Python的ProcessPoolExecutor以相反的顺序给出print和return语句的输出

Question

我在Python中练习使用多处理，因此遇到了concurrent.futures模块。我尝试运行以下代码：

import concurrent.futures
import time


def do_something(seconds):
    print(f'Sleeping {seconds} second(s)...')
    time.sleep(seconds)
    return f'Done Sleeping for {seconds} second(s)'

def main():

    with concurrent.futures.ProcessPoolExecutor() as executor:
        results = [executor.submit(do_something, 1.5) for _ in range(2)]

        for f in concurrent.futures.as_completed(results):
            print(f.result())


if __name__ == '__main__':
    start = time.perf_counter()
    main()
    finish = time.perf_counter()
    print(f'Finished in {round(finish-start, 2)} second(s)')

我希望输出如下所示：

Sleeping 1.5 second(s)...
Sleeping 1.5 second(s)...
Done Sleeping for 1.5 second(s)
Done Sleeping for 1.5 second(s)
Finished in 1.5 second(s)

但是，我得到了：

Done Sleeping for 1.5 second(s)
Done Sleeping for 1.5 second(s)
Sleeping 1.5 second(s)...
Sleeping 1.5 second(s)...
Finished in 1.83 second(s)

return语句出现在print语句之前不是很奇怪吗？ 以下是确切的剪报： sublime_snip

Answer 1

这可能与标准输出缓冲有关。尝试在print语句中传递flush=True，如下所示：

print(f'Sleeping {seconds} second(s)...', flush=True)

您可以看到，这是多处理打印中非常常见的问题：Python multithreaded print statements delayed until all threads complete execution