多处理无法与cmd2模块一起使用，导入问题？

Question

Extending the solution from this question Multiprocessing code works upon import, breaks upon being called , I have injected some multiprocessing code into my project, which has promptly broken.

I think there are import issues. I have two modules. test.py looks like:

print 'omnia praeclara'
import multi4
if __name__ == "__main__":
    multi4.init_manager()
print 'rara sunt'   

and multi4.py looks like:

import multiprocessing as mp
def add():
    print 2+2
def init_manager():
    proc = mp.Process(target=add)
    proc.start()
    proc.join()

Now, this code works fine. It produces the following when run from the command line (Windows):

omnia praeclara
omnia praeclara
rara sunt
4
rara sunt

Which is to be expected (the double printing is explained in the link above).

HOWEVER --- when I make test.py like this:

print 'omnia praeclara'
import multi4
if __name__ == "__main__":
    multi4.init_manager()
print 'rara sunt'   

import cmd2
class Prompt(cmd2.Cmd):

    def default(self, line):
        return cmd2.Cmd.default(self, line)
    prompt = '\n+++ '
    intro = '\n by that remembered or with that forgot.'
    def do_exit(self, line):
        return True

Prompt().cmdloop()

I get

omnia praeclara
omnia praeclara
rara sunt
by that remembered or with that forgot.
+++

This is the command prompt for the cmd2 module. The process calling add does not produce anything. It is hung now. If I type exit into the prompt I get:

+++ exit
4
rara sunt

by that remembered or with that forgot.
+++

So the multiprocessing code finishes, but now I am back into my cmd2 prompt! It is only when I exit again that I really exit.

Clearly, everything is being imported twice. This was referenced in the above link, but is there any way to avoid this? More importantly, how can I get my processes working in the background while I am doing things in cmd2 ?

Answer 1

This is the same issue that's causing the double printing - anything in test.py not protected by the if __name__ == "__main__": guard will get executed in both the parent and child processes. You need to move the call to Prompt().cmdloop() under the guard to prevent it from being executed in the child:

print 'omnia praeclara'
import multi4

print 'rara sunt'   
import cmd2

class Prompt(cmd2.Cmd):

    def default(self, line):
        return cmd2.Cmd.default(self, line)
    prompt = '\n+++ '
    intro = '\n by that remembered or with that forgot.'
    def do_exit(self, line):
        return True

if __name__ == "__main__":
    multi4.init_manager()
    Prompt().cmdloop()