply_async(func, (msg, ))

pool.close()
pool.join()

上面的代码运行将产生如下错误信息：

RuntimeError: 
        An attempt has been made to start a new process before the
        current process has finished its .

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.

从错误信息可以看出进程池相关代码应该放在if __name__ == '__main__'下面，代码修改如下：

# -*- coding: utf-8 -*-

import multiprocessing
import time


def func(msg):
    print('msg: ', msg)
    time.sleep(1)
    print('----')

if __name__ == '__main__':
    pool = multiprocessing.Pool(processes=4)
    for i in range(10):
        msg = 'hello world %d' % i
        pool.apply_async(func, (msg, ))

    pool.close()
    pool.join()