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

【问题标题】：Python's ProcessPoolExecutor is giving output of print and return statement in reverse orderPython 的 ProcessPoolExecutor 以相反的顺序给出 print 和 return 语句的输出
【发布时间】：2020-07-16 00:35:52
【问题描述】：

我正在练习在 Python 中使用多处理，所以我遇到了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)')

我应该期待类似以下的输出：

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 语句之前是不是很奇怪？这是确切的片段： sublime_snip

【问题讨论】：

标签： python python-multiprocessing

【解决方案1】：

这可能与标准输出缓冲有关。尝试在您的打印语句中传递flush=True，例如：

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

您可以看到这是多处理打印的一个非常常见的问题：Python multithreaded print statements delayed until all threads complete execution

【讨论】：