上下文管理器中concurrent.futures.ThreadPoolExecutor中的超时不能正常工作

Question

有人能帮我解释一下为什么在上下文管理器中使用超时时超时不能正常工作吗？

它不使用上下文管理器就可以正常工作，它会在5秒后引发TimeoutException，但使用上下文管理器时，5秒后不会引发异常。

没有上下文管理器的实现

from concurrent import futures
MAX_WORKERS = 20

def download_one(c):
    import time
    time.sleep(100)


def download_many():
    executor = futures.ThreadPoolExecutor(MAX_WORKERS)
    res = executor.map(download_one, [1,2,3,4],timeout=5)
    print(list(res))

    return len(list(res))


download_many()

使用上下文管理器实现

from concurrent import futures
MAX_WORKERS = 20

def download_one(c):
    import time
    time.sleep(100)


def download_many():
    with futures.ThreadPoolExecutor(MAX_WORKERS) as executor:
        res = executor.map(download_one, [1,2,3,4],timeout=5)
        print(list(res))

    return len(list(res))

download_many()

Answer 1

在这两种情况下，超时都能正常工作。在调用list(res)后5秒(=指定的超时值)引发concurrent.futures._base.TimeoutError。但是，在使用上下文管理器( with语句)时，将调用上下文管理器的__exit__方法。在这种情况下，它将等待所有线程完成，然后才会离开上下文并(重新)引发原始错误。

这可以通过在正确的位置捕获和记录异常来演示：

import concurrent
import logging
import time
from concurrent import futures

MAX_WORKERS = 20


def download_one(c):
    logging.info('download_one(%s)' % str(c))
    time.sleep(10)  # Note: reduced to 10 seconds


def sub(executor):
    try:
        futures = executor.map(download_one, [1,2,3,4], timeout=5)
    except concurrent.futures._base.TimeoutError:
        logging.info('map timed out!')  # this is never logged
        raise

    try:
        results = list(futures)
    except concurrent.futures._base.TimeoutError:
        logging.info('list timed out!')  # here it happens!
        raise

    logging.info(results)
    logging.info('sub done')
    return len(result)


def download_many1():  # without context manager
    logging.info('download_many1')
    executor = futures.ThreadPoolExecutor(MAX_WORKERS)
    return sub(executor)


def download_many2():  # with context manager
    logging.info('download_many2')
    with futures.ThreadPoolExecutor(MAX_WORKERS) as executor:
        return sub(executor)


logging.basicConfig(level=logging.DEBUG, format='%(asctime)-15s   %(message)s')

logging.info('start 1')
try:
    download_many1()
except concurrent.futures._base.TimeoutError:
    logging.info('timeout!')
finally:
    logging.info('1 finished\n')

logging.info('start 2')
try:
    download_many2()
except concurrent.futures._base.TimeoutError:
    logging.info('timeout!')
finally:
    logging.info('2 finished\n')

这将输出以下内容：

2019-04-27 21:17:20,578   start 1
2019-04-27 21:17:20,578   download_many1
2019-04-27 21:17:20,578   download_one(1)
2019-04-27 21:17:20,578   download_one(2)
2019-04-27 21:17:20,578   download_one(3)
2019-04-27 21:17:20,578   download_one(4)
2019-04-27 21:17:25,593   list timed out!   # actual timeout after 5 seconds
2019-04-27 21:17:25,593   timeout!          # the timeout you see at the same time
2019-04-27 21:17:25,593   1 finished

2019-04-27 21:17:25,593   start 2
2019-04-27 21:17:25,593   download_many2
2019-04-27 21:17:25,593   download_one(1)
2019-04-27 21:17:25,593   download_one(2)
2019-04-27 21:17:25,593   download_one(3)
2019-04-27 21:17:25,593   download_one(4)
2019-04-27 21:17:30,610   list timed out!   # actual timeout after 5 seconds
2019-04-27 21:17:35,608   timeout!          # the timeout you see 5 seconds later!!
2019-04-27 21:17:35,608   2 finished