Python 進程池 pool

from multiprocessing import Process,Pool
import time,os

def Foo(i):
    time.sleep(2)
    print("in the Proess ",os.getpid())
    return i
def Bar(arg):
    print("-->exec done:",arg,os.getpid())

if __name__ == "__main__":
    print("in the main process",os.getpid())
    pool = Pool(3)
    for i in range(10):
        pool.apply_async(func=Foo,args=(i,),callback=Bar)
        #pool.apply(func=Foo,args=(i,))
    print("end")
    pool.close()
    pool.join()