Python多处理：在并行执行之前和之后串行执行代码答案

【问题标题】：Python Multiprocessing: Execute code serially before and after parallel executionPython多处理：在并行执行之前和之后串行执行代码
【发布时间】：2018-08-05 23:37:56
【问题描述】：

这里的新手：我正在尝试串行执行一些代码，然后创建一个线程池并并行执行一些代码。并行执行完成后，我想串行执行更多代码。

例如...

import time
from multiprocessing import Pool

print("I only want to print this statement once")



def worker(i):
    """worker function"""
    now = time.time()
    time.sleep(i)
    then = time.time()
    print(now, then)

if __name__ == '__main__':
    with Pool(3) as p:
        p.map(worker, [1, 1, 1])
        p.close()

print("Only print this once as well")

我希望这个返回...

I only want to print this statement once
1533511478.0619314 1533511479.0620182
1533511478.0789354 1533511479.0791905
1533511478.0979397 1533511479.098235
Only print this once as well

然而它返回的是这样的：

I only want to print this statement once
I only want to print this statement once
Only print this once as well
I only want to print this statement once
Only print this once as well
I only want to print this statement once
Only print this once as well
I only want to print this statement once
Only print this once as well
I only want to print this statement once
Only print this once as well
1533511478.0619314 1533511479.0620182
1533511478.0789354 1533511479.0791905
1533511478.0979397 1533511479.098235
Only print this once as well

所以它似乎为每个池额外运行了一次打印语句。

任何帮助将不胜感激！

【问题讨论】：

标签： python multiprocessing python-multiprocessing pool

【解决方案1】：

根据观察到的行为，我假设您使用的是 NT/Windows 操作系统。

您看到所有这些打印的原因是因为在 Windows 上使用了spawn 启动策略。当“生成”一个新进程时，会启动一个新的 Python 解释器并接收模块和它应该执行的函数。当新解释器导入模块时，将执行顶级print 函数。因此重复打印。

只需在__main__ 中移动这些打印语句，您就不会再看到它们了。

【讨论】：