if __name__ == '__main__':在一个多进程python程序中的必要性

import multiprocessing, time

def test(i):
    while 1:
        print(i)
        time.sleep(.2)

# if __name__ == '__main__': --- 去掉则会报错
multiprocessing.Process(target=test, args=(1,)).start()
multiprocessing.Process(target=test, args=(2,)).start()

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

#        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.

如果不加 if name == ‘main‘: 则会报错。

子进程会在运行时拷贝当前主进程中的所有内容，这也就意味着当一个新的子进程被创建的时候，该子进程就会复制当前模块，当然也包括了以下两行：

multiprocessing.Process(target=test, args=(1,)).start()
multiprocessing.Process(target=test, args=(2,)).start()

很显然，这样的写法可能形成无限递归式地创建新的子进程。所以为了避免以上情况发生，我们在此引入了 if name == ‘main‘: 。

以上内容参考：
https://*.com/questions/34223502/why-does-this-multiprocessing-code-fail
https://blog.csdn.net/mihanglaoban/article/details/82319892