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

Question

我正在练习在 Python 中使用多处理，所以我遇到了concurrent.futuresmodule。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)')

我应该期望类似以下的输出：

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，这不是很奇怪吗？这是确切的片段： sublime_snip

Answer 1

这可能与标准输出缓冲有关。尝试传递flush=True您的打印语句，例如：

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

您可以看到这是多进程打印的一个非常常见的问题：Python 多线程打印语句延迟直到所有线程完成执行