好吧，我以某种方式做到了。我查看了python文档，并了解到：使用Queue类，我们可以从函数中获取返回值。我的代码的最终版本是这样的：
from multiprocessing import Process, Queue
def multiply(a,b,que): #add a argument to function for assigning a queue
que.put(a*b) #we're putting return value into queue
if __name__ == '__main__':
queue1 = Queue() #create a queue object
p = Process(target= multiply, args= (5,4,queue1)) #we're setting 3rd argument to queue1
p.start()
print(queue1.get()) #and we're getting return value: 20
p.join()
print("ok.")
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