没有 "if __name__ == ' __main_ _':"的 python3.x 多处理循环

Question

我有这个文件(它没有做任何有用的工作，仅供学习):

import multiprocessing,sys
def parent(numproc=2):
    print ('at start')
    childs=[]
    print ('bfore Pipe')
    (parentEnd,childEnd)=multiprocessing.Pipe()
    i=0
    print ('printing i:',i)
    child=multiprocessing.Process(target=child_proc, args=(childEnd,i))
    print ('created child')
    child.start()
    print ('started child')
    print ('joining child')
    child.join()
    print ('joined child')
    print ('exeted from for i in childs')
    mins=[1,2]
    print ('task ended. result: ',min(mins))
def child_proc(pipe,name):
    pass
if __name__ == '__main__':
    parent()

以这种形式它运行完美:

at start
bfore Pipe
printing i: 0
created child
started child
joining child
joined child
exeted from for i in childs
task ended. result:  1

但是如果我放在文件末尾而不是

if __name__ == '__main__':
    parent()

只有

parent()

它落在循环...

at start
bfore Pipe
printing i: 0
created child
started child
joining child
at start
bfore Pipe
printing i: 0
created child
started child
joining child
at start
bfore Pipe
printing i: 0
created child
started child
joining child
Traceback (most recent call last):

为什么？!这个 if 子句有什么不同？

Answer 1

这是 MS Windows 上 multiprocessing 的一个问题:主模块由子任务导入，因此任何代码都不受 if __name__ 保护。 . . 子句再次运行，导致无限循环。