The "freeze_support()" line can be omitted if the program is not going to be frozen to produce an executable

from multiprocessing import Pool
import os, time, random


def worker(msg):
    t_start = time.time()   # 获取开始时间
    print("%s开始执行，进程号为%d" % (msg, os.getpid()))
    # random.random()随机生成0~1之间的浮点数
    time.sleep(random.random()*2)
    t_stop = time.time()   # 获取执行结束时间
    print(msg, "执行完毕，耗时%.2f" % (t_stop-t_start))


po = Pool(3)   # 定义一个进程池，最大进程数是3
for i in range(0, 10):
    po.apply_async(worker, (i,))

print("___start___")
po.close()
po.join()
print("___end___")

运行如上代码时会报以下错误信息：

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.

从错误信息可以看出进程池相关代码应该放在if __name__ == '__main__'下面，那么可以将进程池运行相关代码放在main函数下面，代码修改如下：

from multiprocessing import Pool
import os, time, random


def worker(msg):
    t_start = time.time()   # 获取开始时间
    print("%s开始执行，进程号为%d" % (msg, os.getpid()))
    # random.random()随机生成0~1之间的浮点数
    time.sleep(random.random()*2)
    t_stop = time.time()   # 获取执行结束时间
    print(msg, "执行完毕，耗时%.2f" % (t_stop-t_start))


def main():
    # 定义一个进程池，最大进程数是3
    po = Pool(3)
    for i in range(0, 10):
        po.apply_async(worker, (i,))

    print("___start___")
    po.close()
    po.join()
    print("___end___")


if __name__ == '__main__':
    main()